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Bushings Technology Review: Current Designs & Future Tendencies


Bushings Technology Review: Current Designs & Future Tendencies recent contribution by Professor Stanislaw Gubanski of Chalmers University of Technology in Gothenburg, Sweden, discusses alternative bushing designs. It also looks to future trends as determined by changing market needs and competitive factors.

Damage to 500 kV bushing core from very fast transients.

Bushings are devices that allow high voltage conductors to pass through the earthed walls of transformers, switchgear and substation structures. An integral part of this function involves meeting all the electrical, thermal and mechanical requirements of the application. For example, bushings must provide reliable electrical insulation both internally (against breakdown) and externally (against flashover) of the conductor exposed to the rated voltage and also to periodic service overvoltages – even under contaminated conditions. Another key requirement is providing the mechanical strength needed to support the conductor as well as all external connections, including under short circuit and possible seismic forces. Moreover, the bushing must have the proper thermal design to avoid any overheating of its elements and to prevent the onset of ageing phenomena inside the insulation, both at rated current and during short circuit events. The following technology review, taken from past INMR issues and also incorporating a


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Fires triggered by explosions of oil bushings have been a serious problem in some countries, as in this photo from Brazil.


The bushing is in many ways similar to an insulator or surge arrester in that it is a comparatively low cost component ensuring the safe operation of a comparatively high value asset. For example, while bushings account for less than 5 percent of the cost of a typical power transformer, their catastrophic failure can lead to total loss of the transformer and possibly other expensive apparatus as well. However, in contrast to insulators that contain no potentially explosive internal medium, certain types of bushings can pose a threat not only

Explosion of bushing at this substation in New Zealand saw shards of porcelain pierce the roofs of nearby homes.

to the substation and its personnel but also to the safety of nearby communities. For example, the catastrophic failure of a 110 kV oil-impregnated paper bushing at a substation in New Zealand during the 1980s was so violent that shards of porcelain as well as oil and debris, were launched into homes in an adjoining residential area. Fortunately, occurrences such as this – while dramatic – are also very rare. In fact, the bushing is a ‘workhorse’ that typically ranks among the most reliable components in any modern power network.

- - - Porcelain shrapnel caused damage inside this area  Hole punctured in roof  Bushing which failed Catastrophic failure mode of porcelain housed OIP bushing (bottom)

Figure 1: Comparison of electric field distribution in a bushing without (top) and with field controlling capacitive screens (bottom).

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Modern production of OIP bushings is increasingly automated.

solid insulation of the bushing, essentially forming a system of in-series connected capacitors whose magnitude depends on their geometrical arrangement. Perhaps the most frequently used and effective solution is when series capacitances are maintained at equal levels. The impact of modifying field distribution in this way is illustrated in Figure 1. Inserting metallic screens during manufacture of a bushing can be demanding and at times laborintensive, although modern condenser core winding equipment has made this task increasingly automated. In the case of paper insulated bushings,

The pronounced market preference for OIP style bushings has been maintained by means of a series of subtle refinements made by the leading manufacturers. The basic principle in all bushing design is relatively straightforward: it consists of a cylindrical conductor surrounded by an insulating solid cylinder that is mechanically fixed to the earthed barrier. The distribution of electric field inside such a construction, however, is highly nonuniform in terms of both axial and radial components. The highest stress concentration appears at the so-called ‘triple junction’ between the earthed wall, the insulating cylinder and the gaseous or liquid medium outside the bushing body. This localized high concentration of stress can trigger the onset of partial discharges. These discharges are often referred to as ‘gliding discharges’ since they have a strong capacitive coupling to the bushing’s internal conductor and therefore proceed along the insulating cylinder’s surface. They can lead to tracking along the bushing and even result in flashover.

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Initiation of gliding discharges as well as their subsequent development becomes easier when the unit capacitance of the insulation (i.e. across its thickness) is greater. Therefore, the voltage level for their ignition and propagation (virtually equal to flashover voltage) is determined by this parameter. This stands in contrast to other types of discharges, where the typical controlling parameter is electrode separation distance. Because of such considerations, the best way to increase a bushing’s flashover withstand voltage is by improving the electric field distribution along its surface. This can be achieved in a number of ways although, in the case of higher voltage levels, the most effective means is through capacitive control for AC applications and resistive control for DC applications. Capacitive control is based on inserting metallic screens into the

metallic foils are inserted between the different paper layers. Choosing the appropriate radius and length of these screens then allows for the series capacitance desired. Optimal resistive control of electric field distribution in the case of DC bushings usually involves covering the critical region near the electrode with semi-conducting layers. The aim here is to increase resistance with increasing distance from the earthed electrode.

Alternative Bushing Technologies

In the case of bushings for higher system voltages, there are three principal types of insulation systems currently used throughout the world: oil-impregnated paper (OIP), resin bonded paper (RBP) and resinimpregnated paper (RIP). New systems, without any oil, have now also been developed for use at lower transmission voltages.

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Silicone-housed RIP bushings typically attract attention whenever they are displayed.

Oil Impregnated Paper Bushings Since the main insulation of substation transformers has traditionally been based on oilimpregnated paper (OIP), this same insulation philosophy has carried over and become the most commonly used technology for constructing bushings. In fact, on a global basis at least three out of every four installations of a bushing on a power transformer involve an OIP design. In certain large markets, such as China and the United States, this proportion is probably even higher. This fact is no small achievement for any such long existing technology, especially given the advances made in power system components over the years.

suppliers to offer standardized high creepage porcelains on all OIP bushings they sell. The goal here was to allow the same bushing design to be used across a variety of service environments and thereby reduce the need for end users to stock many different styles of replacement units. More important still, it has allowed bushing manufacturers to streamline their ordering and inventorying of porcelain and reduce both costs and production lead times.

Similarly, over the years OIP bushing suppliers have made design changes aimed at reducing the diameters of the porcelain housings to make them slimmer and also lighter. This has particularly suited the interests of transformer manufacturers who prefer bushings that are easier to handle, both in transport and during installation. While it can fairly be said that OIP bushings once used to be ‘built like battleships’, everything possible has been done since then to make them smaller and neater. Slimmer profiles have offered additional benefits apart from reduced weight: firstly, the porcelains themselves carry a lower cost since decreased diameter can significantly reduce purchase price; secondly, due to slimmer designs, the volume of oil within the bushings could be correspondingly reduced. With this has come progressively reduced concern among users about the perceived risks of oil leaks and fires.

The strong market preference for OIP style bushings has been maintained over the years by means of a variety of refinements made by some of the leading manufacturers. These improvements have allowed this technology to remain attractive to both intermediate bushing customers (i.e. the transformer OEMs) and also to the final users, in spite of their changing needs and demands. For example, for more than a decade now there has been an effort by

Dry RIP bushings (forefront) offer benefits yet have achieved much lower market penetration compared to OIP styles (background).

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Apart from these major design changes on an industry-wide basis, various individual OIP bushing suppliers have added additional improvements in such as areas as better methods of sealing against leaks; designs to facilitate both horizontal and vertical mounting; increased ease of visually monitoring oil levels; easier interchangeability between transformer and switchgear applications; and better mechanical contacts between top terminal and conductor to avoid the potential for heating should the conventional threaded contact become weakened over time. Other refinements have also been developed which allow OIP bushings to be changed out quickly in the field with only minimal impact on the operation of the affected apparatus.

Resin Bonded Paper Bushings Manufacturing RBP bushings has been based on winding layers of resincoated paper around the conductor under heat and pressure to essentially laminate the layers together. This process, however, is inherently difficult to control and therefore such designs have suffered relatively large numbers of failures over the years due to voids and other defects. In spite of this drawback, the RBP bushing style has still found a ready market because of its very competitive price. Presently, use is limited mainly to lower voltage levels since the risk of thermal instability and even runaway due to dielectric losses in the paper is comparatively high. For this same

have over the years also sought to incorporate product refinements to justify the higher price normally associated with this technology. Among the most important developments in this regard has been the application of silicone housings in place of the porcelain that still dominates the vast majority of all OIP bushing designs. Indeed, this is a technical solution which some in the industry have come to view as the ultimate evolution in bushing design and performance. There is no doubt that market acceptance of silicone housings on a bushing has been far greater for RIP than for OIP designs to the extent that more than 90 per cent of all graded

In contrast to high voltage AC applications, the performance of an HVDC bushing is influenced by the resistive properties of its materials and therefore stress control must be appropriately adjusted.

One of the areas within OIP bushing technology where some in the industry feel that there could be growing interest for further development is by incorporating into them better monitoring systems. For example, given the fact that there are hundreds of thousands of such units operating all over the world, one question still to be answered is how intelligent such a bushing design should be in the future, especially in regard to internal monitoring of oil level and other critical service parameters.

reason, operating radial stress in such designs is maintained at a lower level than for other bushing types, usually around 2 kV/mm.

Resin Impregnated Paper Bushings Compared with RBP bushings, significant improvements have been achieved by introducing a technology where the paper insulation is impregnated with epoxy resin and then cured. The resulting insulation system, containing field grading elements, is dry and void free – though great care has to be taken Given the continual refinements to OIP during the curing cycle to avoid bushing designs over the years, one can internal stresses and formation of wonder whether there is any more that cracks, especially as the volumes of can be done to enhance performance material increase for higher voltages. and functionality or to further reduce The typical radial operating stress in cost. Indeed, perhaps this style of RIP bushings is around 3 kV/mm. bushing has today reached a state of design maturity with very little room left As has been the case with OIP for additional optimization. technology, suppliers of RIP bushings

bushings that are silicone-housed have RIP cores. This is because the real advantages of silicone are most evident and accepted by customers when applied to this technology and in some respects even help RIP better compete against OIP styles. Germany, Switzerland and Austria are examples of markets where dry bushing technology incorporating silicone housings has already become widely accepted, mainly due to safety and environmental concerns. The motivation behind the transition to silicone in place of porcelain as a housing for bushings has typically involved such factors as reduced risk to people and apparatus, better pollution performance, easier handling and faster production lead times. For example, there are a lot of applications where a bushing becomes especially vulnerable to pollution,

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such as when it accumulates uneven pollution deposition and becomes at risk of flashover under wetting. In such cases, silicone is often preferred over porcelain. However, one of the biggest challenges in replacing porcelain with silicone on an RIP bushing has been higher cost, especially at voltages of less than 245 kV and where much of the business volume is focused. For RIP units, slimmer diameters compared to most OIP designs have also meant that the porcelain shells being replaced are less costly, only accentuating this difference.

750 kV gas-insulated switchgear bushings.

An early problem when it came to changeover of external insulation away from porcelain was that the new silicone insulators were often specified as a one-for-one replacement for porcelain. This meant that the larger flanges required for porcelain were often also specified for the composite alternative, even though not strictly necessary. Fortunately, this has become less of an issue in recent years since both bushing suppliers and users better understand the need to optimize the entire design. Moreover, a growing level of standardization in this industry has resulted in the sizes and diameters of fittings becoming more uniform. Among the more noteworthy developments in recent years when it comes to the application of silicone housings to RIP bushings has been the introduction of a process which eliminates the classical FRP tube and instead involves molding sheds directly onto the epoxy core. This technology has been available now for almost 20 years and is still seen as most suited for bushings up to a certain maximum voltage.

GIS bushings at 1000 kV substation in China.

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The principal advantage of a direct mold process versus a ‘stand alone’ hollow core composite insulator is potential savings of as much as 5 to 10 per cent of the bushing’s total cost – a considerable inducement in a market as sensitive to price as bushings. This is because direct molding eliminates the need for the costly FRP tube as well as the

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dielectric material that fills the space between the core and the inside of the tube. Notwithstanding these cost advantages, however, the application of direct molding onto RIP cores has still not achieved the widespread use once expected. Apart from the large investment needed by the bushing manufacturer to be able to implement this process in-house, there may also be technical issues to solve, especially at the higher voltages. For example, there must be a very good chemical bond between the core and the silicone so as to avoid any possibility of interface problems. There is also the issue of ‘cold switchon behavior’. When silicone material has been molded directly onto the RIP core, some experts point out that vapor will likely have migrated through the silicone before energization and could become absorbed by the resin body. This may mean that, at least initially, the bushing will have a higher dissipation factor. Yet another potential drawback of direct molding of sheds versus use of a full scale hollow core composite insulator relates to mechanical function. If the mechanical requirements of the bushing exceed those of its RIP core and conductor, the additional mechanical strength of a tube will be necessary to help

carry the load. A tube also offers the Whatever practical refinements are additional benefit of providing a barrier still to be made to RIP bushing against moisture. technology in the future will likely be in such areas as better grading In this regard, it may not be of the condenser layers – making appropriate to draw a parallel them progressively smaller and more between mold-on silicone bushings efficient. There may also be a need for and experience so far with polymeric greater process control to improve the arresters which are now also seeing robustness of manufacturing making growing use of direct molding it a more-controlled and repeatable technology (see page 86). Arresters process, resulting in a consistent are not intended to last 30 to 50 product every time. years and even if they fail, it is often not too serious a problem. If a In the end, a purely technical bushing fails, the whole transformer comparison between OIP and RIP below is in trouble. bushing technologies may not be the deciding factor in customer Apart from external insulation, selection. Given the huge impact there do not appear to be any major of commercial considerations in developments in the way RIP bushing all purchase decisions these days, cores are produced that might reduce success in the future will go to cost. Drying and curing cycles are the those bushing suppliers who offer most critical steps and apparently the most features for the price, cannot be easily shortened. Nor has irrespective of technology. These will the resin that forms the body of the include creepage distance, seismic RIP bushing changed significantly capability, cantilever strength and either in composition or cost. Indeed, total interchangeability for application rather than looking at changes in on transformers or breakers, among the resin body itself, most efforts others. by suppliers have been devoted to improving production logistics. The Similarly, not everyone in the industry entire process for manufacturing these – even among firms offering both OIP bushings is technically demanding, and RIP technologies – is convinced particularly as voltage levels climb that RIP is necessarily always the and this has limited the number of better choice technically. For example, qualified manufacturers at the very one supplier points out that RIP high voltages. technology actually has disadvantages that must also be considered, from

GIS bushing applications in Korea (left photo) and Czech Republic.

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higher cost to greater uncertainty about how the core is ageing over time. This is because, unlike the case for OIP styles, reliable ageing analysis on an RIP bushing cannot be performed in the field. Rather, the unit has to be removed from service and returned to a laboratory for testing. Indeed, given today’s power utility environment, there will be growing concern about how bushings are ageing. For this reason, some predict that the tendency for new bushing types in the future may be more oriented toward gas-filled or gas-impregnated units in place of those relying on organic dielectrics. Other Designs & Considerations Indeed, in contrast to the longstanding OIP, RBP and RIP bushing technologies discussed above, new designs today are making use of pressurized gas, predominantly SF6, as the internal insulation. Metallic screens are then used for controlling electric field stress inside the bushing body. As already discussed, selection of the external housing of the main insulation in a bushing is one of the key factors affecting both service performance and cost. For indoor applications

Bushing applications at Âą 800 kV converter station.

with low contamination and normal humidity, resin-based RIP solutions do not require any additional housing over the epoxy core. This is not the case, however, for either OIP or gasfilled bushings, both of which require either porcelain shells or composite insulators. For safety considerations, the application of the composite solution is often preferred when considering any gas-insulated unit under high internal pressure. Another way to categorize the various bushing technologies available today relates not to the commonalities

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in their design and construction but rather to their main areas of application or applicable voltage levels. For example, probably the most common application of bushings is on power transformers, where the outer parts operate in air, other gas or oil. OIP bushings are currently available up to 1200 kV and higher, while capacitance-graded bushings with epoxy resin impregnated insulation have now been developed up to 1000 kV.

influenced by the resistive properties of its materials and therefore stress control must be appropriately adjusted. For example, it is of vital importance to control the field in the oil part of an OIP HVDC bushing since the ratio between the resistivity of paper and that of oil can be as high as 104.

Use of composite insulators in the design of bushing shells for the highest voltage levels provides not only superior flashover performance Apart from UHV, another specialty but also very high mechanical application includes high current withstand. Another issue relates to bushings (with operating currents up the dynamics of charging under DC to 40 kA) used on the low voltage side and remains an important factor to of transformers and also in generators. take into account, not only during One of the key design requirements in testing but also in terms of impact this case is the capability to effectively on flashover performance, especially dissipate heat. during voltage reversals. Yet another factor to consider in these types of Another broad group of bushing UHV bushing applications relates applications involves connections to to better understanding ageing of gas-insulated switchgear (GIS), most polymeric materials in such an typically as entrance bushings. RIP environment. based condenser cores embedded inside a porcelain housing or The tendency towards increased composite tubes with silicone transmission voltage levels and the sheds can both function well in this resulting growth in the dimensions of regard. For the gas part, a discharge bushings has also made it necessary resistant surface varnish is necessary to better control parameters such as to fulfill the requirement of being seismic behavior. Historically, several resistant to corrosive by-products methods have been used to verify from decomposition of the SF6 gas. seismic capabilities of bushings. In addition, requirements for direct These involve static calculations to connections between transformers and estimate the forces generated during GIS are becoming more common and a seismic event with a given ground special designs of oil to gas bushings acceleration and then comparing have been developed for this purpose. this against the design capabilities of the equipment. Although IEEE Yet another group of bushing 693 and IEC 1463 standards, applications – and one that is now involving shake tables, have been growing rapidly in countries such as used to qualify equipment for seismic China, India, Brazil and South Africa, areas, past earthquakes indicate among others – relates to their use in that transformers and bushings that HVDC converter stations. Nowadays, passed these tests have in some cases manufacturers offer special bushings still sustained significant damage. To for connecting to HVDC transformers, overcome this deficiency, numerical reactors or air-insulated system simulations have been introduced as components up to ± 800 kV DC. a substitute and, depending on future These can operate in a transformer experience, may need to be further oil environment, indoors or for any refined. outdoor connections. As discussed, in contrast to high voltage AC applications, the performance of an HVDC bushing is


The development in bushing technologies over recent years has been quite impressive, if not always

obvious at first glance. Bushings today might look very much like those of the past but, upon closer examination, there are many subtle refinements and improvements that have been made from the viewpoints of scale, functionality, performance and cost. Given the huge impact of commercial considerations in all bushing purchase decisions these days, success in the future may well go to those suppliers who offer the most features for the price, irrespective of technology. Among the driving forces behind this progress have been ongoing efforts by the industry to reduce costs as well as production lead times and to standardize bushings as much as possible to reduce the need for users to stock many different types of spares. Moreover, internal competition, especially between OIP and RIP styles, has also pushed suppliers to seek optimization in both designs, wherever possible. Another factor driving development of bushing technology, especially in recent years, has included growing use of HVDC based transmission as well as increases in UHV AC voltage levels. At the same time, environmental demands for developing oil free as well as low or SF6 free high voltage substations are creating new design challenges for the industry. Today, manufacturers have developed and already offer bushings for voltage levels exceeding 1000 kV and for very high rated currents. On the HVDC side, work on developing ± 1000 kV bushings is now in progress. RIP bushing technology seems to be well understood, allowing manufacture of partial discharge-free condenser bodies for even extremely large units while increased use of new gas compositions (e.g. N2/SF6) in GIL will require elaborating new design criteria. Finally, the growing application of silicone housings in bushing designs has created the need to further study their long-term behavior, especially under combined DC voltage, thermal and mechanical stresses. 

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Bushing technology review: Current Designs & Future Tendencies