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VOLUME 20 NUMBER 2 • QUARTER TWO - 2012

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Issue 96 • Quarter 2 - 2012 • Volume 20 - Number 2

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TRUSTED INSULATORS

Xinjiang Xinneng TIANNING Electrical Engineering Isolating Materials Co., Ltd. No. 45, Taishan Road, Economic & Technical Development Zone Urumqi, Xinjiang, China 830026 Tel: ++86-991-2928153 路 Telefax: ++86-991-2928143 www.tenet-insulator.com路email: tenet_tn@163.com

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TRUSTED SUPPLIER

As one of China’s most respected and qualified insulator manufacturers, we understand that changing suppliers is not something to be done lightly. That’s why we offer our new customers throughout the world the peace of mind that we have been supplying insulators to the highest voltage levels as well as highest quality standards for almost 20 years. Our insulators have provided safe and reliable service on many of China’s most important lines, including critical UHV lines such as the Xiangjiaba-Shanghai ±800kV UHV DC Transmission Demonstration Project. We have supplied as much as 80% of the new overhead network in Western China, including all the key lines for Xinjiang and Northwest Power Grid 750kV Interconnection Project. Look into all the advantages we can offer at all voltages in terms of our unique solid state silastic manufacturing, timely delivery and reduced cost. We hope you will agree that Tenet should be included among your preferred insulator suppliers.

A SUPPLIER OF INSULATORS YOU CAN TRUST. INMR Issue 96.indd 3

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CLOSING IN ON ISSUE NUMBER

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This is the 96th issue of INMR since it was first introduced in September 1993. That’s means that our 100th issue is not that far off and in fact will come out in Qtr 2, 2013 – coinciding exactly with our 20-year anniversary. To mark this milestone, INMR will produce a special commemorative edition featuring a retrospective of 20 years of covering the field of transmission and distribution, in addition to our usual assortment of technical articles. We hope this will be an issue that readers will

find especially interesting to keep as it will also include many of our most memorable photos from nearly 800 technical articles covering more than 60 different countries. Our first two decades have been a wonderful experience and INMR will make the entire year 2013 one marked by the best on-site reporting of the most relevant modern day topics for line as well as substation engineers. The Publisher & Staff of INMR

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2012 Claude de Tourreil Memorial Award for Lifetime Achievement in the Field of Electrical Insulators Several years ago, INMR established an annual award to honor the memory of the late Claude de Tourreil – one of the insulator industry’s most respected and influential personalities. It can fairly be said that his numerous contributions to the advancement of this field are evident even to this day, 6 years after his untimely passing. Past recipients have included Prof. Liang Xidong of Tsinghua University in China (2009), Wallace Vosloo of Eskom in South Africa (2010) and Prof. Ravi Gorur of Arizona State University in the USA (2011). This year, INMR has selected Dr. Igor Gutman of STRI in Sweden to receive this award – a choice we believe Claude would have wholeheartedly endorsed.

While coming from a much different background than Claude, Igor Gutman actually shares a lot in common with him – from his seemingly singleminded dedication to the field of electrical insulators to his years of involvement in the international standards setting community. His combed back whitish mane even resembles the hairstyle I always saw on Claude during our friendship of more than 15 years. After graduating with a Masters as well as a PhD from the State Polytechnic University in St. Petersburg, Russia, Igor embarked on a professional career that has already spanned three decades. He began his first job in 1981 at the Leningrad HVDC Power Transmission Research Institute, where he served as Senior Researcher in the High Voltage Technology Department. Later, in 1994, he joined STRI in Ludvika, where he currently still serves as Senior Specialist and also Manager of Insulation. Igor’s areas of focus at STRI relate mainly to optimizing selection of outdoor insulators as well as defining their most appropriate diagnostics and maintenance – whatever the service environment. This activity has been concentrated mainly on pollution and

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ageing performance of composite insulators and there is little doubt all his work has helped this technology move forward to its current state-ofthe-art. Ice and snow performance of insulation as well as bird-induced flashovers have also been among his interests.

risks in the power industry today is the loss of ‘technical history’ as a whole generation of specialists retire and take with them not-easily transferable knowledge gained over many decades. Thankfully, people such as him are serving as a bridge between the past and the present to help ensure this knowledge is not lost forever.

Igor has published extensively on the topic of insulators, resulting in some 150 conference and journal papers. He is a member of Sweden’s IEC TC 36 “Insulators” and active in a number of working groups within CIGRE/IEC, being Convener of CIGRE WG D1.44. He has been a Senior Member of IEEE since 2005 and last year was granted the Degree of Honorary Professor at the St. Petersburg Power Engineering Institute of I recall a dinner in Ludvika several Professional Development. years ago where Igor explained to me cases of unexplained outages along INMR and its readers worldwide salute transmission lines in Russia and Igor for all his contributions to enhance Finland. These remained a mystery our knowledge of electrical insulators to the system operators until a retired and in the process to helping make maintenance expert was brought in sure that our lights always stay on – who could explain how bird nesting whatever the service conditions. or hunting activity was the principal culprit in each case. Igor remarked at the time that one of the greatest Marvin L. Zimmerman

Editorial

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Creatures Worth Protecting It has been proven that birds first originated as far back as the time of the dinosaurs. And that places them a lot higher on the ‘evolutionary food chain’ than we humans. Yet humans have become the single greatest threat to many magnificent bird species that grace the natural world by their presence. First came glass skyscrapers that birds never really understood as an obstacle and which cause numerous fatal collisions each year. Now, as electrical networks sprout up seemingly everywhere, it is power structures as well as the conductors strung between them that have taken over as the major threat.

We must recognize and then accept that yet another high environmental cost of using electricity is the terrible toll of power networks on the avian world. In this issue is an article that addresses the so-called ‘bird outage’ problem properly and authoritatively – from both perspectives. And that’s a refreshing change. Increasing awareness that this problem, while perhaps impossible to fully eliminate, can at least be mitigated will encourage the search to find the best and most cost-effective solutions. For example, engineers in Spain have set a fine example in the design of some lines in the south that feature special platforms to accommodate nests of migrating storks – far from the conductors. Other ‘environmentally conscious’ lines in that country feature special attachments to shield wires to warn passing birds and avert deadly collisions.

Are such magnificent creatures not worth protecting?

The people who design and operate power networks are among the first to speak out about the problem involving birds. However, there is a tendency for most in the industry to view the threat only from the perspective of how it impacts line performance – not from the viewpoint of the safety of the birds themselves. For example, I recently spoke to an engineer involved in the electrical design of a long DC transmission line that runs over towering mountains. He spoke of the first recorded flashover as having been caused by a huge bird that had bridged the phases with its wingspan. But it was clear that he saw it just as another operational problem.

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It’s long overdue that we recognize and accept that another high environmental cost of using electricity is the negative impact of power networks on the avian world. There’s work still to be done and the power industry should set about doing it.

Editorial

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PREVIEW

of ofthis thisissue issue

Issue 96 0 Quarter Quarter02- 2000 − 2012

8 2012 Claude de Tourreil Memorial Award

10 Perspective

Volume Volume 2000 − Number − Number 20

Utility Practice & Experience

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Algerian HV Grid Operator Focuses on Challenges of Harsh Service Environment

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Tropical Paradise, Yes But Not for Overhead Distribution

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Researching Optimal Insulator Design for High Altitude Lines

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Birds: More Threat to Lines or Threatened by Lines?

Creatures Worth Protecting

18 Editorial

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Make Use of Your Powers of Observation

22 Inside Track on Smart Grid Time to Bring Storage ‘Out of Storage’

24 From the World of Testing There’s Electricity in the Air!

26 Silicone Technology Review Silicones Offers Better Solutions for Cable Accessories

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28 Reporting from Cigre

New Standards on the Way for Post Insulators

30 Transient Thoughts

Insulators

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Modern Pollution Monitoring Principles Allow Better Selection of Insulators for Polluted Service Conditions (Part 2 of 2)

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New Plant to Focus on Composite Insulators

Insulator Stresses Always Log-Normally Distributed?

32 Scene from China

Chinese Power Companies Research Insulator Performance at High Altitude

34 Pigini Commentary

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Selecting & Dimensioning Insulators for HVDC

36 From the Research View Move to DC Will Require New Insulation Test Methodologies

38 Woodworth on Arresters

A Superior Choice to Shield Wire for Most Transmission Lines

40 Focus On Cable Accessories

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Changeover to Polymeric Cables Will Impact AC & DC Accessories

Arresters 102

Review of Proposed New IEC 60099-4 Energy Handling Tests

Cable Accessories 110

Developments in Designs, Materials & Manufacturing of Cable Accessories

Advertisers AdvertisersininThis ThisIssue Issue ABB Components & Insulation Materials 91 Alstom 49 Balestro 61 CESI Group 37 CSL Silicones - SiCoat Outside Back Cover DTR Corp. 63 Dalian Composite Insulator 4, 69 Dalian Insulator Group 14, 15 Dalian Reliable Industrial 75 Dekuma Rubber & Plastic 55 Desma 75 Dextra Power 13 EGU HV Laboratory 61

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Glasforms 13 Hidro Jet 61 Huayi Machinery Group 11 Hubbell Power Systems Inside Back Cover Hübers Verfahrenstechnik 79 Huntsman Advanced Materials 65 Jinan Meide Casting 79 KEMA 25 Nikdim 69 Ofil 73 PK Insulators 9 PPC Insulators 93 Qingdao Highton Machinery 73

Reinhausen Power Composites 105 Rugao Dasheng Line Material 5 SGD La Granja 47 STRI 87 Shaanxi Taporel Electrical Insulation 23 Shanxi Century Metal Industries 69 Shenma Electric Power 6, 7 Sichuan YiBin Global Group SYGG 53 Siemens, Arresters Div. Inside Front Cover TE Connectivity 41 Tianning Electrical Isolating Materials 2-3 Trench Germany 51 Tridelta Überspannungsableiter 39

Uvirco Technologies 63 Vogel moulds and machines 35 Volani Metais 63 Wacker Chemie 27 Wellwin Precision Moulds 11 Wenzhou Yikun Electric 31 Xi’an Gaoqiang Insulation 29 Yizumi Rubber Machinery 20, 21 Zhejiang Fuerte 73 Zhengzhou Xianghe Group Electric Equipment 19 Zibo Taiguang Electrical Equipment 33

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C I N IS T OM SU H IN E EN G OF E X IN T M R

T

he 750 kV/±400 kV Qinghai-Tibet Intertie ranks as one of China’s major network projects for the Western Region and indeed for the country as a whole. Formally launched in mid-2010, it follows other high profile investments for the State Grid Corp., including the 1000 kV UHV AC and ±800 kV UHV DC projects, completed in January 2009 and July 2010 respectively.

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The Intertie consists of 750 kV AC lines that run some 1500 km from Xining to Germu, in Qinghai Province, as well as a circa 1000 km ± 400 kV DC line from there to Tibet’s capital, Lhasa. A 220 kV AC circle grid project in Tibet is also part of the scheme. The project, which includes two 750 kV substations, a 750 kV switching station and two ± 400 kV converter stations, represented an investment of approx. US$ 2.5 billion with the new lines put into operation this year.

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New 750 kV/±400 kV Link to Tibet

What makes this project especially noteworthy, apart from its scale, is that it is the world’s highest overhead transmission line and also the longest HVDC line ever built at such altitudes. For example, the ± 400kV line has an average altitude of 4500 m, with the highest point at 5300 m while more than 900 km of line is at 4000 m or above.

given its extreme cold, permafrost, high UV and fragile ecosystem, high altitude impacted design factors such as preventing corona, controlling electro-magnetic field, selecting suitable insulators, insulation co-ordination as well as the performance requirements of a range of HV equipment.

Apart from the construction challenges across the huge INMR visits Germu and in our Q3, 2012 issue will report Qinghai-Tibet plateau (often called as ‘the third pole’ of on the unique substations operating in this most unique the Earth due to its low air pressure and lack of oxygen) of HV network projects.

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Make Use of Your Powers of Observation

EDITORIAL EDITORIAL EDITORIAL

When I was a youngster of 15 or so, I had already ‘devoured’ every story written about that magnificent sleuth from Baker Street – the illustrious Sherlock Holmes. It was such a disappointment for me when I discovered that there were no longer any of his mysteries left for me to read! It was as if I had been presented with a wonderful meal that should have lasted many years but I had not resisted consuming it all at one time.

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For those who are not that familiar, Holmes was a fictional master detective who relied on nothing more than simple observation combined with the art of deduction to solve crimes that confounded the police. He could stare at a pile of cigarette ash, for example, and predict not only how long the villain had been stalking the victim but also surmise his taste in clothes and music. Today, with sophisticated modeling and imaging we have developed the power to see the world in such minute detail that perhaps some of us are losing the skill of ‘macroobservation’. This is indeed unfortunate. There is no need to cross the forest like an ant – passing every single blade of grass along the way – when a flyover will reveal what we really need to know.

It’s amazing that (with apologies to the people who created the Hubble Telescope) the human eye has never been equaled in terms of its ability to assimilate and process information in an instant. Indeed, with all the scientific techniques that have been developed to assess the condition of a composite insulator, most experts will still tell you that nothing surpasses visual observation. Yet today, especially with the advent of smart grid (which I am certainly not demeaning), there is a growing tendency to feel that everything we need to know about our power networks can be obtained from instruments and algorithms. In this issue is an article that arose quite literally from ‘being bored at the beach’. On school break with my young son in southern Thailand, I walked the country roads around the hotel and just observed what was happening on the local distribution system. And, sure enough, my detective

With all the scientific techniques that have been developed to assess the condition of a composite insulator, most experts will still tell you that nothing surpasses visual observation. work quickly revealed some interesting clues into the types of situations that can adversely affect rural power lines, especially in such tropical environments. I then used the art of deduction to arrive at explanations of how problems might have already developed or were at risk of soon developing. My point is simple. Power engineers should never hesitate to wander around their own installations from time to time – just observing everything with their own two eyes. You may well be surprised at all you can discover about what’s really affecting your network. As Sherlock Holmes might have put it in this case, ‘It’s elementary, my dear Watts!’

Shards of punctured porcelain reveal the real issue affecting this mountain top tower even better than lightning outage rates.

Marvin L. Zimmerman mzimmerman@inmr.com

INMR Issue 96 • www.inmr.com

ISSN 1198-7332 • P.O. Box 95, Westmount (Montréal), Québec, Canada H3Z 2T1 • Telephone: (514) 939-9540 • Telefax: (514) 939-6151 E-mail: info@inmr.com • Editor & Advertising Sales: Marvin L. Zimmerman 中国地区联系方式:余娟女士 电话: 135 1001 6825 / juan.inmr@yahoo.cn Magazine Design: Cusmano Design and Communication Inc • (514) 509-0888 • E-mail: corrado@cusmanodesign.com Contents of this publication are protected by international copyrights and treaties. Reproduction of the publication, in whole or in part, without express written permission of the publishers is prohibited. While every effort is made to verify the data and information contained in this publication, the publishers accept no liability, direct or implied, for the accuracy of all information presented.

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Whenever I visit an overhead line or substation, the people who accompany me often joke that I seem to have a unique talent for spotting whatever is different or somehow deficient. Actually, I do not regard this as a talent at all but rather as a skill first developed in my youth – from my precious time with Sherlock – and subsequently refined over more than 25 years of observing electrical infrastructure.

Editorial

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Time to Bring Storage ‘Out of Storage’

Inside Track on Smart Grid

Today, power generally flows in only one direction: from the power station, via a transmission and distribution grid, to the final consumer. Maintaining balance in such a situation is complex yet still achievable by means of centralized facilities.

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Electricity is normally consumed at the same time it is generated. As a result, it becomes increasingly difficult to maintain power stability and quality as de-centralized power generation (such as roof-top photovoltaics and large wind farms) sporadically feed additional power into the grid. Future power systems will therefore need not only to accommodate bi-directional power flows as well as increased transmission of information but will also have to integrate renewable (often intermittent) energy sources – all the while mitigating congestion and maintaining voltage in the appropriate range. The Smart Grid is expected to allow all this to happen by controlling the demand side as well as the generation side so that an overall power system can be operated more efficiently and also more rationally. It therefore must include control and other technologies as well as IT and communications. But one applicable technology that today remains largely underdeveloped and lacks sufficient attention by industry is electrical energy storage (EES). Reduce Generation Cost, Increase Reliability The role of EES in Smart Grid will be to allow power supply utilities to reduce overall generation costs and achieve higher efficiency given intermittent renewables. EES will also accommodate de-centralized generation by storing surplus energy for use at a later time. Excess production during off-peak hours can then be made available during peak demand periods. This will not only help make full use of generation potential but reduce the cost of electricity as well. Energy storage also helps increase network reliability and provides back-up in the event of power interruptions. In some cases, EES contributes to reducing investment in power system infrastructure, e.g. transformers, transmission and distribution lines through load levelling in certain areas during peak demand periods. In this regard, EES can also enhance frequency control capability. Mitigate Congestion, Maintain voltage EES installed at customer-side substations can control power flow and mitigate congestion or maintain voltage in the appropriate range. Essentially such storage devices help smooth out intermittent generation from wind and sun and modulate excess power fluctuation as well as unreliable power supply. Mobile Power Sources Energy storage devices can also support electrification of existing equipment so as to better integrate it into the Smart Grid. Electric vehicles (EVs) are a good example of this since they have already been deployed in several

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regions of the globe. Some even argue for the potential of EVs as a mobile, distributed energy resource to provide a load-shifting function within a Smart Grid. In this respect, EVs could be not just a new load for electricity but also a potential storage media that could supply power to utilities whenever the price of electricity becomes high. Better Local Consumption Planning Effective storage capabilities will also facilitate energy management in homes and buildings. Given intelligent management of consumption and various economic incentives, consumers can be encouraged to shift their pattern of energy consumption. For example, users could accomplish this by buying energy when surplus power is available (often at a lower price) and storing it for later use. Finally, in micro-grids and de-centralized generation, storage will allow planning and optimizing local consumption. This approach has already been applied successfully in many communities across Japan. Better Smart Grids In summary, examples of how EES will contribute to improved Smart Grids include: • Because increased penetration of renewable energy sources requires more frequency control capability in a power system, EES can be used to lessen imbalances between generation and consumption, thereby allowing network operators to control charging and discharging. • EES can reduce the need for additional investment in power system infrastructure through load leveling at times of peak demand and could also enhance frequency control capability. • EES minimizes the need for costly spinning reserves, as such reducing total generation costs. Storage of electricity generated by low-cost power plants during the night are simply re-inserted into the grid during periods of peak demand. Note: The IEC is actively involved in electrical energy storage and has developed a White Paper that summarizes present and future market needs for these technologies, while providing recommendations to all EES stakeholders. Copies can be downloaded without cost at: http://www.iec.ch/whitepaper/energystorage/

Richard Schomberg IEC Chairman of the Smart Grid Strategic Group Chairman, TC 8 – Systems aspects for energy delivery Chairman, PC 118 - Smart Grid user interface Responsible for Smart Energy Standards at EDF-Group

Editorial

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There’s Electricity in the Air! Switching in power systems is most often thought of as something carried out by heavy-duty specialized devices such as circuit breakers. One therefore tends to forget that a large number of switching operations on highvoltage networks are in fact performed by disconnectors – open-air devices that outnumber virtually every other piece of apparatus at a HV substation.

from the world of TESTING

These are relatively simple mechanically, with two contact blades that slowly move away from each other whenever instructed. Their basic function is to make a visible separation between different parts of the electrical supply system, typically to isolate a particular component for maintenance.

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Disconnectors are often involved in ‘live’ switching as well. This means that there is some current involved, even when the relevant network section is not loaded. The current in this case flows as a result of ‘parasitic capacitance’ and, though only a few tenths of an ampere, it can appear quite spectacular. At separation of the contacts, the current will simply not cease – mainly because the voltage across the blades at this point immediately breaks down the hot air gap. Only when a substantial contact distance is reached (namely one that the available voltage cannot bridge), does current stop flowing.

Disconnector arc switching becomes especially complex when the arc operates in the seemingly safe environment of a closed GIS installation and needs to switch even lower current, i.e. few milliamps. While there is no longer any ‘sound and light show’, there are still neat short arcs that are even more repetitive than in air. And there lies the problem! The successive breakdown events in an SF6 environment create very rapid trains of transients that roam with virtually unlimited freedom throughout the enclosure. At any opportunity to exit, e.g. at bushings, they partly escape the GIS and continue on outward. Now they are called very fast transient overvoltages (VFTO) and become a serious concern – especially as the equipment voltage rating increases. For UHV equipment of 1100 kV and 1200 kV, VFTO transients reportedly reach even higher levels than lightning impulses.

During the intervening seconds, an electric arc becomes visible – namely, ‘electricity in air’. The length of this free burning arc can attain several meters because the arc heats up the air and thereby creates a more conductive path for itself. Only wind has the potential to destroy the ‘party’, much like in breakers where far more powerful fault arcs are cooled by blasts of gas. The nature of these free-burning arcs was recently studied at the University of Eindhoven as part of a PhD project whose goal was to find ways to increase disconnector current switching capabilities. Based on tests at various laboratories, it was discovered that disconnector arcs are highly repetitive in nature. There is also a natural tendency for any arc to selfextinguish at current zero. However, the poor insulation of the heated air immediately upon extinction usually cannot resist the voltage imposed by the circuit and the arc re-ignites for another cycle. The research helped clarify that the interaction between the arc and electrical transients of the circuit can be quite intense. This interaction then has an impact when it comes to testing. In the previous IEC standard on disconnector switching, the test-circuit was not defined. After recognizing its potential impact on disconnector arc behaviour (especially duration), a new IEC document (TR 62271-305) was developed which details the test-circuit and therefore makes testing less arbitrary.

GIS disconnector testing has been under discussion for a long time and, with the advent of IEC 62271-102, the ‘dust seemed to have settled’. Nevertheless, testing of such ‘bus-charging’ switching duty remains a real challenge in every respect. Transients with mega-hertz frequencies have to be measured inside the GIS enclosure and laboratory HV sources then have to be protected against the evil of a ‘hit-and-run’. This calls for very special skills and equipment. KEMA, for example, has already conducted testing with equipment up to 550 kV but we are increasingly aware of the risks during installation.

Our research work also showed that the electricity flow through air appears in two distinct modes: a short-lived ‘machine gun’ like firing arc across the shortest point between contacts; and a highly erratic one that burns softly like a huge flame and expands rapidly upward in air, often covering meters in length.

And so it is that specialists when it comes to testing switchgear with hundreds of kA can find themselves challenged by disconnector switching with no more than a few milliamps.

The repetitive nature of the ‘machine gun’ type arc (though pleasing to the eye and ear of any well-rooted HV engineer)

Professor Rene Smeets Rene.Smeets@kema.com

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has its drawback. The rapid and frequently collapsing voltage imposes steep surges on nearby equipment, sometimes leading to reports of damage. The erratic arc version can also reach too close to neighbouring conductors, thus initiating faults.

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Silicones Offers Better Solutions for Cable Accessories Silicone elastomers, gels, fluids and pastes are perhaps best known in INMR for their increasing use in insulators, surge arresters and coatings. But these same polymers are also finding growing application in a sophisticated new generation of MV and HV cable accessories. Modern power cables consist of much more than a conductor surrounded by a layer of insulation. Therefore, the industry is looking for superior solutions when it comes to key accessories such as terminations, joints and plugs. And it is here that silicone elastomers exhibit characteristics that make them ideal to meet these new demands.

Silicone Technology

REVIEW

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When it comes to the growing concern for safety and the environment, new accessories based on silicone elastomers have the advantage that their decomposition products are non-toxic while the silicones themselves are not only non-hazardous but also virtually non-flammable. From the performance perspective, these materials have desirable mechanical properties such as high elasticity that provides accessories with an advantageous stress-strain relationship and the equivalent of a low-tension set. The dominant physical property of silicones in the case of terminations and joints, for example, lies in their high elasticity that enables them to effectively seal any interface between cable and accessory. This eliminates risk of partial discharges and guarantees short-term dielectric properties. Moreover, these properties remain virtually the same over a wide temperature range. Finally, silicones are ideal for outdoor applications due to their well-known hydrophobicity as well as their resistance to arcing, tracking and weathering – including to ozone and radiation.

Cable joint design consists of middle electrode and two field control electrodes encapsulated by insulating material. Control electrodes at end of joint body are made from either an electrically conductive or a high permittivity silicone elastomer.

In general, the design of a cable joint consists of a middle electrode and two field control electrodes encapsulated by the insulation material. The control electrodes at the end of the joint body are made from either an electrically conductive or a high permittivity silicone. The joint’s insulating silicone body is then covered by a layer of semi-conductive material in order to build up electrical contact to the cable’s conductive shield. Similarly, terminations generally consist of either conductive or high permittivity silicone stress control elements as well as the silicone insulating body.

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There are two established techniques for field installation of silicone joints and terminations: slip-on and cold shrink technologies. In the case of the former, the accessory is mounted by sliding it onto the cable with the help of a lubricant such as silicone paste. Cold shrink technology relies on a different principle whereby the joint or termination is expanded beyond its original size and stored on a support core until ready to be installed. During the two-step assembly, the termination is first moved onto the cable and then mounted easily by removing the core. For instance, it’s simple to adjust the expanded termination and move it to the desired location. The mechanical properties of the silicone must then be able to withstand any subsequent expansion or shrinkage of the joint. The installation of a termination or joint creates a change in the insulating system at the crossover from the cable to the accessory. It is therefore necessary to integrate some field control device, without which the field would reach a maximum at this point and exceed the tolerance of the surrounding insulation. This could lead to partial discharge activity and reduce lifetime of the joint. With proper grading, however, the PD onset voltage can be increased by a factor of 10. Capacitive or geometric field grading is a common technique in this regard and sees an electrically conductive element connected directly to the outer conductive layer of the cable. The special shape of the deflector smoothens the field by widening the electric field lines and achieving manageable tangential field strengths. This reduces electrical stress on the insulation material and increases long-term performance of the termination or joint. Refractive field grading is another option and makes use of a thin layer of a high-permittivity silicone elastomer. Based on deflection of the field lines by materials with different relative permittivities, the deflector prevents any concentration of field. Field grading with high permittivity silicone allows the manufacturer to design accessories with very slim profiles. Grades of silicones are already available for the realization of these different terminations and joints, including cold shrink and slip-on accessories with either capacitive or refractive field grading. Nevertheless, R&D is ongoing, spurred on by the trend to higher voltage applications and HVDC. Novel costeffective silicone compounds and new types of accessories are therefore already ‘in the pipeline’ to meet the needs of growing cable cross sections. All in all, it can fairly be said that silicone elastomer-based accessories offer sophisticated solutions for today’s requirements in both MV and HV cable applications. Such applications benefit from the insulating and electrical conductivity characteristics of these materials, their outstanding mechanical performance and their stability against outdoor weathering.

Dr. Georg Simson Georg.Simson@wacker.com

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New Standards on the Way for Post Insulators New UHV AC as well as DC projects around the world have suggested the need for certain additional product standards for insulators. In 2011, IEC TC 36 circulated a questionnaire to manufacturers and utilities to better assess their views on new standards required specifically for hybrid insulators and composite hollow core station posts. Responses supported such a need and, at the recent General Meeting of TC 36 in Melbourne, it was decided to develop a Technical Specification (TS) for hybrid insulators. Then, after a positive vote for a composite hollow core station post insulator standard in November of last year, Subcommittee 36C (substation equipment) set up a project team (PT 62772) to develop such a document as well.

REPORTING FROM CIGRE

Hybrid insulators combine different technologies to achieve an optimized solution for certain applications, with station posts currently the major area of interest. This can result in improved service performance compared to either porcelain or composite solutions alone (see Table below).

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Qualitative Comparison of Relevant Properties of Three Technologies for Station Posts (Solid Core) Property

Porcelain

Composite

Hybrid

Weight

1

5

3

Pollution Performance

1

5**

3…5*, **

Torsional & Bending Stiffness

5

2…3

5

Seismic Performance

1…2

5

1…2

Brittleness (Resistance to Vandalism)

1

5

3…4*

Depending on whether full or partial coverage of porcelain rod by housing Only valid for housing materials with hydrophobic behaviour Rating system: 1=worst, 5=best *

**

The principal achievement of a hybrid solution is matching the mechanical stiffness of porcelain with the pollution performance of a hydrophobic housing. Working Group (WG) 12 of TC 36, under project leader Jens Seifert, will compile the Technical Specification to be titled Polymeric HV Insulators for Indoor and Outdoor Use - General Definitions, Test Methods and Acceptance Criteria. Following the typical construction of hybrid insulators, a relationship can be deduced versus existing widely-used IEC standards. (see Fig. 1) During the first meeting of this WG, it was decided that the new standard should also include hybrid porcelain hollow core insulators. There will even be a reference to potential cap & pin and long rod hybrid designs, although no suitable applications have yet been identified. Recent findings from CIGRE WG D1.27 in regard to ageing tests under AC or DC will be part of this document but mitigation measures (such as creepage extenders, booster sheds or RTV coatings) will not.

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Technology

Mechanical Function

Test philosophy for hybrid insulators based on present IEC standards for station posts.

External Electrical Function

Ceramic Core (typically)

Interface Between Core & Housing

Polymeric Housing

Mechanical Tests as per Product Standard IEC 60168 Ceramic Station Posts (including Mechanical Type Test)

Interface Tests as per IEC 62217 (Common Clauses) & Product Standard IEC 62231 Polymer Station Posts

Housing Tests as per IEC 62217 (Common Clauses) & Product Standard IEC 62231 Polymer Station Posts (including Electrical Type Test)

With IEC 62231 for composite station posts (Composite station post insulators for substations with a.c. voltages greater than 1000 V up to 245 kV – Definitions, test methods and acceptance criteria), a relevant standard for this application has existed since 2006. However, at the time the standard was first being prepared, large solid core diameters of 88 mm and higher were in the early stage of development. This limited the standard to a nominal voltage of only 245 kV. Nevertheless, clever solutions were developed for UHV station post applications by using tripod or multi-pod structures. This way, dimensional limitations of the solid core diameters could be compensated for and existing experience with line posts (i.e. 1st Edition of IEC 61952 published in 2002) or with solid core station post insulators (i.e. IEC 62231) could be better exploited. Such solutions even offered advantages over those relying on a hollow core – the main being that no internal space would have to be filled. In addition, because the construction concept is modular, transportation to the work site became easier, even for insulator assemblies of 10 m or more. Still, there are applications (e.g. for disconnector switches) where a singlepiece structure using the tube concept provides operating benefits. For such applications, the product standard for hollow core insulators (i.e. IEC 61462) would be regarded as applicable. However, a polymeric station post is a selfstanding product and the creation of a standard equivalent to IEC 60273 for porcelain station posts would become necessary. This new standard will be called Composite hollow core station post insulators for substations with a.c. and d.c. voltages greater than 1000 V - Definitions, test methods and acceptance criteria and will make reference to existing IEC standards 62217 and 61462. A critical detail will be the large interface formed between the internal surface of the tube and the external surface of the solid filler inside it, typically some type of foam. With the design philosophy that such a solid filler essentially transforms the hollow core into a solid core, no longitudinal cracks, e.g. by de-bonding, will be permitted. De-bonding can occur under excessive deflection or if there is shrinkage of the solid filler under low temperatures. In such cases, the shear stress on the interface might exceed the bonding strength between filler and internal tube and an interfacial crack can develop in both radial and axial directions. The new standard, to be developed under project leader, Norbert Mikli, is expected to provide useful test methods in this respect as well.

Dr. Frank Schmuck frank.schmuck@sefag.ch

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Transient Thoughts

Insulator Stresses Always Log-Normally Distributed?

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At a recent IEEE Working Group meeting in Tampa, Florida, I described various problems while developing an application guide for insulators used under winter ice and snow conditions. The WG Chair succinctly summarized my contribution in the minutes as follows: “everything in the world is log normal”. After reflection, I have come to the following conclusion: as long the above is prefaced by, “everything in the lab is normally distributed”, this assertion is indeed an excellent summary of the problems we face doing insulation coordination calculations to predict risk of failure of a line or substation. The chart below illustrates what I mean.

ground resistance, the value a = 1.9 (associated with a natural log standard deviation of about 0.9) has been reported in several different studies. The values for ice accretion have a broader range of a = 1.3 to 2.1. In all cases, however, these exponents are less than the value used for lightning of a = 2.6.

High voltage lab personnel are quite familiar with processing flashover and withstand voltage values, e.g. from the up-anddown method for impulse waves. This leads to a well-defined estimate of insulator strength, shown as the narrow probability density function (P.D.F.) on the right. When considering several insulators in parallel, the mean strength value shifts a bit to the lower left, but the peak narrows even further.

The IEC 60071 standard on insulation coordination gives some advice on this by considering the natural variability of lightning waveshapes, the possible points of attachment, the random phase angle associated with the AC line voltage as well as the COV of the lightning impulse flashover process. However, the standard also notes that the risk integral evaluation becomes impractical with more than two random variables. Still, by focusing on the most influential and significant random variables and fixing the others, a satisfactory failure estimate can be computed. To some extent, lightning backflashover calculations already implement this advice. The strong correlation between the peak current (Ipk) and maximum rate of current rise (Sm) is exploited by fixing an equivalent linear front time, (Ipk/Sm) in calculating peak insulator voltage. However, at present, simple backflashover calculations ignore the random variable whose wide statistical variation may have the most influence on the result – namely tower-to-tower variation in soil resistivity. This second factor can be incorporated by considering a two-dimensional matrix of probability intervals, and computing the insulator stress for each derived combination of peak current and footing impedance. A model for the probability of lightning impulse flashover, using an exponent a = 33 to correspond to the COV of 5%, completes the improved calculation.

In contrast, the P.D.F. of the ‘stress´ variable has a skewed, asymmetrical distribution with a broad range of values. For lightning, there is a natural variability of parameters from flash to flash since the distribution of peak impulse currents relates directly to the peak impulse voltage associated with each flash. This stress variability is expressed using a log-normal distribution with a median and a log standard deviation based on natural logarithms of individual data values. In the case of a peak current of the first negative return stroke (Ipk), this can be approximated using a simplified expression that the probability (P) of exceeding Ipk is 1/(1 + ( Ipk / median)a), where the median peak current is 31 kA and the exponent a is taken as 2.6. The simplified expression for probability is useful in the range of 0.01 < P < 0.99 and has other helpful characteristics. First, when the exponent a < 3, it is related to the standard deviation of the log normal distribution as follows: SD (lnx) = 1.71/a. Second, when the exponent is large (say a > 16), it is related to the coefficient of variation (COV) of a normal distribution by a = 166/COV. The coefficient of variation is the standard deviation of the narrow peak in the figure divided by the mean and then multiplied by 100 percent and is typically in the range of 5 to 10% for electrical strength of insulators. Finally, the simplified expression for P can be manipulated easily to give a stress or strength associated with a pre-defined probability level such as P = 0.02 for the 2% stress or P = 0.10 for the 90% strength. The equation P = 1/( 1 +( x / median )a ) provides a satisfactory fit to many other observations of natural stress, whether x is ice thickness or weight, pollution level, precipitation conductivity or soil resistivity. Each region will have a different median annual value (say of ESDD), but pollution observations seem to share about the same value of exponent a = 2.3. For soil resistivity and

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This poses an interesting question. If statistical distributions of pollution levels are log normal (and in fact broader with higher log standard deviations than of those used for lightning currents), why would we try to use normal statistics for calculating the interference area for probability of failure in the figure?

Under icing conditions, there are actually three rather than two independent environmental stress variables in the flashover process. These are the pre-existing insulator contamination level (or ESDD), the thickness of the ice accretion per unit length on the insulator and the electrical conductivity of the applied water, corrected to 20°C. In large countries such as Canada and China, icing and pollution conditions can change considerably over distances of 1000 km or more. For example, urban areas and regions downwind of pollution sources are vulnerable to flashover at line voltage from accretion of rather thin ice layers and partial bridging by icicles across the dry arc distance. By contrast, full bridging of insulators by icicles occurs more frequently in rural areas that may have lower precipitation conductivity. It is important to develop insulator selection advice that anticipates and accommodates such a broad statistical distribution of input parameters. Evaluation of the interference area and flashover risk could be extended to sum a threedimensional matrix of threat levels, estimating the critical flashover level for each combination. However, simpler guidance is preferable. One approach is to develop an equitable way to distribute the flashover risk among the three parameters. This would offer the benefit of keeping the probability estimates within the range of actual observations, rather than extrapolating to an extreme ice thickness based on a limited data set of 10 or 20 annual values. And, by selecting log-normal distributions for all parameters at the outset, I believe we can accommodate our broad range of environmental conditions in a more practical way.

Dr. William A. Chisholm W.A.Chisholm@ieee.org

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Chinese Power Companies Research Insulator Performance at High Altitude In China, regions with an altitude of 1000 m and higher account for about 60% of the total land area, meaning that HV transmission lines will inevitably pass across them. The problem in these situations is that high altitude features low air pressure and thin air. It is well known that voltage decreases with altitude, no matter whether it is air gap discharge voltage, conductor corona inception voltage or pollution flashover voltage. The higher the altitude, the more problematic this phenomenon becomes for the external insulation of power lines and equipment.

SCENE FROM CHINA

Insulators that perform well at sea level are not necessarily equally suitable when it comes to operating in high altitude areas.

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The article on page 70 of this issue provides a general discussion of this research and an overview of the type of results obtained. For example, it was found that compared with areas near sea level, insulator pollution flashover voltage at altitudes of 1000 m and 2000 m will be reduced by 5.7% and 11.4% respectively.

Tsinghua University also discovered that there are two factors involved in this reduction of pollution flashover voltage as air pressure changes. One is the volt-ampere characteristic of electric arc changes with air pressure while second is the influence of electric arc bridge flashover across the insulator’s profile. In high altitude areas, such a span bridge is far more likely to occur and this is what imposes more demanding Tsinghua University and Chongqing University started studying requirements when it comes to shed geometry. Those insulators the influence of air pressure (altitude) on the pollution flashover that perform well at sea level are not necessarily equally characteristics of insulators as far back as the early 1980s. At suitable when it comes to operating in high altitude areas. the time, research facilities to simulate high altitude for test purposes were still relatively modest. However, construction Further research is clearly necessary to identify optimized of UHV transmission projects in China over recent years insulator profiles suitable for high altitude applications. has greatly pushed forward research activities on selecting external insulation in high altitude areas. With this has been Prof. Guan Zhicheng a rapid development and of research facilities, from largeTsinghua University, Shenzhen Campus scale air pressure tanks to pollution test laboratories set up at guanzc@tsinghua.edu.cn high altitudes, including one in Kunming at 2100 m and another in Tibet at over 4000 m. Both test bases are now available to evaluate pollution flashover characteristics of e.g. suspension and station post insulators with different materials and geometric profiles. Together, the new air pressure tanks and high altitude test bases complement each other in function – the former to closely study the influence of air pressure on insulator pollution flashover characteristics, the latter to verify these test results and allow them to be directly applied during project design.

New ± 400 kV line to Tibet operates at an average of 4000 m.

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Tsinghua University, Chongqing University, China EPRI, China Southern Grid and Xi’an High Voltage Apparatus Research Institute, have all now conducted their own tests on the tendency of insulator pollution flashover voltage to decrease as altitude increases. Test objects have included porcelain, glass and composite insulators with a variety of profiles while pollution severity has ranged from high to low, including both AC and DC flashover testing. Separate statistical analyses were then made of the test data obtained, including, for 80 test samples of AC suspension insulators.

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Selecting & Dimensioning Insulators for HVDC After publication of Guidelines for the Selection and Dimensioning of Insulators for AC Applications (CIGRE Brochure 361-2008), work has been underway over the past several years to generate basically the same type of document for insulators used in HVDC. Now, after more than 10 meetings (the first in Beijing in 2005, the most recent in Prague this past February), CIGRE WG C4.303 has finalized the draft of such a Guide and the relevant Brochure is in the editing phase.

P I G I N I Commentary

It was indeed a challenge to develop a document acceptable to all 22 WG members from 13 countries and representing a mix of manufacturers, electric power utilities and testing laboratories. Part of the difficulty derived from the fact that technical knowledge and service experience when it comes to pollution in DC is much less than for AC. As such, it was necessary to conduct a thorough analysis of relevant laboratory and field experience, while also aiming to benefit from recent R&D activities in the area of UHVDC.

34

Another hurdle was to arrive at a common perception among manufacturers and users about the critical role pollution plays when it comes to DC systems. In the end, there was consensus that pollution in DC is a much more important factor in design than in the case of AC, with a direct influence on insulator length, system cost and feasibility of UHV.

is also recommended. For this, the concept of USCD may have to be considered in order to arrive at a suitable design that takes into account the influence of creepage factor (CF) on insulator performance.

As shown in Fig. 2, USCD is not necessarily constant but rather increases linearly with CF due to a corresponding decrease in the efficiency of the profile. As a result, withstand voltage (U) per • the magnitude of switching overvoltages in DC systems (in p.u.) is unit arcing distance tends to reach a point of saturation when CF generally lower than for AC; increases due to excessive creepage distance being ‘forced’ along • contamination deposition is generally higher on insulators a given insulator length. This means there is a risk of either a large subjected to DC voltage; underestimation or overestimation of U (assuming a constant value • for the same voltage stress, creepage distance required for DC, of USCD) unless the assumptions are retained over only a limited at any given insulator contamination level, is higher than for AC; range of CF values. In the upcoming Guide, the dominant role of pollution in insulator design under DC is acknowledged for the following reasons:

As a result, the simplistic approach (as adopted in the case of AC) cannot be directly extrapolated to DC. In the new Guide, the accuracy that can be achieved utilizing different design approaches is analyzed, starting from the most qualitative to the most accurate and quantitative. In particular, a simplistic approach is illustrated that can be used to obtain basic indications for preliminary design. According to this simplified methodology, pollution severity to be assumed in DC design is evaluated starting with pollution data measured on either non-energized or energized AC insulators, usually of standard cap & pin type. Correction factors extrapolated from service experience and laboratory test data are then applied to translate available information on pollution severity to a DC scenario taking into account:

The selection Guide for DC then proposes that the most appropriate insulator design can be obtained through a statistical determination of the distribution of pollution severity as well as knowledge of insulator performance under the specific pollution conditions. This approach permits statistically evaluating the insulator dimensions that are necessary to achieve desired system reliability. The upcoming new Guide, created through the diligent and efficient work of WG C4.303, will represent an important step forward when it comes to optimizing HVDC insulator design.

Alberto Pigini pigini@ieee.org

1. the influence of DC energization on non-uniformity of the contamination layer, (i.e. much greater than for AC); 2. type of contaminant (e.g. soluble to non-soluble ratio); 3. insulator geometry and material. The resulting accuracy of the pollution severity evaluated in DC therefore depends on how accurate are the correction factors being applied. As far as insulator performance is concerned, the approach makes reference to so-called ‘representative’ curves, averaging out experimental data that are typically quite spread (as shown by Fig.1). The new Guide points out that approximations involved in the simplified approach risk leading to inaccurate results which could in turn have great impact on system cost and reliability. In terms of final design, the Guide recommends obtaining direct indications of relevant pollution severity from systematic measurements on DC energized insulators. This can be accomplished before line construction begins using a number of field stations installed at representative sites and equipped with energized DC insulators. Direct evaluation of the performance of the insulator selected under the specific contamination condition

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Fig. 1: Unified specific creepage distance (USCD) versus pollution severity expressed as SDD (salt deposit density). Range of results on disc insulator of different profiles. Comparison with average curve assumed for preliminary design.

Fig. 2: Example of the dependence of USCD and U (withstand voltage for unit arcing distance) on ratio between the insulator creepage distance and the arcing distance (CF) for homogeneous insulators.

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Move to DC Will Require New Insulation Test Methodologies It certainly appears that future development of electrical power networks will not be possible without incorporating more and more DC based systems. Indeed, apart from recent well-known projects in China and India, Europe and the U.S. are now also preparing for an HVDC solution. The advantages that arise from such a transition are manifold and have already been presented to readers in various INMR editorial columns and articles.

36

However, creation of HVDC networks will require developing reliable switching equipment – something still regarded as a ‘missing link’ in the plan. Moreover, up to now, these connections have been designed predominantly as point-to-point. The technology still has to be developed to allow multi-terminal connections to reduce transmission losses as well as the number of station components required. Research in this area is therefore ongoing. In the U.S., there’s already a plan to connect the country’s three asynchronous power systems through a DC hub named Tres Amigas Superstation. This facility will regulate the direction and level of power flows while using some of the latest technologies in the field, including voltage source HVDC converters, a DC superconducting cable ring and an energy storage battery system. Once in operation, the station will allow integration of large solar and wind resources from the western and central regions of the country and also provide the capability to better manage real time power fluctuations. The Department of Energy even aims to further develop this new system by building a network of 765 kV AC lines with a number of AC-DC-AC links. All the above solutions of course call for extremely high security of supply since the amounts of power being transmitted will be huge. This calls into question what will then be needed from the viewpoint of reliability of insulation. Extensive experience up to now points to considerable differences in physical phenomena under DC when compared

Indeed, a number of costly mistakes have already been made by not applying established design criteria and test methods when selecting insulation systems for DC applications. In many cases, these were the result of lack of sufficient knowledge. There is obviously still work to be done to increase our understanding of all DC related phenomena and one of the important activities in this regard will focus on establishing new test methodologies. While CIGRE is now working in this direction, it is noteworthy that the dominant approach still seems to rely on adopting modified test procedures that already exist for AC. I don’t necessarily see anything wrong with this approach as long as there remains a resemblance to the real operating conditions of the insulation. Let me mention two relevant examples: the first refers to the evaluation of tracking and erosion resistance of polymeric materials for outdoor applications. A number of test laboratories have adopted the original procedures of the so-called inclined plane test (according to IEC 60587 and ASTM D2303). Diverse materials samples have been tested this way at different voltages and under both positive and negative polarities. The outcome of this work clearly demonstrates that the extent of material damage observed during such DC tests can be far greater than under AC stress. China, for example, has already introduced its own national standard for the modified procedure, while CIGRE is still moving toward this goal within Working Group D1.27. All of this is ongoing in spite of experience indicating that widespread damage due to tracking has seldom been observed on polymeric insulators in service on most HVDC lines worldwide. A second example, this time in my view still lacking a coordinated international approach, relates to withstand testing under DC voltage. Our experience indicates that applied test voltages (especially in the withstand case), deposit electric charges on the solid insulation surfaces. These, in turn, may be large enough to modify local electric field distribution and this way influence the results of subsequent tests. When polymeric materials are involved, the time needed for the deposited charges to decay can be as long as a few days. It is therefore necessary to either wait quite a while between each voltage application or alternatively to apply special procedures for charge neutralization. This would make such testing time consuming and expensive. Similar problems can also appear during standard impulse testing of polymeric insulation systems. The commonly applied ‘up-and-down’ procedure used during these tests assumes that results of each impulse application are independent of one another – something that is no longer true when charges remain on insulator surfaces. Again, our experience indicates that varying the time interval between successive impulse applications might well affect test outcome. Appropriate actions and broader discussion will therefore be needed to address these effects when proposing new as well as when revising old international standards.

Prof. Stanislaw Gubanski Chalmers University of Technology stanislaw.gubanski@chalmers.se

Photo: INMR ©

From the Research View

In the case of Europe, ABB’s Gunnar Asplund proposed a solution as far back as the mid 1990s with the goal of helping to expand electricity trade across the continent. This would be accomplished by creating an extensive HVDC network to permit transferring excess energy from different regions – wind energy from the west, hydro energy from the north and solar energy from the south – all for utilization in parts with heavy demand load. His vision is shared today by many partners and driven by plans to install huge offshore wind farms in the North Sea as well as thermal solar plants in the Sahara and Middle East.

to AC. These include types of stresses and resulting distribution of electric field, accumulation of space and surface charges, breakdown and flashover phenomena as well as insulation material ageing and deterioration.

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A Superior Choice to Shield Wire for Most Transmission Lines I recently attended a conference on line and substation design where there was a most thought-provoking presentation. In it, the speaker challenged attendees, and especially the next generation of transmission line engineers, to build lines with much more emphasis on their ‘sustainability’. Of course, there are many possible definitions when it comes to the issue of sustainability. So the speaker proposed three basic principles for sustainability when it comes to designing overhead lines:

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Perhaps not surprisingly given my chosen field of interest, I was struck right away by how much arresters can contribute to the sustainability of transmission line design and operation. For example, the speaker addressed the topic of losses when he suggested that more conductors should be used. While this would translate into lower operating temperatures, he did not specifically address losses due to shield wires. When I asked him about this, he replied: “the only good shield wire is no shield wire.” In fact, I’ve been thinking precisely about this for a while now and I realize it indeed becomes feasible when applying line arresters to provide the necessary lightning protection. Therefore, I was particularly drawn to another presentation during that same event that estimated what losses actually were due to shield wire, both with and without ‘segmentation’. Basically, segmentation is an installation option where shield conductors are insulated from the towers and terminated every mile (circa 1.5 km) or so in order not to form a continuous conductor over the line’s full length. (Such a process may generate issues when there’s an optical fiber involved, but that’s not the issue now). As it turns out, losses due to the overhead shield wire are a function of load current, i.e. the higher the load current in the phase conductors, the greater will be the losses due to shield wire. Simple enough. However, in order to project life cycle losses on any new line, the analysis needs to assume some load current. In this case, the presenter applied conservative numbers for a 345 kV line and his subsequent analysis proved fascinating.

Basically, he chose an average current of 1000 amps on a line of 100 miles (160 km) that’s equipped with two shield wires. The right shield wire has an integrated optical fiber and is 0.61 ohms per mile while the left is aluminum clad steel and 2.4 ohms/ mile. The resulting steady state currents induced into these shield wires would then be 41 and 70 amps respectively. That translates into roughly 708 kW of steady state losses. The presenter then went on to demonstrate that, given this scenario and the typical 30 year life of towers and shielding, the cumulative losses due to the shield wires would have a present value of $ 3.8 million, based on an energy cost of $ 50/MWh. That’s right – a life cycle loss of nearly $ 4 million! If, for example, externally gapped line arresters that are approximately equal in materials and installation costs to shield wire had been installed to protect this line, there would be no energy losses at all. Not only does such a design solution seem attractive from an economic perspective, it’s also more sustainable environmentally because it requires fewer materials and costs less to operate. Now, before considering such an option the first thing most line designers will ask is: “What is the reliability of a lightning arrester?” That’s a fair question. If we consider the most common type of line arrester design that doesn’t include a series gap, the answer becomes difficult. But if we consider an EGLA where the arrester is never energized at line voltage, the answer is easy. Since there is no external leakage current and no electrical stress imposed on an EGLA’s active parts over its lifetime, reliability is inherently high. A typical failure rate, for instance, could be on the order of only one unit per million, i.e. for every million arresters installed, only one will fail in any year. Indeed, my estimate in that the reliability of EGLAs is uniformly high and their service lifetimes cover many decades. I can also say that of all the ‘ancient’ arresters I’ve observed all over the world, the externally gapped types are the ones that seem to always still be in service. In other words, if an EGLA is used to replace the shield wire, the issue of arrester reliability itself becomes essentially irrelevant when estimating line reliability. I therefore pass on a challenge to all young (and old) line designers who read this column: design your next lines with more emphasis on sustainability. Consider EGLAs in place of shield wires and save a lot on operating costs and the impact to the environment.

Photo: INMR ©

Woodworth on Arresters

• use less materials (apart from conductor); • avoid materials or processes that pose risks to the environment; and • install as much conductor as required to keep losses to a minimum.

Not only is a line arrester design solution attractive from an economic perspective, it’s also more sustainable because it requires fewer materials and costs less to operate.

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Jonathan Woodworth Jonathan.Woodworth@ArresterWorks.com

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Changeover to Polymeric Cables Will Impact AC & DC Accessories The future development of cable accessories is being influenced mainly by the continuing technological shift from paper to polymeric-insulated cables. Moreover, the impact of this shift is not the same for AC as it is for DC.

F

CUS ON CABLE ACCESSORIES

At the present, the large majority of power transmission and distribution is handled by AC systems due to their comparative ease of transformation at the different voltages. In the case of submarine as well as lengthy land cables, however, HVDC systems offer key advantages including higher capacity, reduced losses, no need for capacitive charging currents and better control over both load flow and reactive power transmission.

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Looking at AC, cable accessory designs and requirements have depended primarily on whether they were to be used for low, medium or high voltage applications. For example, when it comes to LV cables, the wholesale changeover from paper to PVC or PE-insulated cables took place some 40 to 60 years ago and there has been little new to report since that time. Accessories here employ mainly heat shrink technology for straight joints and cast resin technology for branch joints. At the MV level, the ascendancy of polymeric cable technology worldwide took place over a more recent time frame. XLPE and EPR cables began to be widely installed even before the 1980s though in some countries this process started later. One especially notable feature at this voltage level has been the huge diversity of alternative types of polymeric cable. While for the first 80 years only four basic paper cable constructions seemed sufficient (i.e. belted, metalized paper screened, three-core single lead sheath and single-core), once polymeric MV cables began to appear on the market at least a 100 different types of construction were soon being offered. Indeed, the numbers were so large that an effort was mounted as part of a joint project (EuroMVcable) presented at the 2007 Jicable Conference by a working group from France, Germany, Italy, Spain, Sweden, Denmark and the UK with the goal of dramatically streamlining this high number. After careful analysis of the many available MV cable designs, four optimized cable construction variants were proposed. Unfortunately, response to the proposal across different countries was not uniformly positive. As a result, a large number of different cable types and accessories are today still being applied over this voltage range, whether using heat shrink, slipon or cold shrink technologies. When it comes to HV, XLPE cables are presently used up to as high as 500 kV and EPR cables up to 150 kV and together these are increasingly replacing older paper technology. Comparing two alternative 110 kV cable constructions, for example, the costsavings offered by XLPE become readily apparent. Accessories for HV polymeric cables are based mainly on silicone because of its hydrophobic properties when applied to terminations and its softness and gas penetration rate when used in the manufacture of joints (see article on page xx). In the case of DC cable systems, the HV range is really the only application area and here the major problem for XLPE cables is formation of space charges in their insulation material. This can then lead to eventual breakdown due to modification in electric field distribution, especially in the case of traditional line-commutated converters (LCC) that change load flow by reversing voltage polarity. This remains the reason why paper-insulated cables still offer performance advantages for HVDC. While a changeover to maintenance free and more environmentally friendly polymeric cables is clearly on the way, this will apply mainly for DC systems that use modern voltage source converters (VSC). Here, change of load flow is

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Table: Proposed Construction Characteristics of Four Optimized MV Cable Variants* Proposed 4 Optimized Cable Variants Core, Al 150 mm2 Insulation

Solid

Ø = 13.2 mm

Stranded

Ø = 13.9 mm

XLPE 20 kV

4.30 mm

XLPE 30 kV

6.33 mm

HEPR 20 kV

4.60 mm

Bonded

External semiconduct. Strippable Screen

A

C

D

✓ ✓ ✓

0.5 mm 0.5 mm

B

✓ ✓

Aluminium foil

Aluminium foil + copper wire

Copper wire

Outer sheath

MDPE

2.5 mm

LLDPE

2.2 mm

Polyolefin Z1

3.0 mm

✓ ✓

✓ ✓

* David, J.-M.; Regaudie, V.; Fara, A.; Simeon, E.; Benard, L. Diefenbach, I.; Lesur, F HARMONIZING MV CABLE: RESULTS OF THE EUROEAN PROJECT ‘EUROMVCABLE’, Jicable, Paris 2007

accomplished rather by inversion of current direction without changing voltage polarity and this results in much lower stress on XLPE type cables. Nevertheless, the space charge problem remains an important element of design of DC XLPE cable accessories and this is leading to application of EPDM for joints in place of the marked preference for silicone in most AC applications. So, while classic paper cable is being phased-out with a corresponding influence on the types of accessories needed, specialized applications for this technology will remain. The most important of these are line-commutated converters where currently available XLPE cables are still not able to offer superior performance.

Professor Klaus-Dieter Haim University of Applied Sciences Zittau/Görlitz, Gemany KDHaim@hs-zigr.de

110 kV paper-insulated cables (left) compared to XLPE cable.

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UTILITY PRACTICE & EXPERIENCE

Part 1 of 2

Algerian HV Grid Operator Focuses on Challenges of Harsh Service Environment If someone had to rank transmission service environments across the globe in terms of the severity of their pollution challenges, Algeria would surely find itself near the top of the list. Covering a vast expanse of some 2.3 million sq km, it is the largest country in the Arab world, Africa and the Mediterranean basin. The north has a seacoast stretching over 1200 km while the immense Sahara to the south takes up more than 80% of its land mass. This combination of ‘sea and sand’ is typical of the North African ‘Maghreb’ and defines the dominant pollution exposure within which the transmission system must operate. However, the already severe service environment in Algeria is only made worse by the presence of alkaline lakes that dry out by mid-summer and generate a mixed blessing of valued salts that contribute yet more pollutants. Another challenge is wide temperature fluctuations that can vary from a searing desert heat of 50°C during summer to winter lows of freezing and below. This past year, for example, saw unusually heavy snowfall and icing near major population centers. Yet another problematic climatic feature are prevailing winds that carry salt spray inland from the Mediterranean each summer. In the south, siroccos blow choking dust in from the desert, often at gale force speeds. And finally, if all these forces of nature were not enough to deal with, heavy industrial contamination blankets large parts of western & eastern Algeria from flaming chimneys at huge petrochemical complexes. This first of a two-part article by INMR contributor and T&D Insulation Specialist, Raouf Znaidi, reports on how management and maintenance staff at the Algerian national transmission grid operator are dealing with this combination of challenges, all the while striving to meet the growing reliability needs of an economy undergoing rapid industrial expansion. The article also highlights some of the criteria that guide decisions in developing countries when it comes to evaluating and adopting new transmission technologies.

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Photos: Courtesy of R. Znaidi and GRTE

Fergani. Strategy in place to improve key performance indicators for transmission.

GRTE’s Chairman & General Manager, Abdelaziz Fergani, states that electricity demand in Algeria has recently been growing by some 8 percent each year and this has required SONELGAZ to embark on an ambitious program to expand its generation facilities as well as transmission and distribution networks. Another factor behind recent investments in the electrical sector is a high-profile interconnection project between Europe and North Africa that

involves one of the country’s longest 400 kV lines.

being allocated to building, operating and maintaining a high-quality and reliable transmission infrastructure.” Says Fergani “in order to deal with Prior to his appointment as economic expansion and growing Chairman, Fergani’s career spanned domestic energy needs, GRTE must all levels at SONELGAZ so he has annually invest in the range of 60 had first-hand exposure to the to 70 billion DA (equivalent to over types of problems that need to be US $ 1 billion) or about twice our overcome in order to attain this goal. previous budget and up to double our “Pollution on insulators as well as turnover as well. This investment is severe weather events,” he explains,

Photos: Courtesy of R. Znaidi and GRTE

The Société Algérienne de Gestion du Réseau de Transport (GRTE) was created in 2004 when state-owned Société Nationale de l’Electricité et du Gaz (SONELGAZ) was restructured into separate units, each specializing in different areas such as generation, transmission, distribution, engineering, line & substation construction, R&D, etc. GRTE is one of the largest of these subsidiaries and charged with operation and maintenance of circa 22,000 km of overhead transmission lines as well as more than 240 substations.

Live line washing from helicopters has aimed to improve efficiency of maintenance.

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Table 1: GRTE Transmission Network (as at end 2011) Voltage (kV)

Length of System (km)

Type & Number of Insulators

Specific Creepage USCD (mm/KV). (IEC 60815 TS)

Glass

Composite

Glass

Composite

400

2089

35

Not yet

49.5

Not yet

220

11,555

18/20

EPDM & SiR

47/52

80

150

69

12

None

56

None

90

565

8

None

-

None

60

7939

5/7

EPDM & SiR

38/54

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Number of Substations

243

the Algerian transmission network. This plan will see implementation of successive remedial measures including growing specification of composite insulators, widespread use of live line working procedures (apart from on lines equipped with composite insulators) and introduction of more efficient helicopter-based live line cleaning. “All these strategic measures, including ongoing training programs for operating personnel,” remarks Fergani, “will benefit our utility and indeed the SONELGAZ Group as a whole.”

Algeria’s Transmission System

Heavy industrial pollution also affects Algerian transmission lines. For example, Fergani explains that GRTE has now implemented a strategic action plan with the objective of improving key indicators (KPIs) that track performance of

Photos: Courtesy of R. Znaidi and GRTE

“have both traditionally had a negative impact on our transmission lines. Dealing effectively with these challenges has therefore become one of our top priorities.”

GRTE’s transmission network consists mostly of 220 kV and 60 kV lines as well as a growing 400 kV network that includes a high-profile 1200 km line interconnecting with neighboring Tunisia to the east and Morocco to the west – a regional electricity super-highway referred to as Autoroute électrique EstOuest. Ultimately, the goal is to also connect these grids to southern European countries such as Italy and Spain. There is also a relatively small overhead network at 90 kV and 150 kV.

400 kV lines equipped with strings of 35 aerodynamic glass discs while EPDM and silicone insulators find growing application at 220 kV.

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Table 1 outlines this network and the main type of line insulation specified at each voltage level. Maintenance Director, Fathallah Soukeur, explains that transmission lines in Algeria have traditionally been insulated by a succession of different designs of glass insulator strings, from standard profile to anti-fog type to special anti-fog (with deeper under-ribs) and most recently to aerodynamic discs. However, in spite of a typical unified specific creepage distance (USCD) of as much as 57mm/kV, the harsh pollution service environment has led to unacceptably high numbers of flashovers – especially during the late night and early mornings (see Table 2). In 2011, for example, more than 2900 outages were recorded, mostly at 60 kV and 220 kV, resulting in a global outage rate for the entire transmission system of some 13%. This, in spite of a combination of pollution countermeasures undertaken, including over-insulation, live line washing and growing use of composite insulators.

Table 2: Outages & Outage Rate at GRTE - 2008 to 2011 2008

2009

2010

2011

Total Outages

1899

2276

2258

2936

Outage Rate/100 km

9.71

10.93

10.43

13.11

Table 3: Number of Outages by System Voltage (2010 & 2011) Voltage

2010

Rate/100km

2011

Rate/100km

400 kV

37

2.19

61

3.02

220 & 150 kV

867

7.54

1301

11.11

60 & 90 kV

1324

15.67

1574

18.16

Total

2228

10.30

2936

13.11

Table 4: Key Performance Indicators (KPI) of GRTE Transmission System 2009

2010

2011

END (in MWh) Undistributed energy

6793

16,818.6

4542

TIM Average interruption time (minutes)

88.85

209.5

50.93

Number of outages/100 km

10.93

10.43

13.11

Availability (%)

97.29

98.61

97.91

SAIFI

5.11

3.50

3.38

SAIDI (min)

209.4

123

118.2

Still, notwithstanding the increased overall outage rate per 100 km, the effort to improve key transmission performance indicators has already begun to show results. For instance, the average interruption time (TIM) from all types of outage has decreased significantly – from 89 and 209 minutes in 2009 and 2010 respectively to less than 51 minutes in 2011.

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Photos: Courtesy of R. Znaidi and GRTE

Lahcen Louaked, Head of GRTE’s Maintenance Department explains that, in an effort to best overcome historic problems with pollution, special additional countermeasures have been tried. These have included adapting line insulation design to each specific environmental constraint. In some cases, this policy has seen one to two standard or fog type glass insulators added to existing strings

Tower on 400 kV line features 35 aerodynamic profile glass discs.

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Collapse of 400 kV towers from powerful winds.

while in other cases entire strings were replaced with aerodynamic glass discs (often referred to locally as ‘Mexican style’ due to their broad sombrero-like profile).

Unusual Weather & Operating Events Soukeur and Louaked. Pollution and strong winds both impact Algeria’s power grid.

Photos: Courtesy of R. Znaidi and GRTE

“In order to prevent such tower collapses in areas prone to excessive winds,” says Soukeur, “a decision was made in co-operation with experts from a European power utility to reduce the initial 400 m span between 400 kV suspension towers to only 200-300 m and also to alternate more tension and suspension structures. In some cases, we have even changed the configuration of certain towers.”

Examples of foundation damage from wind and corrosion.

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Soukeur and Louaked point out that pollution has certainly not been the only problem negatively impacting Algerian transmission lines. For example, the networks around Algiers as well as well as in the south and east of the country have suffered from problems including extreme icing, sandstorms and even spiraling winds of as high as 180-200 km/h. Cataclysmic events of this type have caused major outages such as that resulting from the collapse of 18 towers on one 400 kV line as well as 25 pylons on a 60 kV line – leading to an outage that affected some 50,000 residential customers for two weeks in June of 2010.

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Louaked adds that collapses of some lattice structures have also resulted from corrosion in their foundations. For example, two steel pylons in

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a desert area of the southeast collapsed even though still under guarantee by their constructor. Independent diagnostic tests revealed that the collapses were due to unexpectedly strong winds combined with corrosion. Louaked explains that footings on the pylons of the affected line (Tougourt-ElOued) were extended by metal grids buried into the sandy soil – an alternative to a standard cement and gravel foundation and used by SONELGAZ for some projects due to a cost savings of 25%. However, in this case, strong southerly winds created a sandblast effect that virtually denuded the support structure. The stability and mechanical strength of the towers was so diminished that they toppled under the impact of the first strong winds. In this particular case, the contractor replaced the previous metallic grid foundation with a new conventional one of cement and gravel and included an anti-corrosion paint as well.

Growing Use of Composite Insulators

Louaked, who will soon retire after a career at SONNELGAZ that spanned more than 20 years, reports that GRTE first started using composite insulators only about 10 years ago. Initial applications were mostly on a trial basis and for some of the most problematic 60 kV and 220 kV lines – especially those whose routing crossed either mountainous terrain or uneven forested land.

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For example, he notes that prior to the initial application of composite insulators in 2002, the Alest-Kouba and Alest-Ofaye 220 kV lines often became virtually inoperable, particuarly at night, due to as many as 190 outages per year. “However,” explains Louaked, “since installation of over 4400 EPDM suspension insulators, we recorded a surprising improvement of up to 98% on these lines. In fact, the only recorded outages were those affecting preexisting glass cap & pin strings that had not been replaced due to our reluctance to install the composite insulators on tension towers.” “Today,” concludes Soukeur, “the superior pollution performance of composite insulators is quite clear and we have accumulated enough

service experience that we are confident with this technology.” The only unknown, he adds, is their expected service life in the Algerian type of environment. Says Soukeur, “from our perspective, some prudence is still required and we keep in mind what would happen if all our composite insulators began to fail over the same short time frame. What could we or indeed any other power company do to suddenly replace them all?” Louaked points out that, apart from performing well under pollution, EPDM and silicone rubber insulators also offer attractive economic benefits compared to traditionally used glass strings when it comes to acquisition, installation and especially maintenance (see Table 5).

“Since the first application of EPDM suspension insulators on problematic lines, we recorded a surprising improvement in line performance of up to 98% and the only outages recorded affected pre-existing glass cap & pin tension strings.”

Therefore, while caution is still being applied due to uncertainty surrounding life expectancy, they are now being examined for growing use mainly in suspension applications.

Actual

Targeted

Figure 1: Impact on annual outages rate/100 km after installation of composite insulators: Actual (red) vs. targeted.

Figure 2: After installing composite insulators, number of outages reported on two problematic lines decreased from 190/year to less than 5/year between 2000 and 2010.

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Apart from uncertainty about service life, another issue that has acted to slow the transition from glass to composite insulators has been the comparative ease of inspection of the alternative technologies. “While it is always easy and requires neither special training nor costly inspection equipment to detect failed glass discs,” notes Louaked, “this is unfortunately not yet the case for composite insulators.” He goes on to emphasize that ease of inspection is widely considered as the main advantage of glass compared to porcelain or composite designs and, in the latter case, it is still difficult to detect any kind of internal defect. This means that performing live line work with such insulators is not yet recommended nor has any procedure been standardized at GRTE for their live inspection and maintenance. According to Louaked, GRTE’s selection strategy for composite

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insulators is based predominantly on axial distance and interchangeability between these and the glass strings being replaced – not on creepage distance alone, which in most cases is actually longer for composite types than for glass strings.

Photos: Courtesy of R. Znaidi and GRTE

In 2010, notes Soukeur, the total population of composite insulators on transmission lines in Algeria amounted to just over 11,500 units. These are in service mainly on the 220 kV and 60 kV systems and represent 5.3% of the total network. “Considering their relative benefits and drawbacks and given our present system needs,” he says, “our current target is to grow this proportion to about 10% by the end of 2012-2013.”

Ease of inspection has been a dominant reason in Algeria for the preference for glass.

Table 5: Comparison of Acquisition & Installation Costs of Composite versus Glass Insulators Glass

EPDM & SiR

System Voltage

Number of Towers

Acquisition & Installation Est. Costs (US$ 000)

Acquisition & Installation Est. Costs (US$ 000)

Potential Savings (US$ 000)

Potential Savings (%)

220 kV

646

1108

685

423

38

60 kV

130

57

49

8

13

Total

776

1165 100%

734 63%

431

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Table 6: Comparison of Installation & Maintenance Costs of Composite versus Glass Insulators on Two Lines

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Installation Costs (US$ 000)

Lines/ No. of Towers

Glass

Composite

Marsat/Reliz 43 Towers

44

28

Marsat/Ously 69 Towers

71

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Savings US$ 000

Maintenance Costs (US$ 000) Glass

Composite

16 (36%)

13

0.6

12.4 (95%)

26 (63%)

20

1

19 (95%)

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GRTE Chairman Fergani indicates that, according to investment plans for the coming two years, 2 million aerodynamic glass cap & pin insulators as well as 6000 silicone rubber suspension insulators have already been ordered. These are to be installed on new 60 kV, 220 kV and even some 400 kV lines and will represent an acquisition cost equivalent to some Euros 40 million and 650,000 respectively.

Live Line Washing

Among past remedial measures commissioned by GRTE to combat the impact of pollution and reduce the number of flashovers was regular hand washing of insulators. However, this proved slow and costly. As a result, a program of training operating and maintenance personnel in more efficient techniques was instituted in co-operation with a Canadian power utility. This included progressive application of live line working as well as washing, first from robot-controlled trucks and more recently from helicopters. Soukeur explains that washing from helicopter was first implemented by GRTE in 2010 and used during the traditional washing period from June to October mostly on polluted lines across mountainous terrain and forests. This first program was considered a success since almost 4700 glass strings were washed in a relatively short time frame. Moreover,

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10 percent of all insulators on Algerian transmission liens will soon be composite type.

Table 7: Axial & Unified Creepage Distance (USCD) Criteria at GRTE, 220 kV Composite Versus Glass Insulators Glass System Voltage

SiR

Number of discs

Type or Reference

USCD mm/kV

USCD mm/kV

Axial Distance (m)

18

U160P

70

80

3.060

18

U160A

46

57

2.628

18

U120A

46

60

2.340

21

U120A

53

68

2.73

20

U120A

51

59

2.600

18

U120P

56

58

2.628

220 kV

Table 8: Development of Composite Insulator Usage at GRTE (2002-2012) Year

2002

2006

2008

2010

Proj. 2012/2013

Total Network Length

14,635

17,403

21,683

22,232

2,000,000 Glass discs

Number of New Insulators/Year

4400

1704

1312

3801

6000 SiR units

Length of Lines with Composite Insulators (km)

306

475

913

1184

Evolution

2%

3%

4.2%

5.3%

10% (objective)

Table 9: Growth in Application of Live Washing from Helicopters by GRTE

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Live Line Washing

2009

2010

2011

Development 2011/2010

By truck (No. glass strings washed)

4,503

4,684

3,730

-21%

By Helicopter (No. glass strings washed)

-

4,170

12,872

+ 209%

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using helicopters saved the high cost of moving heavy trucks and logistical equipment into areas without ease of access by road. Says Louaked, “helicopter-based live line washing ended up costing 7 times less than from trucks, simply because of the efficiency with which the work could be done.” For example, he indicates that the average number insulators strings washed each day by helicopter has been about 200 whereas in the case of tanker trucks this figure rarely exceeded 15 for each washing crew. That means that washing 200 strings by truck over the same time

“From our perspective, some prudence is still required when it comes to composite insulators and we keep in mind what would happen if all such insulators began to fail over the same short time frame.”

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“In this context,” he adds, “we are quite interested to develop a new and more effective relationship between SONELGAZ Group and our suppliers. This will be based on a spirit of partnership and therefore take into account any related local investment, often supported by government incentives.”  Part 2 of this article will appear in INMR Q3, 2012 Photos: Courtesy of R. Znaidi and GRTE

Application of helicopter-based washing by GRTE more than doubled between 2010 and 2011.

period would require 13 work teams, each composed of 5 trained linesmen as well as an equal number of trucks and water pumps. Last year, more than 12,800 glass strings were washed from helicopters and, based on this, Soukeur predicts that application of this maintenance technique will continue to grow.

Purchasing Strategy for Insulators

Fergani explains that the purchasing policy followed by GRTE with regard to strategic items such as insulators aims to avoid overdependence on any one supplier or product. “With this goal in mind,” he says, “the first or pre-qualification phase of suppliers is based on their complying with our technical requirements that follow international standards, specifications and type tests This is then followed by a final qualification procedure based on the financial offer of each pre-qualified supplier.”

Photos: Courtesy of R. Znaidi and GRTE

According to Fergani, the first ranked supplier is the one offering the lowest pricing. But instead of limiting the final qualification entirely to this first ranked supplier, GRTE proceeds to divide up the insulator order between either the first and second ranked or between the first, second and third ranked bidders using either a 60%-40% or 50%-30%-20% ratio. The one condition is that the second and third ranked suppliers must first align their pricing with that of the lowest bidder, who is the real winner of the tender. Looking to the future of relationships with suppliers of components such as insulators, Fergani points out, “formal purchasing procedures and rules in Algeria generally require a minimum of 6 months from the time an international tender is issued until reception of the items that are ultimately purchased. Given this,” he continues, “we have asked ourselves what we could do if faced with the need to receive the order much sooner, such as should there be an urgency to replace many failed insulators in a short time. That’s why”, he explains, “within our regular tender we typically ask all companies who choose to participate to sign a cooperation or joint development agreement associated with the item being tendered.”

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UTILITY PRACTICE & EXPERIENCE

Tropical Paradise, Yes But Not for Overhead Distribution

Lightning flash density around Thailand. Near the coast and out at sea, activity tends to be low. Levels to the north and south are similar to those found in Florida, USA - with ground flash density of 10 to 14 per km2 per year. (Data from NASA OTD Version 2.2 records).

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This picturesque, palm-filled scene from southern Thailand conveys a sense of nothing but serene, natural beauty. Yet, lurking hidden in this same image are some of the most important stresses that adversely impact overhead distribution â&#x20AC;&#x201C; coastal pollution, high humidity, frequent lightning, intense solar radiation, abundant wildlife and, sadly, occasional vandalism as well. All these present serious challenges to operators of overhead distribution networks in terms of unplanned outages and potentially high maintenance costs.

Photo: INMR Š

The photos on the following pages provide evidence of the types of problems that can occur and also remark on some of the solutions applied by the system operator given the demanding service environment.

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All photos: INMR Š

Porcelain insulators are brittle and can quickly fracture due to stone throwing or gunshot. The insulators in these images show typical outcomes of vandalism.

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All photos: INMR Š

High levels of contamination on both upper and lower surfaces can quickly overwhelm porcelain disc insulators leading to pollution flashover, as clearly evident in these photos. Damage to glaze from successive flashovers will leave surfaces increasingly rough leading to increased concentrations of localized pollution deposition and even greater risk of future similar flashovers.

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All photos: INMR ©

Biological contamination is a problem often associated only with polymeric insulation yet can occur with equal ease on porcelain surfaces in a hot, damp environment. Mold growth can reduce effectiveness of external insulation due to build up of persistent wetting. Another issue in a tropical setting is onset of corrosion and possible damage to cement work from leakage current collecting here whenever porcelain is wetted by dew or rain.

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Deterioration due to high UV is another potential problem. Cutouts above show evidence that fuse tubes are well on the way to failure caused by long-term exposure to strong sun. Initially, the paint starts to disappear followed by the polyester layer itself. If these cutouts had to operate, they could possibly explode the tube and need to be replaced – not a serious problem if there is a backup tube. If not, there could be a long-term outage.

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The limited angle of glaze damage and its presence on all sheds is likely the result of impulse flashover and resulting power arcs. Since there is no bonding to the insulator bases, leakage current flows through the concrete, which typically has low resistivity. However, the framing looks as if it was adapted from a wood construction and there is no impulse strength in the concrete crossarm. Note the loss of glaze on bottom shed of porcelain in photo at bottom, making it even more vulnerable to contamination build up.

All photos: INMR Š

Also, these post insulators seem to be dimensioned much smaller in terms of dry arc leakage distance than the four accompanying standard discs. This should normally be reversed since post leakage should be higher than series discs in contaminated areas such as this.

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Wildlife, such as monkeys or large birds, are an issue affecting distribution lines in most tropical countries and remedial measures often include specialized protectors over terminals of, e.g. arresters. However, these are not always effectively installed.

Note: On porcelain bushing at left there is evidence on the second shed from the bottom of a so-called ‘glaze worm’ – a signature of long-term damage from minor arcing (as opposed to flashover).

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For example, the wildlife protector on the arrester in this photo is virtually worthless against larger animals and birds. A monkey or similar animal that comes close to this top lead will not survive if they are also touching a point of ground potential such as an earthed concrete pole. Similarly, animal guard covers have been installed over a nearby transformer to prevent an animal fault across the terminals but the rod gaps (which probably came already equipped on the transformer) are not protected. To be effective, the top terminals of these rod gaps should have insulation as thick as on the high side phase conductors.

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ď ´ This photo shows that

All photos: INMR Š

both 30 kV arresters have failed and operated their disconnectors. The top arrester also has its hot lead cut away so that it is completely isolated from the system. The bottom arrester, however, still has the high voltage applied at the top and therefore also at the bottom since the disconnector is open. Note: Sheds on the insulating hanger in this installation seem highly under-dimensioned. Once the disconnector operates, the full system voltage stress across the insulating hanger will likely cause it to flashover sporadically within a short time.

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Photos: INMR ©

 Three EPR shed

 Silicone sheds on this 30 kV polymeric arrester that protects the transformer shown above still maintain good hydrophobicity, even in a demanding location right beside the sea. There is no evidence of mold and the discoloration may rather have been due to an external flashover. Note creepage distance of insulating hanger, which is the minimum that would pass IEC or IEEE standards.

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material arresters from one manufacturer look almost new and obviously in good shape while older design from another supplier (second from left) shows evidence of superficial surface changes. Note: Fuse tubes on cutouts show one with some paint still on and one with most paint already gone.

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Hydrophobicity of the top shed of these IEC Class 2 or 3 arresters protecting cable connected on the other side of structure is impressive.

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Note: All have insulated bases with ground leads directed down the structure to probably 1 to 3 condition monitors. Since they are protecting underground cable, power system operator has specified a higher quality arrester for this application and one that lends itself to condition monitoring as well.

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UTILITY PRACTICE & EXPERIENCE

Researching Optimal Insulator Design for High Altitude Lines 70

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T

ransmission lines the world over often have to traverse mountain ranges or plateaus situated at least 1000 m above sea level. In China, for example, it is estimated that some 60% of the country lies at this altitude or higher. In such cases, design of transmission and distribution systems has to take into account

Areas with low air pressure and thin air impact the operation of electrical lines since voltage decreases with altitude – whether air gap discharge voltage, corona inception voltage or pollution flashover voltage. Moreover, the higher the altitude, the more problematic this will become in terms of the performance of line insulators, or indeed any high voltage insulation. It is well known that, as altitude increases, changes in air pressure, temperature and humidity will all exert some influence on discharge voltage. However, it is the change in air pressure that is the most significant as well as the most consistent of these variables in terms of its effect. When it comes to evaluating insulator performance, pollution flashover voltage is obtained when the humidity surrounding the test object reaches saturation. The influence of atmospheric humidity can therefore be ignored. Similarly, the regularity of temperature change as altitude increases is not always obvious. Pollution flashover testing is performed in a fog chamber where temperature is often different from ambient. Accounting for the

the fundamental changes in insulation performance that are associated with high altitude. This article, contributed by INMR Columnist, Professor Guan Zhicheng of Tsinghua University’s Shenzhen Campus, discusses these changes as well as recent research conducted in China on this topic.

influence of temperature on pollution flashover of insulators is relatively complex and remains a subject of debate. Indeed, no temperature correction factor is presently applied when it comes to insulator pollution flashover voltage.

Recent UHV transmission projects in China have accelerated the pace of research in the area of external insulation at high altitude. Given the above, current research on how altitude impacts insulator performance tends to focus on the influence of air pressure – not humidity or temperature. The correlation between altitude and this parameter is shown in Table 1 below (obtained from actual measurements).

Table 1: Changes in Air Pressure & Relative Density with Altitude Altitude

Sea Level

1000 m

2000 m

3000 m

Average Air Pressure (MPa)

0.1013

0.0897

0.0794

0.0704

Ave. Air Pressure (mmHg )

760

673

596

528

Average Relative Air Density

1.0

0.901

0.812

0.732

*

1 mmHg = 133.3224 Pa

*

Experts from a variety of countries, including Japan, Russia, and Canada have studied the influence of air pressure on insulator pollution flashover voltage and proposed that the correction for air pressure be expressed using an equation. In it, the parameter, n, reflecting the value of the air pressure correction, is obtained by testing. Most agree with an n value of 0.5 for a normal design of insulator and 0.6 for an antipollution design under AC voltage or 0.35 in the case of DC. U(p) = U(p0)(p/p0)n As early as the beginning of the 1980s, Tsinghua University and Chongqing University began research into insulator pollution flashover characteristics under high altitude conditions of low air pressure. Utilizing relatively small-scale tanks, pollution flashover tests were conducted to better understand and evaluate the influence of air pressure (altitude) on the pollution flashover characteristics of a variety of different types of insulators. Recent UHV transmission projects in China have only accelerated research into how best to select external insulation at high altitude. Along with this has come rapid improvement in the facilities available for this purpose. For example, two largescale air pressure tanks have been built and put into service in Beijing and in the central city of Wuhan. In the case of the Beijing facility, the tank body (consisting of a circular metallic structure of 20 m diameter and 25 m height) can simulate an altitude of up to 5500 m. Moreover,

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it is capable of performing not only icing and melting tests on insulators but also full-scale UHV pollution flashover tests under both AC and DC. The new facility in Wuhan, which has the same tank dimensions but is made of reinforced concrete, has also conducted numerous insulator pollution and icing flashover tests at simulated high altitudes.

The air pressure tanks in Beijing and Wuhan as well as the new high altitude test bases in Kunming and Tibet serve to complement one another. For example, the air pressure tanks artificially simulate high altitude and low air pressure to evaluate its influence on insulator pollution flashover behavior. Test results from the two high-altitude test sites can then verify these findings. At the same time, insulator

have all conducted tests on the tendency of insulator pollution flashover voltage to decrease as altitude increases. Pollution severity during the course of these tests has ranged from low to high, including both AC and DC flashover tests. A lot of test data has been obtained in the process and, while the actual figures may differ somewhat from one test to the next, basic tendencies have been the same. For example, all these tests have confirmed that the value of the exponent n, (quantifying the influence of air pressure on pollution flashover voltage) depends not only on the voltage being applied, but also on type of insulator (whether porcelain, glass or composite), its

Photo: INMR ©

China Southern Power Grid – one of the two major Chinese grid operators – has set up a pollution laboratory in the southern city of Kunming at an altitude of 2100 m. The hall there measures 26 m x 28 m x 30 m and can carry out flashover testing at up to ± 1000 kV DC and 800 kV AC. In fact, Tsinghua University and the China Southern Power Grid’s

For its part, the State Grid Corp. of China has built a test base in Yangbajing, Tibet, where the altitude is more than 4000 m. This facility includes a pollution laboratory and fog chamber of 9 m x 9 m x 11m and can do testing for ± 200 kV in DC and 200 kV in AC.

Pollution test hall and line near Kunming operates at 2100 m.

pollution flashover experience under the high altitude conditions of the test bases can be directly applied during project design. In China, universities, research institutes and power grid operators

n value

Technical Centre have recently begun a joint research project at this facility to study the pollution flashover characteristics of ± 800 kV full-size suspension as well as station post insulators having different materials and profiles.

Figure 1: Results of high altitude performance tests on four different suspension insulator designs.

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shed profile and the severity of the pollution. In this regard, tests were conducted by Tsinghua University on four different designs of suspension insulators, with findings for the resulting exponent n values for the different profiles shown below. Designs of these insulators were classified as: (a) XS-4.5, (b) XP-16, (c) XP3-16 and (d) XWP2-16. Test results showed that the n value for the type a insulator (with a relatively simple profile and no edges on the lower surface) is comparatively small. Moreover, n values for this insulator vary quite a bit under different pollution severities. It was also found that the

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n value of the anti-pollution type d U(p)=(1-Kh)U(p0) is not necessarily higher than for the common type b and type c insulators. The physical meaning of K in this case is to show the percentage of Tsinghua University, Chongqing pollution flashover voltage decrease University, China EPRI, China for every increase of 1000 m in Southern Grid and the Xi’an High altitude, while h expresses the Voltage Apparatus Research Institute relevant multiple of 1000 m. have done a lot of tests separately to Comparing the two formulas, the obtain n values of polluted insulators. following correlation between K and Statistical analyses of these n values n can then be obtained: measuring influence of air pressure on pollution flashover voltage (and K= {1-(p/p0)n }/h which included an AC pollution flashover test as well as a negative Based on the data from Table 1, the polarity DC pollution flashover test) correlation between K and n can be proved interesting. There were a total calculated as per this formula and is of 80 test samples of AC suspension shown in Figure 3. insulators, and the distribution of n values obtained is shown in the histogram of Figure 2. These n values Insulator designs follow a normal distribution, with the average being 0.49. that perform well

electric arc bridging the insulator profile has two distinct components: stable electric arc bridges and also electric arc breakdown in air. The first type – stable bridging electric arcs – are not easily extinguished and move due to external forces such as electromagnetism and thermal buoyancy. As the arc root moves, there is increased risk of a shortage of enough discharge distance, leading to flashover.

A similar approach was then applied to AC post insulators and here the average n value was 0.48 (i.e. very close to that for suspension insulators). The average n value for DC suspension insulators, however, was 0.27, which is significantly lower than for AC.

near sea level are not necessarily equally effective when operating in mountainous areas.

The influence of altitude (air pressure) on pollution flashover voltage is usually expressed using the formula referred to earlier, which is basically an empirical formula to statistically process test data. The physical meaning of n is not that clear. Tsinghua University has therefore proposed that the equation below be used instead.

For example, given the n value is 0.5, compared with flat areas (near sea level), the pollution flashover voltage of insulators at altitudes of 1000 m and 2000m will be reduced by 5.7% and 11.4% respectively. The diagram also assigns a more direct picture of the physical meaning of an n value.

The second type is a ‘span bridge’. In this case, the passage of the arc is not the result of partial discharge but rather caused by breakdown of the air gap outside the insulator profile. This arc span bridge leads to a growing shortage of sufficient discharge distance and therefore a decrease in flashover voltage. Research found that the more shed profiles protruded and the shorter the distance between them, the higher the probability of the arc span bridge and the more significant the decrease in flashover voltage.

Research at Tsinghua University has found that there are two basic reasons behind change of pollution flashover voltage as air pressure changes. One is that the volt-ampere characteristic of an electric arc changes with air pressure and second is the influence of the electric arc bridge flashover across the insulator’s profile, or sheds.

In high altitude areas, this ‘span bridge’ between insulator sheds is the type more likely to occur and therefore this imposes a more demanding requirement in terms of ideal insulator profile. In other words, those insulators that perform well near sea level are not necessarily equally suitable for service in mountainous areas.

Research measuring the influence of shed profile on insulator pollution flashover characteristics at the high altitude test base in Tibet found that

All this suggests the need for ongoing research to identify those insulator profiles that are optimized for demanding high altitude applications. 

Figure 2: Distribution of n values obtained from testing 80 suspension insulators.

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Figure 3: Correlation between K and n.

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UTILITY PRACTICE & EXPERIENCE

BIRDS:

More Threat to Lines or Threatened by Lines? A

s industrial contamination is coming under more and more scrutiny and control in many countries, the issue of flashover outages attributed to birds (often included under the category of ‘unexplained outages’) is becoming a growing problem. As service environments improve, more wildlife and in particular birds return to play a greater

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role in what can adversely affect an overhead network. The following article, from Igor Gutman of STRI (Sweden), Evgeny Solomonik of NIIPT (Russia) and Wallace Vosloo of Eskom (South Africa) examines the topic of how birds and power lines co-exist in what is mostly a mutually threatening relationship.

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Introduction

Recent service experience from countries such as Germany and Russia suggests that up to 70-80% of outages on overhead lines can be directly attributed to birds. Even though these types of outages are almost always characterized by successful automatic re-closure (i.e. from 94 to 97% of the time), power utilities are still concerned. In older days, their concern was due mainly to old type oil circuit breakers that required maintenance after each cycle of about 15 operations. At present, the worry is due more to requirements for power quality and availability, since many utilities now set their own internal rules on average outages permitted each year per 100 km of line.

justifiably – as ‘devices of mass avian destruction’. Estimates of the numbers of birds killed each year are shocking and one authoritative source reports that 7 million die annually only in the European part of Russia. Another reference, based on a thorough investigation, refers to a density of electrocuted birds at about 15 for every 10 km of line. Especially dangerous in this regard are lines with vertically installed pin-type insulators because the conductor sits on the insulator, not below as in a suspension string.

Experience from countries such as Germany and Russia suggests that 70 to 80% of all outages on overhead lines can be directly attributed to birds.

What makes this situation all the more tragic is that the threat posed to birds by distribution lines has been documented for years now. Moreover, there is adequate Examples of bird knowledge on how to solve or least electrocution on largely reduce the problem by distribution lines. insulating the conductors close to the It is important at the early stage of tower using protective devices (e.g. any discussion on problems of birds insulating covers) of different shape and power lines to distinguish the situation for distribution voltages from and design. that in the case of transmission. The two are vastly different problems with The principal requirement for such devices, usually made of polymeric much different possible solutions. materials, is that they have a service Photo courtesy of the author life similar to that of the components In the case of distribution systems, they are intended to protect, e.g. the central problem is electrocution conductors, insulators, arresters. In of birds. Indeed, ornithologists this regard, they must be resistant to Experience with such devices has as well as bird lovers the world deterioration from weathering and UV. generally been positive and the only over regard these lines – perhaps issue is the cost necessary to provide sufficient protection in areas known to have high concentrations of birds. (Interested readers who wish to know more about such devices are referred to a more thorough review applied to surge arresters in an article titled, Wildlife Protective Devices for Arresters, appearing in INMR Q1, 2010).

Photo: INMR ©

Unlike the widespread carnage of birds along distribution lines, transmission networks face a challenge of an entirely different sort. Here, flashovers of suspension strings by streamer-type excretions are relatively common and can affect lines of up to 500 kV. In these cases, the birds fly away basically unharmed (although stressed by the effects of the spectacular flashover) while network equipment has to be relied on to quickly restore the line.

This tranquil scene may prove a recipe for disaster.

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Examples of how lines and structures can be made safer for birds.

Photo: INMR ©

Photo courtesy of the author

Photo: INMR ©

to 2 m and move at speeds of 2 to 5 m/s. The angle of the streamer to the cross-arm can even be 60-70° yet still lead to flashover.

Engineering Overview of the Problem

The problem of outages on overhead transmission lines due to streamertype flashovers is caused by typical bird behavior after perching on a tower cross-arm. Before taking-off, they usually empty their bowels and release up to 60 cm3 of excrement mixed with conductive urine at one time and coming out at quite a pressure. This creates a conductive path that bridges the air gap between tower structure and conductors resulting in a flashover either in parallel to or a little apart from the insulator string. Detailed investigations of this streamer have been made in HV laboratories (where researchers once even chained birds to mockup towers) as well as in zoos using high-speed cameras. This research confirmed that the continuous part of the streamer can attain lengths up

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Excrement is non-conductive in the dry condition. But the resistivity of the half dry excrement has been found to vary from 800-2000 Ω∙cm. In its natural state, this figure will vary slightly by species, e.g. storks (220-610 Ω∙cm); eagles (190-770 Ω∙cm); and the lowly chicken which serves as a reference point (260850 Ω∙cm). It should be taken into account, of course, that the average temperature of a bird is 41.5°C and that actual resistivity will be less than measured at lower ambient temperatures.

At Stellenbosch University in South Africa, a special bird excretion flashover simulator was even developed and experiments performed at a resistivity of about 70 Ω∙cm. This figure was selected after analysis of samples from the martial and black eagles and the mixture consisted of cellulose, salt and tap water. The results were then used to investigate the physics of bird streamer flashover.

It certainly seems clear based on the above that, from an engineering point of view, at least, the problem of bird streamer flashover on transmission lines has been identified and confirmed. The issue then is how best to protect a line from birds perching in the vicinity of insulator strings and periodically flashing them over. And this requires asking, why Experiments confirm that, given the above parameters, it is indeed possible are the birds even there at all? to flash over strings of up to 500 kV AC and ± 400 kV DC. In Russia, for example, research has shown that flashover occurs at a resistivity of about 600 Ω∙cm (using actual excrement collected at the zoo) and a dielectric strength of 70 kV/m, which is less than the maximum operating voltage for the 110 kV insulators tested. Similarly, in the U.S. it was found that, at a resistivity of about 120 Ω∙cm (from samples of excrement mixed with raw eggs and salt), a 500 kV suspension cap & pin string flashed over at 320 kV, or about 1.1 nominal operating voltage. It is worth noting that in these experiments the streamer could be as far as 70 cm away from the string yet still affect it.

Birds: morE thrEat

Research has confirmed that the continuous part of the streamer can attain lengths of up to 2 m and move at speeds of 2 to 5 m/s.

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Photo courtesy of the author Photo: INMR ©

Photo courtesy of the author

Example of bird streamer flashover affecting middle phase.

Resistivity of bird excrement can vary widely by species.

Streamers are typically released just after landing or before take-off.

Bird Streamer Flashover: The Perspective of the Bird

unknown outages while the newer one did not. The reason was that the first line was already occupied by most of the large local birds, which then never moved to the second line.

According to ornithologists, there is an obvious reason why transmission towers attract large raptor type species such as owls, vultures, eagles, etc., i.e. the usual culprits in causing streamer type outages. The structures provide them with ideal platforms for hunting, roosting and nesting. Indeed, the cross-arms of a typical 110 kV tower are located at the optimal height for buzzards to hunt prey along the right-of-way.

A similar example from South Africa showed that only half a line suffered from unknown type outages while the other half did not. In this case, the reason was that the problematic half was under control of a couple of eagles that settled near its end while no other raptors watched the other half. The above demonstrate that it is worthwhile to study the behavior of birds occupying any portion of a transmission line. In most cases, birds come to rest and sleep towards dusk and take their place on the cross-arm or in the nest. Sometimes, they empty their bowels for the first

Moreover, once birds establish a roosting site, they continue to use it year after year. In Finland, for example, two lines of identical design and sharing the same corridor, but constructed at different times, had much different operating histories. The older line suffered from

Stork nests on „ tower in Algeria.

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In summary, transmission towers offer a natural place for birds to rest and hunt. The only issues left to resolve are: How to be certain that the flashovers are indeed bird-induced and, if so, what to do about it?

Clues for Utility Maintenance Personnel

As discussed, the peak number of outages (probably from bird-induced flashovers) take place in the early morning. This normally occurs in clean or slightly polluted areas during conditions of high humidity, as insulators are wetted by morning dew. Thus, it is not always easy to pinpoint the exact cause of the flashover. For example, it could well be a classical pollution flashover or one related to a problem with E-field. Recommendations to ascertain whether or not birds are involved in the outage are: 1. Inspect the problematic line by

Photo courtesy of the author

Test set-up used in South Africa.

time right after landing – thereby providing the first peak of outages around 10 to 11 PM. Then, the birds empty their bowels a second time in the early morning (4 to 6 AM), most probably to reduce their body weight before take-off. They fly away leaving behind yet another potential outage to be dealt with.

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3. 4.

5. 6. 7. 8.

distribution voltage: modern scientific and practice experience” was held in November last year. Organized by the Russian Bird Conservation Union, this meeting brought together ornithologists and engineers as well as representatives of the country’s environmental prosecutor’s office.

begun to step up enforcement of the Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act. All migratory birds are protected under the Migratory Bird Treaty Act. In a landmark case, a utility was sentenced to three years probation for electrocuting 17 eagles and hawks near Rangely, Colorado. The utility ultimately pleaded guilty to six violations of the Migratory Bird Treaty Act and the Eagle Protection Act. Under the settlement it agreed to pay $100,000 in fines and restitution to the U.S. Fish and Wildlife Service.”

Unfortunately neither CIGRE nor IEC yet perform any activities in this area although a proposal for a new working group on the topic was put forward to Study Committee B2.

1. Preventing Outages Due to Roosting

In Russia, a scientific workshop “Problems of bird electrocution and safety on overhead power lines of

The main device to prevent birds from roosting on a tower cross-arm is

Orange River – Ranch Line Frequency of Occurrence

2.

helicopter, checking for traces of bird contamination on the surfaces of insulators or on the cross-arms. Nests can also be observed on particular towers; Contact local ornithologists for any maps that show density of different species in the area. Local biology students can be of great help in the field to observe bird activity near any affected line; Establish presence of any nearby agricultural land, fish farms or sugarcane fields; Check seasonal pattern of outages and relate this with natural departure/arrival of migratory birds and time of birth of their juveniles. In the European part of Russia, for example, there is typically a sharp increase in outages during August, related mostly to juveniles being taught how to fly and feed; Check daily pattern of outages and relate this with typical morning peaks; Verify percentage of successful auto re-closures (typically around 95% in case birds are involved); Monitor below the line for dead birds or burnt feathers; Check the appearance of flashed insulator strings. In the case of bird-induced outages there are usually traces of the arc root on the conductor (at 1-1.5 m from the insulator), on the cross-arm above the insulator or at the top one or two cap & pin insulators.

14% 12% 10% 8% 6% 4% 2% 0% 12

13

14

15

16

17

18

19

20

21

22

23

00

01

02

03

04

05

06

07

08

09

10

11

Examples of typical times for birdinduced outages.

Solving the Problem in ‘Bird Friendly’ Ways

A number of guidelines/standards exist with recommendations in these situations, including the recently published IEEE Guide for Reducing Bird-Related Outages, Eskom Transmission Bird Perch Guidelines, Russian Regulations on Design and Reconstruction of Electrical Installations, PUE (for lines and substations).

Traces of arc root at the cross-arm provide confirmation of bird streamer flashover.

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Photo courtesy of the author

Still, engineering bodies remain under pressure by environmental groups. For example, the IEEE Guide notes “Disturbed by the continuing large numbers of birds electrocuted and colliding with power lines, the U.S. Fish and Wildlife Service has

Appearance of insulator string provides obvious clue as to cause of flashover.

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Perch guards are available on the market but must be dimensioned for each type of bird, which is extremely important for species with long legs.

known as a perch guard (also known as bird discourager). Such perch management has been successfully applied in the U.S., Russia, Sweden, China, Germany and other countries to prevent problems due to bird streamers. Experience in South Africa has demonstrated just how effective these can be. Perch guards are available on the market but must be dimensioned properly for each species â&#x20AC;&#x201C; a factor that is extremely important for birds with long legs. Unfortunately, no practical recommendations yet exist for such species-specific perch guards.

Photo: INMR Š

Photo courtesy of the author

To reduce cost and maximize effectiveness of this measure, it is best to ensure collaboration between engineers and ornithologists to define the most suitable locations for each different type of tower. Care should also be taken to cover the complete area potentally occupied by birds, since they will quickly take over any space left open.

Photo courtesy of the author

Typical perch guards and result of their use in South Africa (number of outages decreased dramatically).

Many utilities still use mostly metallic guards made of wire or rods, which could harm birds while landing. This argues in favor of flexible guards made of plastic. Again, as for distribution lines, these must be weather and UV-resistant, with some periodic maintenance cycle to monitor for signs of deterioration. There are cases known where contamination to insulators is also due to other excretions apart from streamers, although these are less a risk due to their lower conductivity. The solution here is to use a top insulator of larger diameter or install a plastic shield over it.

2. Preventing Outages Due to Nesting

Birds (like power engineers) tend to very conservative and will try to build their nest on the very same tower chosen the year before. There are anecdotes from a power utility which removed the nest of a white stork from the first tower entering a substation yet each time the

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can measure 2 m in diameter and weight up to 50 kg. To remove such a construction might require one or two linesmen for a few hours.

Photo courtesy of the author

In fact, experience demonstrates that, if anything, removing nests is the incorrect approach. This is because, during initial construction and subsequent re-construction, birds use branches and metallic wire, which can either fall causing an outage or which can expand downward from the nest, reducing the air gap and causing even more trouble.

Broader top shed protects rest of insulator from bird excretions. stork returned to this same tower. After a ‘battle’ lasting 3 years, the substation was ‘bombarded’ by storks bringing branches and metal wire and the nest was left on the first tower as a sign of submission by the utility. (This is probably not entirely true, but the power company had to provide some explanation why they lost the battle against the storks). Many power supply utilities worldwide report on how many nests have been taken down before the birds return. This is not an easy task since the nest of a stork

Therefore, if birds have already built their nest, it is better not to touch it before the birds vacate. Instead, the structure should, if anything, be protected by fence to keep out e.g. snakes, cats, which will be attracted to the nest. And if any reader doubts that snakes can reach the top of transmission towers, they might be surprised to learn that it can be and is indeed often accomplished.

and should be somewhat higher than the original nests to make them an attractive alternative. Then, once the nest has been re-located, the original tower should be equipped with bird guards to discourage future renesting. Again, to establish the best construction of alternative platforms, there should again be close cooperation between line engineers and ornithologists.

Summary

In general, power companies should do whatever is possible to protect birds from overhead lines at distribution voltages and to protect overhead lines from birds at transmission voltages. At present there is a plenty of experience and proposals. However, these are not that well structured by specific bird species, which may be a key issue in their relative success or failure.

Unfortunately neither CIGRE nor IEC yet give much attention to the Moreover, once the birds have raised issue of bird-induced flashovers even though it is becoming a their offspring and left the site, the nests should be carefully re-located. progressively more important reason for transmission outages. To In the long run, this would be the solve the problem, there must be most successful and ‘bird-friendly’ approach, namely providing the birds close collaboration between power engineers and ornithologists and the with a nearby nesting alternative two should not regard each other as – usually some kind a pole with a the enemy.  platform installed near the existing nest site. Such alternative platforms should be built alongside the line

Photo: INMR ©

Stork nest was tolerated on this tower (left) but not on one having cable connection, where nest was relocated to a nearby nonpower structure.

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INSULATORS

Modern Pollution Monitoring Principles Allow Better Selection of Insulators for Polluted Service Conditions

Failures of insulators due to pollution flashover can prove very costly, causing potentially long outages and requiring expensive and time-consuming maintenance.

(Part 2 of 2)

This article, the second of two-parts, was contributed by Igor Gutman of STRI in Sweden and Wallace Vosloo of Eskom in South Africa. It introduces practical field and laboratory experience with different pollution monitoring techniques as well as presents principles to convert from one parameter to another. It also discusses how to use these parameters for insulator dimensioning according to the requirements of the IEC 60815.

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By most accounts, one of the leading causes of such failures is improper specification of insulators in the first place – to the extent that the designs selected are unable to cope with all the stresses imposed by pollution in the environment. Therefore, it is vital for engineers at power supply companies to correctly understand the real pollution characteristics of the service environment into which insulators will be placed into service.

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Photo: INMR © Photo: INMR ©

Breaker housings at left were not well specified for service at a 110 kV substation near seacoast and subsequently had to be coated with RTV to prevent flashover. Newer breakers at same substation (top) feature porcelain housings with alternating sheds and higher specific creepage and do not have coatings.

Leakage Current Measurement Leakage current flowing on an insulator depends mainly on the characteristics of the pollution on its surface as well as its geometry and material. For this reason, measuring and analyzing leakage current can be useful to estimate pollution severity and risk of flashover. Leakage current is usually measured by collecting the current at the ground end of energized insulators. The three most common methods for this purpose are: 1. Surge counting The number of leakage current pulses (or surges) exceeding some threshold level can be recorded during a period of time. This is important since the numbers of pulses as well as their amplitudes increase when approaching the last stage of the pollution flashover process.

recording the highest peak current over a given time interval. This parameter can then be related to risk of pollution flashover. 3. Measuring accumulated charge Accumulated charge measurement is performed in the same way as leakage current measurement. However, instead of focusing on the values of the highest peaks, the signal is integrated to represent

accumulated charge – a parameter more related to the ageing process on the insulator’s surface. Over the years, a number of different pollution monitors to measure leakage current have been developed. Products have ranged from simple surge counters to advanced multi-channel devices that even incoporate integrated meteorological stations.

Leakage current device installed in South Africa (left) and Norway (right) allows continuous sampling of leakage currents on 9 energized insulators and can record peak currents (positive and negative), average currents (positive and negative), RMS currents, accumulated charge and energy loss.

2. Measuring peak current Information about pollution severity on an insulator can be obtained by

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Rotated Component Matrix* 1

Component 2 3 4

COM25MAX

-.142

.129

Humidity

.249

.790

No rain (0 mm)

-.977

-.109

Light rain (< 1 mm)

.972

.125

-.136

5 .737

Wind from sea (45-260 degrees)

-.788

.292

Most dangerous wind from sea (260-285 degrees)

.766

.193

.266

-.206

Low wind (< 1 m/s)

-.125

-.112

-.951

.135

-.717

.594

.276

Medium wind (1-5 m/s)

.208

Strong wind (5-7 m/s)

-.152

Storm (> 7 m/s)

.291

-.802 .953

.199

Extraction method: Principal Component Analysis. Rotation Method: Varimax with Kaiser Normalization

Fig. 1: Example of application of MVA method: (top) Matrix of factors showing that pollution event consist of wind plus humidity plus light rain. (bottom) Same illustrated by one pollution event.

These days, many power supply companies are looking for simple and robust systems for pollution monitoring and warning in place of complicated â&#x20AC;&#x2DC;research-typeâ&#x20AC;&#x2122; systems for measuring leakage current.

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A standard such system now typically includes an integrated weather station to record wind speeds and direction, humidity, rainfall, temperature and UV-B radiation. All sampled values are then saved at some user-defined interval, typically every 30 minutes. Recorded data is retrieved using a serial port, which also serves to configure the instrument. Using such advanced devices, insulator researchers have been able to obtain a large volumes of leakage current and corresponding weather data. These have helped to better understand the physics of the pollution flashover process on different types of insulators. Sophisticated mathematical approaches have also sometimes been necessary. For example, multivariate analysis (MVA) methods has been used since they are appropriate to study and analyze data structured on many interrelated variables. MVA software was successfully tested in an ageing and pollution performance study performed on seven insulators exposed to pollution at the Kelso Test Station (on the Indian Ocean coast of South Africa) and at the Dungeness Test Station (on the southern coast of the United Kingdom. An example of applying MVA is shown in Figure 1. A classical coastal pollution event is characterized by strong wind from the sea followed by some kind of wetting. However, in the case of a silicone rubber insulator, high humidity alone is usually not sufficient to lead to high current. Therefore additional wetting in the form of light rain is usually needed. This conclusion is supported by recording a typical pollution event on a silicone rubber insulator. Sensitivity of MVA can be very high. For example, during the prepollution phase of the pollution event at the Dungeness Test Station, salt is transported inland by strong

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All Photos: INMR ©

Silicone housing on bushing exposed to coastal pollution in New Zealand. Pollution accumulated on porcelain removed from same site (right).

winds. The most dangerous wind direction (associated with the four most severe pollution events and leading to the highest currents) has an angle of between 0 and 20° to the shore. This is due to a wall of sand that protects the site from the perpendicular (90°) direction. The description of the pollution event at the Kelso Test Station on South Africa’s Indian Ocean coast is slightly different. During the pre-

pollution phase at Kelso, salt comes in not only from the sea but also from the land since wind direction is almost parallel to the coast. Another practical application of leakage current data would be to convert it into pollution flashover performance curves. This approach was tested by applying STRI’s IST program (discussed in Part 1 of this article) to 10 uniquely different insulators at Eskom’s Koeberg

Insulator Pollution Test Station (KIPTS) on the Atlantic coast near Cape Town. Results proved quite reasonable in predicting outage rate compared to actual experience, i.e. number of blown fuses), (see Table 1). Results of such field station testing can be used practically well beyond simply assigning a ‘pass/fail’ grade to each insulator tested. For example, they can help estimate the pollution flashover performance of a complete overhead line located in the same area and equipped with these same insulators (geometry and material).

Photo: INMR ©

These days, however, many power supply companies are looking for simple and robust systems for pollution monitoring and warning in place of complicated ‘researchtype’ systems for measuring leakage current. This is because uprating/ upgrading existing overhead lines requires accurate evaluation of pollution levels.

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Wind direction is an important variable in understanding pollution deposition events.

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Table 1: Example of Application of Leakage Current Data Material

Calculated number of flashovers for 1 insulator per year

Number of fuse operations* at KIPTS after 1 year

Long rod

SIR

10-5

0

36

Long rod

SIR

10-9

0

B-2

24

Long rod

SIR

10-4

0

C-1

31

Long rod

SIR

-5

10

0

C-2

20

Long rod

SIR

0.5

0

D-1

36

Long rod

EPDM

0.3

0

D-2

35

Long rod

SIR

-2

10

0

E-1

29

Long rod

Porcelain

1.3

4

E-2

29

Long rod

RTV SIR

10-2

0

F

27

Cap & pin

Glass

2.5

4

Code

Specific creepage distance, mm/kV

Type

A

31

B-1

Example of single-channel devices for leakage current monitoring: South Africa (top), United States (bottom). Photo taken from ISH paper D-3, 2009).

Pollution performance curves based on this data and used in specialized software program give reasonable agreement with field experience.

Indeed, attempts to use the systems described above for this purpose have not always been successful because they are expensive and need maintenance at least once a year. Moreover, there were communication problems in many applications. Therefore, single-channel systems (for peak current only) applicable to both overhead lines and substations are now most in demand and under

Example of different cells used for measuring hydrophobicity transfer.

active development. The goal is to have relatively inexpensive and reliable devices cover large areas and allow data collection and transfer via the Internet.

Specific Techniques Applying to Composite Insulators Specific diagnostic techniques for composite insulators (along with practical examples from field and

laboratory investigation) include: • Hydrophobicity measurement; • Hydrophobicity transfer (localized ESDD); • Rapid flashover procedure to obtain dielectric strength of naturally polluted insulators. 1. Hydrophobicity Hydrophobicity measurements are performed according to IEC TS 62073 and comparatively simple.

Figure 2: Examples of HT measurement and their spread for composite insulators removed from different sites and lines.

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3. Rapid Flashover Procedure (to obtain dielectric strength of naturally polluted insulators) Because relatively few insulators with natural contamination are normally available from service, it would be time consuming to use the standard ‘up-and-down’ testing procedure to obtain flashover voltage. The rapid clean fog test method can then be used instead with the advantage that it is directly applicable to both naturally and artificially polluted insulators

Photo: INMR ©

The insulator is tested by consecutive application of voltage until the minimum withstand voltage of the U-shaped curve is obtained. Flashover levels generally decrease before resistance of the insulator starts to increase due to the washing effect. The minimum voltage level obtained is then considered to represent U50%. As a result, only one test is required per insulator, level of pollution and degree of hydrophobicity. Glass disc insulator from Skagerrak DC line.

2. Hydrophobicity Transfer (localized ESDD) Hydrophobicity transfer (HT) is a measure of a material’s ability to recover hydrophobicity. This process is due to diffusion of low molecular weight species (LMW) in the silicone rubber bulk material through the pollution layer to the surface. These then encapsulate any pollution particles, including salt. Even if the polluted surface appears hydrophilic, part of the pollution layer is penetrated by LMW silicones and effective resistance increases. HT is defined as: HT = ESDD – ASDD ESDD

where ASDD is Apparent Salt Deposit Density (or localized

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ESDD). Both ESDD and ASDD are measured with a small cell filled with deionized water where the bottom is the surface of the polluted insulator. ASDD is measured initially as current through the cell when the encapsulated pollution has not yet dissolved. After 5 minutes, or when current has stabilized, the bottom surface of the cell is scraped with a glass rod to set any encapsulated pollution free. This is then the measure of ESDD. This parameter provides a good indication of the ability of a composite insulator to recover under different environments. A compilation of HT values and their spread for different AC and DC silicone insulators is presented in Figure 2 (high HT is considered > 0.5).

This method is included in CIGRE Brochure 481 (issued in December 2011) and was successfully applied at STRI for testing: • AC porcelain support insulators with industrial pollution; • DC glass cap & pin insulators with light inland pollution; • DC composite insulators with light marine pollution; • DC composite insulators with light inland pollution; • AC composite circuit breaker housings with heavy industrial/ marine pollution. This method can be applied to create pollution performance curves to be used in the STRI IST program, as was done for insulators replaced from the Skagerrak and Cabora Bassa DC overhead lines. It can also be used to quantify the ability of a composite insulator

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Fig. 3: Examples of high level of repeatability for rapid clean fog test method for two different naturally polluted composite insulators.

to recover hydrophobicity. Such methods are presently under review by a new CIGRE working group D1.44 ‘Testing of naturally polluted insulators’. The test’s excellent repeatability is illustrated by the results from two different naturally polluted insulators (see Figure 3).

Summary Recently issued IEC 60815-1 requires accurate site severity evaluation performed over a minimum period of one year to identify the maximum stress level and corresponding site pollution severity (SPS) class. Such a process can then be defined as pollution monitoring and is the starting point

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in the process of selecting and dimensioning outdoor insulators for service in polluted areas. Although a software program called the Insulator Selection Tool follows the dimensioning process of IEC 60815 and has been verified against experience from the field, accurate pollution input data for this program is crucial. Among the latest trends in pollution monitoring to generate such inputs are: • Expanded application of simple, robust and reliable devices (e.g. dust deposit gauges and singlechannel leakage current sensors);

• Novel as yet not standardized methods that take into account specific features of insulators (e.g. hydrophobicity transfer / localized ESDD in the case of composite insulators); • ‘Smart’ (time and cost-effective) methods to obtain flashover voltage of naturally polluted insulators, e.g. when only a few test objects are available (rapid procedures). These will be standardized in the future’ • Further development of specialized software programs for ultimate application by power companies (e.g. combining technical and economic issues in one program). 

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INSULATORS

New Plant to Focus on Composite Insulators

One of the world’s newest insulator plants is about to come on-stream in the coastal city of Dalian, in northeastern China. The sprawling facility, which belongs to the Dalian Insulator Group, consists of several large manufacturing halls and will concentrate on solid and hollow core composite insulators as well as insulator fittings.

All Photos: INMR ©

INMR visits the plant to report on the reasons it was built and to examine some of the new injection molding production equipment that is already in place and being commissioned.

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But about two years ago, continuing sales growth made it apparent that the still new facility would soon need to be expanded. Plans were then put forward for an investment in a separate new factory at a nearby location that would concentrate exclusively on composite insulators. This would have the benefit of focusing all injection molding technology at one production site while also allowing the existing porcelain plant to itself expand to meet increased demand for such insulators.

All Photos: INMR ©

Dalian Insulator Group is one of China’s oldest and largest suppliers of porcelain insulators and in recent years has also begun to supply composite line insulators up to the highest transmission voltages. Both have been manufactured at the company’s production base, which was re-located several years ago from its original site near Dalian’s center to an industrial development zone outside the port city.

New composite insulator plant in Dalian.

Ren Guiqing, a Director and senior executive at Dalian Insulator Group, explains that the investment in an entirely new plant is a reflection of management’s belief that the market for composite insulator technology will continue to expand. Says Ren, “we feel that the superior anti-pollution performance of composite insulators will be of growing importance in coming years, especially in developing countries

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where there are rapidly rising levels of industrial activity.” Another issue, according to Ren and Vice General Manager, Yang Luguang, is the cost advantage offered by composite insulators, which they point out can vary from 1/3 to as much as 2/3 less than the purchase price of equivalent porcelain disc insulators, depending on voltage.

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All Photos: INMR ©

Perhaps the most compelling argument in favor of an entirely new composite insulator manufacturing operation has been recent sales growth in this product sector, which Yang notes has been exceptional. “Since 2006,” he reports, “our sales of composite line insulators have increased by a factor of 15, most of which has been to satisfy demand from customers in export markets. So, it is really the market itself that is driving our new manufacturing investment.”

Plans for the new hollow core line will concentrate solely on molding housings over tubes purchased from external suppliers. However, Yang who has recently shifted

responsibility from international sales to production of composite insulators and fittings, expects that the new facility will eventually supply its own tubes as well. “In the short term, we feel that our needs can be met by outside tube suppliers,” he says. “But as sales grow, being able to make our own tubes will obviously give us greater control over delivery lead times as well as product quality.”

All Photos: INMR ©

220 kV composite insulators highlight a growing market segment in China and internationally.

Yang predicts sales growth for composite insulators will remain strong – all the more so since Dalian Insulator Group has now developed and will soon introduce its own range of hollow core insulators. This is a market that he believes is also about to expand. “In spite of the fact that prices for hollow core composite insulators are generally higher than for porcelain,” he explains, “we still see a growing interest for them based on inquiries coming from our OEM customers in Europe and elsewhere.”

“Our sales of composite line insulators have grown by a factor of 15 since 2006. So, it is really the market itself that is driving our new manufacturing investment.”

Manufacturing slip on polymeric sheds at new plant.

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All Photos: INMR ©

Two new 1100-ton molding machines will focus on producing solid core insulators from 220 kV to 1000 kV.

The new facility is expected to soon be in full operation but there is already limited production of polymeric sheds for those insulators manufactured using shed-by-shed assembly technology. Also, a range of new injection molding equipment is already in place and the process of final assembly and commissioning of these machines has begun. Liu Zengyuan is Production Manager at the new plant and explains that the driving issues these days from the perspective of composite insulator manufacturing include ensuring a high conformity rate as well as low cycle time. Both factors will determine machine and worker productivity. Conformity rate is a good measure of the level of quality control and Liu notes that one problem that can lower this is air becoming trapped in the silicone material as it moves from the stuffer and plasticizing unit into the mold cavity. The function of the plasticizing unit is to render the highly viscous HTV rubber to a state where it can better flow into the mold while also removing any air bubbles that might develop in the process. From the plasticizing unit, the silicone compound is transmitted to a barrel for injection into the cold runner system that ensures the same pressure at each injection nozzle and also optimizes utilization of the silicone material by eliminating wastage.

All Photos: INMR ©

Liu examines test specimens, molded without primer for easy removal from rod, during commissioning of new injection machines. Any rod displacement within mold cavity during molding cycle risks lowering conformity rate. This machine, with 800 mm x 2600 mm platen size, has the mold closing from top down to minimize such problems.

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Another potential quality issue that affects conformity rate, observes Liu, is rod displacement caused if the FRP core with crimped on end fittings shifts slightly from the center of the mold cavity due to excessive machine vibration. “Mold closing by the top section moving down helps avoid this potential problem,” he explains, “since it guarantees that the rod will remain in its desired position before injection. But it is also important to monitor and control temperature and pressure of the injected silicone.”

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Liu alongside stuffer and plasticizing unit that delivers silicone material into mold cavity.

machine was subsequently increased to two based on high expectations for their performance. Wang Shihai, General Manager of Yizumi’s Rubber Machinery Div., points out that the productivity of injection molding machines to manufacture composite insulators has improved greatly over recent years. Original machines, he notes, were mostly adaptations of equipment used to mold other items

All Photos: INMR ©

“As order sizes grow, the ability of our molding equipment to handle large volumes efficiently and with minimal downtime as well as high conformity rate becomes critical.” Wang and Liu examine flashing on batch of test insulators.

Apart from quality control, equipment issues that can adversely impact productivity are hydraulic and mechanical problems, including oil leaks from nozzles onto the mold during routine maintenance. Injection from bottom nozzles is one way to eliminate any risk of such leaks. Says Liu, “as the order sizes we receive from customers grow, the ability of our molding equipment to handle large production volumes with minimal downtime as well as high conformity rates becomes critical. These are therefore the main considerations that influence our

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but often had less than optimal results when used to manufacture insulators. By contrast, the latest generation of such machinery is At the time of the visit by INMR, being custom built to meet today’s management from one of the equipment suppliers – Yizumi Rubber production goals for insulators. These include the capability to easily Machinery – were also in the plant, handle large volumes while also overseeing start-up of new 1100maintaining the highest possible ton and 500-ton molding machines. conformity rate and efficiency. Liu explains that these units were selected specially to allow large According to Wang, the growing quantities of 220 kV line insulators international market for composite to be molded two pieces in a single insulators is changing the production shot or alternatively 500 kV units strategies of many suppliers. For in two shots or 750 kV insulators in example, as he sees it, equipment three injection cycles. He adds that suppliers today also have to develop the original order of one 1100-ton selection process for each molding machine we buy.”

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While most original injection machines used by this industry were adaptations of equipment used to mold other items, the newest generation of such injection molding machinery is being custom built to manufacture large volumes of insulators with the highest possible efficiency. Wang reviews controller operation during machine commissioning.

All Photos: INMR ©

machines that can cope with different formulas of silicone rubber as well as with the requirements of producing items to be used over a wide range of different voltages. “Many insulator manufacturers use HTV (HCR) silicone,” he notes “but each has their formulation of this material with its own physical characteristics, such as hardness. The machine’s plasticizing and injection system, which is based on the normal parameters of silicone, must therefore be able to adapt easily to different materials.” Wang also remarks that MV, HV or UHV solid or hollow core insulators all have special requirements that need individualized solutions. “A molding

HTV silicone rubber mass is dropped into stuffer for plasticizing before being pumped into mold cavity.

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machine today,” he observes, “must be able to maximize production efficiency whether producing the same item or a number of different products in high volumes.” Composite insulator quality is a key factor impacted by an injection machine. Some potential problems in this regard are relatively easy to see, such as excessive flashing, trapped air, shortage of material, uneven color or surface scratching. But other problems are harder to detect by appearance alone, including non-concentricity of the core, rod damage, poor adhesion, uneven curing and scorching. “Not every manufacturer gives sufficient attention to all these possible problems,” observes Wang. “So, it is best to minimize their occurrence by using machines that offer precise control over factors such as pressure, speed and injection volume.” As example, Wang mentions one case where a large manufacturer elsewhere in China used old molding equipment to produce 220 kV insulators in a single shot. After purchasing a latest generation machine, product conformity rate increased by 23% while cycle time fell by 30%. He also points to another case, this time in India, where a new manufacturer began producing 11 kV composite insulators using an adapted 400-ton standard molding machine. Daily

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output was some 900 pieces while the amount of flash and waste for each averaged almost 36 g. After installing a new 360-ton machine, daily output increased to 2300 pieces while waste on each fell to only about 5 g. Wang believes that such examples demonstrate that using specialized machinery that incorporates the latest technology offers a quick return on investment and will be a key driver for insulator manufacturing in the future. Other purchase considerations when it comes to machine selection, says Dalian Insulator’s Liu, include ease of maintenance so that all work can ideally be done at ground level without requiring any climbing or working from a height. This same type of thinking also impacts worker safety and ergonomics. For example, filling the stuffer is a task that can pose some risk and also be physically demanding on the worker who has to continually lift and push up a heavy mass of silicone raw material. Wang explains that new machine designs help overcome this traditional problem by incorporating a stuffer unit that requires twohanded operation, for greater safety, and where the silicone material can be dropped in conveniently from above. Yet another safety feature is a protective curtain to prevent injury to workers. “That kind of design thinking,” he remarks, “helps make a machine more user friendly.”

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Says Wang, “equipment companies such as ours already know silicone rubber and molding quite well. In our case, we have also begun trying to better understand the needs of the insulator industry so that we can adapt this knowledge to meet all their requirements.” Wang and Sales Manager Nancy Liu say that this philosophy has already paid off. Although Yizumi started supplying this market sector only in 2009, they report that the company has already shipped some 150 machines specifically for molding composite insulators. They claim that some of their technology applied to insulator manufacturing has even begun to influence development of other injection machinery for this market. Says Wang, “more technical co-operation within our industry is something we support. It can only benefit all composite insulator manufacturers.” Production Manager Liu seems to agree as he points to new equipment from other suppliers, now also being readied for final commissioning. “I have seen a lot of improvement in the type of molding machinery we buy when it comes to the issues that count most in our factory, such as quality, efficiency and safety. Now,” he adds, “we will need to instruct our workers in all these changes so that they can best benefit from these developments.” Liu goes on to mention that the plan is to use the workers who are involved in these first commissioning steps to provide the machine training to others who will also work in the department.

All Photos: INMR ©

All Photos: INMR ©

Back at the main office, Vice General Manager Yang Luguang, remarks that all the work now being done to start up the new composite insulator plant in an optimized manner will take time. But that he feels will not pose a problem. “It will still be some years before the international market for this technology attains its full potential,” he observes. “When that happens, we will also be ready.” 

Other new molding equipment at new plant also includes 1800-ton and 2400ton clamping machines for hollow core insulators.

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ARRESTERS

Review of Proposed New IEC 60099-4 Energy Handling Tests

Perhaps the most significant change yet in IEC 60099-4 promises to be good news for engineers involved in the specification and application of surge arresters. Not only is there a change in the way an arrester’s energy handling capabilities are tested, but a better way of classifying arresters is now also emerging. INMR columnist and arrester specialist, Jonathan Woodworth, a member of IEC Maintenance Team (TC 37 MT4) that proposed the new tests, has prepared an article that outlines his perspective on the changes as well as how these tests should be interpreted and applied for maximum benefit. Photo: INMR ©

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Introduction It has long been apparent to both suppliers and users of arresters that the line discharge classification system used to quantify an arrester’s energy handling capability has not been ideal. After review by members of IEC TC 37 MT4, the following major issues were identified: 1. Line discharge classification could not be fixed using the present standard and therefore a better system to classify arresters was needed; 2. Existing tests do not provide for a standardized way to establish and verify energy handling capability. As such, arrester manufacturers have had to develop their own methodologies to calculate product data, typically resulting in energy ratings that can vary from one supplier to the next; 3. To perform transient studies of their systems to determine protective needs, arrester users require data that is consistent and realistic; 4. Impulse and thermal withstand characteristics are still tested (indirectly) in the present standard using the same procedures and are not discernible from one another. It has become clear that it would be desirable to modify these tests to provide a means to independently verify an arrester’s thermal withstand and its impulse withstand. Previously, the two were intermingled in the operating duty cycle tests and in the transmission line discharge (TLD) tests. The proposed new document to improve these perceived deficiencies is currently at the CD (Committee Draft) stage, which in turn will be followed by a CDV (CD for voting) and an FDIS (Final Draft International Standard) – whereby experts from across the world can provide their input and comments via their respective IEC National Committees. The 3rd Edition of IEC 60099-4 (expected to be published no sooner that mid 2013) will then contain substantive changes in the area of energy handling capabilities as well as in the means of classifying arresters. For example, to address the desired changes in energy testing, two tests have been developed that separately quantify impulse type surge durability and thermal withstand capability.

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The issues surrounding improved arrester classification have been resolved by adopting an IEEEstyle system whereby arresters are classified as either station class or distribution class. Each has a specific set of tests to pass, which then define arrester class and replace the former line discharge classification.

Important New Definitions

Table 1: Arrester Classification Arrester class

Station

Nominal discharge current

20 kA

10 kA

Switching impulse discharge current

2 kA

1 kA

Qrs (C)

≥ 2.4

≥ 1.2

Wth (kJ/kV)

≥ 10

≥4

Qth (C)

Distribution Class Arresters Arrester class These include arresters intended Nominal discharge current for use on distribution systems, Switching impulse discharge current typically Us ≤ 52 kV, that are meant to protect components primarily Qrs (C) from the effects of lightning. Arrester Wth (kJ/kV) classification is assigned based Qth (C) on the test series applied during type tests. Note: Distribution class arresters can have nominal discharge Thermal Energy Rating (Wth) currents (In) of 2.5 kA; 5 kA or 10 kA. The energy, given in kJ/kV of Ur, which can be dissipated by an Station Class Arresters arrester or arrester section during a These are arresters used at substations thermal recovery test without leading to protect equipment against to thermal runaway. transient overvoltages and typically (but not necessarily) intended for Repetitive Charge Transfer Rating (Qrs) use on systems of Us ≥ 72.5 kV. The The charge, in coulombs (C), in the classification is assigned based on form of a single event that can be the test series applied during the type transferred through an arrester at test. Note: Station class arresters will least 20 times (at intervals that allow have nominal discharge currents of cooling to ambient temperature) 10 kA or 20 kA. without causing mechanical failure or unacceptable degradation of the Thermal Charge Transfer Rating (Qth) metal oxide varistor blocks. This is the charge, in coulombs (C), that can be transferred through an Charge Transfer arrester or section of arrester during a A unit of measure that quantifies the thermal recovery test without causing current flow through an arrester and thermal runaway. is calculated as the integral of the

Distribution 10 kA

5 kA

2.5 kA

≥ 0.4

≥ 0.2

≥ 0.1

1.1

0.7

0.45

current over the time of the surge event (in coulombs).

New Arrester Classifications Classification of an IEC rated arrester will be based on the data provided in Table 1. If an arrester is tested according to the tests in the selected column and passes all levels, it can then be rated at that level. This will replace the previous line discharge (LD) class scheme. With the proposed new system, there is no possibility that e.g. a 10 kA station arrester could be classified as 20 kA, as might have been the case in the previous LD system by increasing arrester discharge voltage. Not only must energy dissipation (kJ/kV) now be at an acceptable level, but the charge transfer (in coulombs) must also be acceptable in order to classify an arrester at the next higher level.

Repetitive Charge Transfer Rating (Qrs)

Example of arrester with energy withstand exceeded.

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Test Rationale This test is applied to all non-gapped MOV type arresters with the only difference between station and distribution class arresters being in the wave shape of the 20 impulses in the rating test. The Qrs test has been designed to test the capability of an arrester to withstand discharges such as lightning or switching surges and, being performed only on disks, does not need a thermal equivalent section.

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Examples of arresters with energy rating exceeded. for the repeated impulse, the residual 3. One 8/20 current surge of voltage at In and Vref are measured. 0.5 kA/cm2 peak current Both are measured again after the test as part of the pass/fail evaluation. The last impulse is intended to stress the disk one more time to make sure It is worth noting that for the that the final Vref or residual voltage first time in arrester testing, the impulse did not lead to any internal arrester’s reference voltage is used in cracks in the material that might determining pass/fail criteria. During otherwise have gone undetected. research and development of this test, it was discovered that small The disk is considered passed if it changes in Vref are an indication of has not exceeded the 5% change limit of Vref and residual voltage at degradation in the varistor material. Therefore a change in Vref of greater In and is not physically damaged. If than 5% is a test sample failure. one sample fails, it is permitted that Similarly, residual voltage is sensitive 10 more samples be tested, but this Test Procedure to degradation and a change in time no failures are allowed. The repetitive charge transfer test residual voltage of greater than 5% is replaces the long-duration current also considered failure. Rating Considerations impulse withstand test whereby The Qrs characteristic will be 3 samples are tested with 18 The repeated impulse is a switching quantified in terms of charge impulses each with no failure surge for station class arresters (coulombs) and not energy permitted – a methodology that with the amplitude set by the dissipation (joules). Charge has yielded little in the way of statistical manufacturer at 110% of the unit’s been chosen as a test basis for information about failure probability. desired charge transfer rating. For the purpose of better comparing The new test, by contrast, will distribution class arresters, an 8/20 different models of MOV arresters. offer higher assurance of arrester impulse is utilized while transmission Energy values can be calculated performance by being applied only line arresters use 200 µs to peak from this information by multiplying to the individual disks that make surge. Each sample is impulsed 20 the charge and the related switching up the full arrester. In the case of times in 10 sets of two impulses impulse protection level. station class arresters, a switching about 1 minute apart. Ample time surge or half sine surge is applied. for cooling is allowed between each For the Qrs test and final rating, it is For arresters used on systems of impulse set to allow the disk to expected that the values should fall 52 kV or less (i.e. distribution class return to ambient temperature. between 0.5 and 25 coulombs. The arresters), a half sine wave of 200 µs rating will be expressed in coulombs is used for the test. At the end of this test sequence and not as a class or level. However and once the disks have returned each class of arrester does have Test Procedure Details to ambient temperature, three final a minimum requirement to meet, The test procedure is written for both tests are applied: as stated in Table 1. The rating is station and distribution class arresters determined as 90% of the repeated with the only difference being in wave 1. Vref, impulse level during the 20 shot shapes and amplitudes. In preparation 2. Residual voltage at In series. Since it is the express goal the of the IEC maintenance team to separate thermal energy and impulse energy handling capabilities, 10 sets of two impulses each are to be applied in succession. This is believed to be an acceptable number that will not drive the disk to a temperature that could damage its materials. Moreover, cooling is allowed between sets to ensure that the result is not a one or two shot rating but rather a rating that can be sustained over many surges during the arrester’s service life.

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New Operating Duty Test For station class arresters (Ur ≥ 54 kV), this will verify the thermal energy rating (Wth) For distribution class arresters (Ur < 54kV), this will verify thermal charge transfer rating (Qth)

be expressed in joules for station class arresters with typical rating of 54 kV or more and coulombs for distribution class arresters rated < 54 kV. Where a station class arrester is rated below 54 kV, it can be tested and rated with a thermal energy rating, i.e. tested in the same manner as those whose ratings are equal to or greater than 54 kV.

The initial Vref and In tests in the sequence are to set the baseline for evaluation after the thermal stresses.

The next two high-current impulses surges are meant to electrically degrade the discs (while a dielectric test has become a separate new test) and the current amplitude of Rationale these impulses is the same as in the This new operating duty test is present operating duty test. This way designed to quantify the energy it will become possible to distinguish The characterization and conditioning dissipation or charge transfer between a thermally prorated section part of the test can be performed on necessary to raise an arrester’s and a dielectrically prorated section the disks in still air. However, they temperature to a level where it is not (which is not the case in the existing can also be tested in a dielectrically stable under operating conditions. arrester standard). pro-rated section to avoid the need For a high voltage arrester, the unit of for subsequent dielectric testing. measure can be joules or coulombs. (The ‘conditioning’ part was removed The fourth and final set of energy For arresters used on distribution inputs to the arrester is the ones because it is really a remnant of old systems, the unit of measure will be used to rate it. Prior to the last set gapped silicon carbide technology coulombs or charge transfer. of impulses, the arrester must be and in any case a CIGRE study has heated to 60°C unless the arrester is shown that 20 impulses have no Again, the use of charge transfer for UHV application (in which case impact on electrical ageing). The eliminates the potential confusion the temperature is determined using thermal recovery part of this test caused by joule ratings with respect another test sequence). As per the must be performed on thermally to residual voltage. As discussed, it is test procedure, energy inputs are as pro-rated sections. A temperature the explicit goal of the developers of follows: sensor must be integrated into the this test to separate the thermal rating sample such that the temperature of an arrester from its impulse rating. Arresters of Ur ≥ 54 kV of the active part can be measured. (for system voltages Us ≥ 72.5 kV) If not dielectrically equivalent, then Test Procedure another test is necessary to qualify This test will become the new Rated energy injection within the dielectrics. (Note: This separate operating duty test. The thermal three minutes by one or more test is not part of this overview) energy/charge transfer rating will long-duration current impulses or by unipolar sine half-wave current impulses or, in case of non-gapped types (NGLAs), by lightning discharge impulses. Arresters of Ur < 54 kV Rated charge transfer within one minute by two lightning current impulses 8/20 μs.

Figure 1: Repetitive charge transfer rating test sequence.

Figure 2: Thermal rating test sequence (new operating duty test).

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Within 100 ms from the application of energy or charge, a voltage equal to the elevated rated voltage (Ur) shall be applied for 10 s. Thereafter, a voltage equal to the elevated continuous operating voltage (Uc) shall be applied for a minimum of 30 minutes to demonstrate thermal stability. The resistive component of current or power dissipation or temperature or any combination of these shall be monitored until the measured value is reduced appreciably (i.e. success), or a thermal runaway condition becomes evident (i.e. failure).

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Table 2: Comparison of the Old and New Energy Rating Levels Old LCD

Required minimum test energy kJ/kV

Corresponding new thermal energy rating Wth kJ/kV

Estimated current at old LD test A

Charge calculated with the same current and duration as old LDC to give the required minimum energy C

Correspond new repetitive charge transfer rating Qrs C

Approximate range of system voltage kV

1

1.0

2

277

0.56

0.4

â&#x20AC;&#x201D;

2

2.1

4

538

1.10

1

up to 300

3

3.3

7

721

1.78

1.6

up to 420

4

5.0

10

962

2.75

2.4

up to 525

5

6.9

14

1118

3.75

3.6

up to 800

The thermal charge transfer or thermal energy rating will be 100% of the sum of the thermal charge transfer or thermal energy rating of these two impulses. Also, the sample must not experience a change in residual voltage at In of more than 5%. Rating System There are minimum rating requirements for station and distribution class arresters, however the actual thermal rating will not be mandated by this test. Station class arrester will have a thermal rating as given by the manufacturer and tested per the above test. Station class arresters will have thermal energy ratings (Wth) from 4 kJ/kV-Ur to 30 kJ/kV-Ur. Distribution class arresters will only have thermal charge ratings and must meet the minimum requirements as per Table 1.

Comparison of Old & New Classification Systems

selecting the arrester with a thermal energy rating (Wth) that is above the system response. The prospective energy that a system will require of an arrester can be determined using transient analysis software, but if that is not available a simplified formula is in IEC 60099-5, as follows:

and is derived based on the assumption that the entire line is charged to a prospective switching surge voltage and is discharged through the arrester at its protective level during twice the travel time of the line (for switching surges). W:

energy in Joules that will be dissipated by the arrester for the given surge level; L: line length; c: speed of light; Z: line surge impedance; Ups: arrester residual voltage at the lower of the two switching impulse currents; Urp: representative maximum switching voltage.

Steep Front Residual Voltage Test This test has new methods required to reduce the potential of misunderstanding the data. Disk Ageing Tests In the past, this test was a procedure that allowed for increasing watts at the end of the test. With the new standard, disk ageing with an upward trending watts loss will no longer be allowed. As a result, there is no required voltage adjustment for ageing in the thermal evaluation tests. 5000 hr Ageing Test of Housings This test is no longer suggested as an alternative to the 1000 hr test. UHV Arresters Requirements and tests for UHV arresters (for highest system voltages Us > 800 kV) are introduced.

Conclusions

With publication of this new standard, industry will have a test Annex K of the new standard shows standard that finally reduces the a detailed comparison of the old and ambiguity of energy rating and energy new classification methods. Table 2 is an example of the potential rating For example, if the calculated energy testing in general. Moreover, arresters having different designs can be and the equivalent rating from the dissipated by an arrester using the properly compared and evaluated. current system and comes directly above formula is 7 kJ/kV, then the from this annex. The old LDC Class 1 desired thermal energy rating (Wth) of is very similar to the new distribution the arrester should be a minimum of Specifiers of arresters can request a specific energy rating and all class arrester and the old LDC 2, 3, 7 kJ/kV. potential suppliers submitting quotes 4 and 5 will become station class will now be offering the same type arresters. Other Tests Changing in the of arrester. Users of arresters can Upcoming Edition of 60099-4 therefore be confident that they are Selecting the Right Station Several important additional changes applying the correct arrester for the Class Arrester Energy Rating application at hand. are coming with Edition 3.0 of With this new system, the required 60099-4 including: energy rating of an arrester can At the same time, arrester be determined by first calculating manufacturers can now run a test Temporary Overvoltage Test the level of energy the system will This is now a mandated test and not that is standardized and meaningful discharge into the arrester and then to both specifiers and users. ď ¸ an option as in the past.

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CABLE ACCESSORIES

Developments in Designs, Materials & Manufacturing of Cable Accessories

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T

o achieve the optimum balance of cost and performance, accessories for MV and HV cables are increasingly being produced using tailor-made polymeric materials as well as specialized manufacturing processes. At the same time, developments in cable systems themselves are also impacting the design of accessories that are being developed for use both now and in the future. This article, a consolidation of contributions by Jack Qin (China) and Jens Lambrecht (Germany) of Wacker, Rainer Röder of Gardy Technology, Gerhard Mais of Vogel Moulds and Machines and Samuel Ansorge of Pfisterer (all from Switzerland), provides an overview of general market trends for various cable accessories as well as key properties required of the elastomeric materials being used to manufacture them.

Overview

Although one still finds a variety of alternative cable technologies in service worldwide – from gas-insulated to oil-filled – cross-linked polyethylene types have begun to dominate new applications since the late 1980s. At the same time, accessories for these XLPE cables have been undergoing considerable development and improvement over that period, especially during the past decade. The first generations of accessories, for example, were liquid-filled while today more ‘environmentally friendly’, dry types are gaining in preference. Other features demanded these days of cable accessories include flexibility and ease of handling. For example, terminations for GIS or transformer applications are normally installed on-site, meaning that the GIS or transformer would then have to be completed and tested there as well – something that is not always convenient. Recently available pluggable terminations eliminate the need for additional on-site testing and also offer advantages in terms of installation – independent from that of the GIS or transformer that need only be equipped with pre-installed sockets. Customers can then decide later in the project whether a cable or overhead line termination is more suitable. Pluggable bushings for overhead line connections as well as more compact pluggable arresters are now also available, while connections can be made in every cable position – whether horizontal, vertical or somewhere in-between. The ‘heart’ of a pluggable connector system is the socket, which must be capable of providing field control and designed to meet the requirements of different applications. While it will be placed into GIS and transformers, the socket operates in oil or SF6 that have different permittivity factors. Therefore the design of the field control elements must be capable of covering a range of different field strengths, depending on application. HV terminations with silicone housings represent a growing cable accessory segment. „

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Future developments when it comes to cables will concentrate on higher voltages, larger cross-sections and new applications. The challenge here is that while cables are already available up to 800 kV, production, transport and handling for such voltages are all very demanding. Achieving increased current capacity will require cable conductor cross-sections to increase from the 2500 mm² that is fast becoming common to 3000 mm² or even more. The trend toward renewable energy sources is also driving a growing need for DC cable links up to 300 kV to connect offshore wind farms to the grid. Accessories for such applications will require different material properties and manufacturers will have to gain the relevant field experience to develop products suited for this emerging market.

Trends in Cable Accessories

Most MV and more and more HV cable installations today are insulated using extruded polyethylene.

Among the key features being demanded today of cable accessories are flexibility in application and ease of handling. Here, the inner aluminum conductor is surrounded by a grading layer made from electrically conductive polyethylene. The main insulating layer and outer conductive grading layer are made of polyethylene, while the outer conductor is covered by an external sleeve. The use of medium and high voltage cables of different lengths requires their connection and integration into substations and overhead lines. It is here is that cable joints and terminations find main application. Whenever a cable is connected to an overhead line or an electrical installation, the outer conductor and grading layer must be removed and the cable insulation covered by a termination. The main tasks of the termination in this regard include:

ƒ Sectional view of typical XLPE cable.

• Grading the electric field at the end of the removed outer grading layer; • Covering the polymeric insulation; • Providing the external creepage needed for the pollution conditions of the service environment. At the MV level, terminations have virtually all been made of polymers such as EPDM and silicone for decades and a similar transition to polymeric housings seems to be the trend at higher voltages as well. Silicone-housed HV terminations offer a number of advantages over porcelain including increased safety, less environmental risk and faster installation. For example, depending on voltage class, the time required to install such terminations has been estimated to be as much as 20 percent less than for porcelain housings. When it comes to MV terminations, there has been steady process of design improvements over the years resulting in more cost-effective products with slimmer profiles. Similar advancements in field grading technologies are expected to result in slimmer HV terminations as well.

Cut away of one design of 35 kV termination (left). Example of typical termination design schematic (right).

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But cables are not connected only to an overhead line and other installation. More often than not, the connection is to another cable such as when a damaged section must be repaired or a branch needs to be made. This can be accomplished using different

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Selected recent applications for terminations

types of joints, whose principal functions include: • Providing proper insulation, especially with respect to the radial electric field; • Grading the electric field at the ends of the cable’s outer grading layer; • Covering the connector while also providing the required mechanical strength.

New, compact and more costeffective solutions can be found in the area of cable joints as well.

compression apparatus on site. There are fewer parts as well, which makes them less costly to produce.

As is the case for cable terminations, the potential for more rapid and less costly installation is now also driving development of pre-molded joints for HV applications. These are increasingly replacing pre-fabricated joints since there are fewer parts and a built-in compressive force is included that eliminates the need for separate

Apart from these types of developments in terminations and joints, a third type of cable accessory – the connector – is now also offering improved and more cost-effective cable solutions. Connectors are used to plug cables into electrical equipment, such as switchgear, transformers, measuring devices, etc. Accurately fitted parts of molded silicone are connected to epoxy resin counterparts to achieve a safe and reliable connection that can be quickly separated whenever necessary. The number of connector designs and added auxiliary equipment is very broad. For example, plug-in arresters can be mounted to ground lightning impulses and this same technology can even be adapted to produce connectors for busbars.

Cut away view (left) of a typical HV termination and schematic of how it grades electric field distribution (top).

Material Requirements & Properties Elasticity, erosion resistance and long-term stability under electrical

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stress are among the most important characteristics required of the materials used to manufacture cable accessories. In the case of joints, reliable electrical interfaces with epoxy fitting parts are guaranteed because of silicone’s pliable nature as well as its inherent ability to generate and maintain lasting mechanical pressure. Example of typical design evolution of 20 kV outdoor termination. First generation (left) consisted of thick walled RTV silicone molded part with over-molded conductive field grading part. Limited mechanical properties of silicone required several sizes to fit all cable cross-sections since it was impossible to slip termination over fitting. Latest designs (right) consist of thin-walled liquid silicone rubber (LSR) and easily slip over prepared cable. Hose-like integrated field grading allows much slimmer design.

In this regard, the superior electrical insulation properties of silicone elastomers, combined with the fact that they resist ageing and feature stable long-term elasticity, is fast making them the material of choice for many cable accessories. For example, in the case of joints, reliable electrical interfaces with epoxy fitting parts are guaranteed because of silicone’s pliable nature as well as its inherent ability to maintain lasting mechanical pressure. The latest generation of dry type cable joints and terminations share a common feature – namely the insulating component including field control elements. This ‘heart’ of every dry type accessory is normally made of silicone and the parts prefabricated and usually tested at the facilities of the manufacturer.

These types of requirements are increasingly being met by specialized grades of silicone elastomers and the use of the right grade for each application is important both in terms of service performance and optimized manufacturing. For example, stateof-the-art field grading for the MV range can be achieved using a silicone with a relative permittivity of about 15. Conductive grades are then used for field grading in HV applications. Similarly, erosion resistant grades of liquid silicone rubber (LSR) or high consistency rubber (HCR) with hardness in the range of 25 to 40 Shore A have been found ideal for cable terminations. Moreover, their high elasticity allows easy assembly without need for additional tools. The most noteworthy benefit of silicone elastomers when it comes to outdoor cable applications such as terminations (especially under polluted conditions) is their intrinsic hydrophobicity. This property is already well understood by the power industry and allows for more economic designs with shorter creepage and less maintenance requirements in comparison with materials such as porcelain.

 Multiple-layer-extruded body of typical silicone MV joint (top). Example of new compact MV joints (bottom) produced as multi-layer silicone with integrated connectors.

Pre-fabricated cable joints for 400 kV „ (top right) are increasingly being replaced by pre-molded joints with integrated stress relief devices (example at left designed for 161 kV).

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Manufacturing Needs

Silicone elastomer joints are generally manufactured either by a variety of molding processes or by multiple-layer-extrusion technologies. Cable accessories for all voltage levels and applications these days are increasingly based on silicone, featuring stress cone inner insulation with non-conductive silicone and then the electrically conductive silicone. When it comes to cable terminations, there is either a direct over-molding of the outer insulation or alternatively use of a hollow core insulator housing. In the past, these types of accessories were manufactured mostly using RTV silicone. However, due to pressures to reduce costs and processing times, use of LSR is now increasing. This material has shown good results from a cost perspective as well as in electrical and mechanical design. For example, bonding between the different silicones – conductive and non-conductive – can be achieved through the material’s intrinsic properties, without need for any external bonding agent or primer. Different manufacturing techniques are used, often depending on voltage level of the accessory. For example, a horizontal mold parting line on the silicone stress cone at MV levels is generally accepted. In such a case, typical manufacturing equipment consists of dosing and mixing units for a two-component LSR in a ratio of 1:1. A standard clamping machine (as also used for MV epoxy parts) is close by. The sleeves to be manufactured have a dedicated manufacturing process while the molds too feature a specialized design. The production area can then be set-up with a special venting system as well as closed loop manufacturing and testing processes. Cable systems required by power utilities are moving toward higher voltages and, with this trend, the requirements from the viewpoint of

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material electrical properties and automated production processes only increase. One particular design of cable termination with stress cone and external insulator, for example, required a more specialized manufacturing set-up since the thickness of the silicone would normally have required a long vulcanization time. But in this case a patented system called AVT was used whereby the two components of the silicone were pre-heated in a heat exchanger and this resulted in a 30 to 50% shorter curing cycle. Any unwanted back grinding was also minimized and this reduced quality control costs as well. In general, as the cable industry receives more and more orders for cable joints and terminations at HV voltages, e.g. up to 500 kV, optimizing production of stress cones that may require up to 80 liters of silicone will become a growing need. Using RTV silicone or even LSR in a non-optimal process could take many hours and it is therefore becoming ever more important to implement automated processes. Their common basic goal is to reduce overall cycle time by integrating each of the individual process steps for the silicone body into one controlled manufacturing process, including: mold preparation; pre-heating; mold filling with LSR; controlled curing; cooling the mold to room temperature; opening the mold; automatic extraction of the core from the silicone body; and cleaning the mold in preparation for the next cycle. In order to achieve high product quality when it comes to large volume HV cable accessories, optimizing process parameters will be critical, meaning controllable inner mold pressure (with no back grinding) and very good material flowability. This way the final accessory will offer the best mechanical and electrical properties as well as the perfect adhesion between the field control part and the insulating silicone.

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T-connector features three layers of special silicone rubber grades: inner conductive layer; insulating layer; and over-molded outer conductive layer. Examples of connectors for 10 kV applications (right).

Pre-molded silicone cable joint (foreground) features silicone insulating body (blue) and electrically conductive silicone outer layer (black). Mounted joint (background).

First generation silicone-housed terminations feature 14 weathersheds whereas an equivalent porcelain insulator would need much longer creepage and some 27 sheds.

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In the case of joints, reliable electrical interfaces with epoxy fitting parts are guaranteed because of silicone’s pliable nature as well as its inherent ability to generate and maintain lasting mechanical pressure. Typical manufacturing schematic for cable joints and stress cones.

Principle of cold shrinkable joint. Pre-fabricated parts are widened and put onto a spiral support. Once support is removed, widened body shrinks down to diameter of cable. This technology requires unique mechanical properties as well as long term elastic behavior and relies on latest generation of silicone elastomer materials.

Summary & Outlook

Applications for cable accessories these days are becoming ever more demanding with respect to mechanical properties as well as the compression set of the materials being used. At the same time, voltage levels are getting higher while overall life cycle costs (including raw materials, production, field assembly and maintenance costs) are all under pressure to be as low as possible. In this type of environment, one type of accessory should ideally be applicable over a range of different cable cross sections. For the future, as cable voltage levels climb, there will be a growing need for further tailor-made solutions to manufacture large silicone-based accessories in the optimal way. In this respect, there will also be a need for ongoing co-operation between the industry that offers cable terminations and joints, manufacturers of the related production equipment and the silicone material suppliers. 

Typical manufacturing set-up and mold for MV accessories made with LSR.

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Complete 3 phase cable joints manufactured of LSR.

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