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16 - 22 September, 2013


20

YEARS

76

INMR Issue 100.indd 76

Q2 2013

UTILITY PRACTICE & EXPERIENCE

2013-05-15 4:16 PM


DDGs & Reference Insulators Help Utility Monitor Pollution Across South Africa

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ew factors more influence performance and life expectancy of insulators than the service conditions where they are installed. Knowing the levels of pollutants that deposit onto insulators during the course of the year is therefore the best way for power engineers to quantify and plan for the stresses that will affect reliability of overhead lines and substations. One utility that has suffered from recurring pollution flashovers over the years is South Africa’s Eskom – among the largest power companies in the world

and alone the generator of 45% of all electrical power on the African continent. With a vast network operating under the effects of coastal and industrial pollution as well as periodic bush fires, Eskom has for decades invested in carefully monitoring its service environment. INMR accompanies Eskom technical specialist, Robert Faraday Watson, on such a data-gathering trip across more than 4000 km of varying landscapes and pollution sources.

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Photos: INMR ©

Watson sprays sides of DDG (top and bottom right) to allow dust along sides to collect in bottom receptacles.

If you receive an email from Robbie Watson, it will invariably end with these words, “Those who measure, know!” Well, for Watson and his colleagues, this is not simply a slick slogan but in fact an expression of the dogma that influences much of what they do in practice.

engineers select the proper insulator creepage needed at substations. And, extrapolating pollution data from one site to the next allows us to understand how pollution is affecting our lines as well.”

Watson remarks that pollution flashovers, such as massive ones that Once a month, for over a decade blacked out much of the Western now, Watson sets out on the same Cape in 2000, have been recurring problems over the years. The only long journey to collect data at way to anticipate such events and selected points scattered across a vast area. Each trip is arduous yet react in a timely manner, he reasons, one that he recognizes provides vital is by constantly monitoring ESDD input to reliable operation of the grid. levels that affect insulators. In the case of Eskom, this is done using a Says Watson, “all the information I far-flung network of substations and gather ends up helping our design

other selected sites where directional dust deposit gauges (DDGs) as well as strings of reference glass insulators are installed. It is Watson’s responsibility to visit these installations across the Western Cape and conduct analyses of what he finds. Insulation specialist, Wallace Vosloo, has worked alongside Watson for over 20 years and was among the team that ‘procedurized’ testing at the Koeberg Insulator Pollution Test Station (KIPTS) for many of the components that Eskom purchases. Apart from insulators, these include all items relying on external

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Collection Tubes Adjustable Guys Collection Jars

Support Column

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DDG installed early in the construction cycle of newly built 765 kV Sterrekus Substation provided input to dimension insulation at this site.

“The information I gather helps design engineers select the creepage needed at substations and also allows us to better understand how pollution is affecting our lines.”

Photos: INMR ©

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insulation, such as CTs, VTs, autoreclosers, arresters and cable accessories. Vosloo notes that while the use of DDGs first originated in England, Eskom perfected their application over the years beginning in the late 1970s and early 1980s. “One of the keys to using them,” he explains, “was being able to make sense of the data gathered. Once this was done, the information proved invaluable in developing the whole philosophy behind IEC 60815.” The standard DDG used by Eskom is comprised of four vertical tubes, each with a large slot of standard dimension cut into its sides and arranged with proper identification to face all four directions on the compass. A removable cylindrical receptacle at the bottom of each tube then collects all wind blown deposits that enter these slots. Before removing containers for

analysis, the sides of the DDG are typically sprayed with water to ensure that whatever particulate matter adheres to them is washed down into the appropriate container and included in the analysis. Watson explains that among the advantages of the latest generation of DDGs used by Eskom is that they do not require an energy source, are relatively simple and cheap to install and are made from rugged plastic components. This makes them not only durable but also less likely a target for theft, as occurred in the past with earlier metallic versions. He and Vosloo note that a DDG’s efficiency in collecting dust is greatest for relatively coarse particles and correspondingly less for very fine particles that the air stream typically sweeps around the gauge. Nevertheless, they also point out that this same physical process is inherent in dust deposition on

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Receptacles facing different directions collect varying amounts of particulates, allowing prevailing wind direction and sources of pollution deposition to be identified.

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Wind blown contaminants, such as biological matter at 400 kV Aurora Substation, can settle onto sides of DDG slots and must be washed down by spraying before analysis.

any objects in the path of wind and therefore offers a valid simulation of how contamination deposits on insulator surfaces. Not surprisingly, the volume of material collected in each receptacle depends mainly on wind direction and is obviously greatest when it blows directly into the slot and lowest when it comes from a perpendicular angle. This means that the DDG is also effective in indicating from where contamination most impacts a particular substation and thereby helps identify local pollution sources of greatest concern. Conversely, wind tunnel tests to see if contaminants could be blown out of a DDG confirmed that particles larger than 300 µm diameter remained unaffected, even at windspeeds up to 90 km/ hr. Moreover, very small particles decreased only marginally with no significant impact in measuring net pollution levels – especially if one considers that such high winds

Photos: INMR ©

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will clean insulators of these tiny particles as well. Watson reports that he conducts his analysis of each DDG by adding

The DDG indicates from which direction contamination most impacts a substation and also identifies the specific pollution sources of greatest concern.

enough de-mineralized water (i.e. less than 5 µS/cm) to make up a solution totalling 500 ml in volume, allowing for any water that may already be in the container from rainfall. After shaking to disolve the contents of each receptacle, he uses a portable meter to measure resulting conductivity, which varies directly according to volume and nature of pollutants inside. The resulting Pollution Index established by the DDG is then taken as the mean of the reading from all four directions, normalized over the 30day period between readings. Another application of the DDG is filtering and weighing the resulting solution to assess levels of nonsoluble pollutants (NSDD). The second aspect of Watson’s regular visits to test sites across the Western Cape involves taking readings of pollution accumulating on the surfaces of reference insulator strings composed of 7 standard glass shells. Each disc in the string is analyzed at different time intervals

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with the top and bottom units functioning only as ‘dummies’. Measurement begins with Watson wrapping the metal cap of the disc with tape to prevent its deposited contamination from impacting the reading on the actual insulation surface. The glass is then washed thoroughly in a tub of de-mineralized water of constant one-liter volume before a meter reads conductivity of the resulting solution. Temperature of the water is also recorded to correct conductance values to a standard 20°C. Knowing the area of the glass disc (in this case 583.9 cm2 at the top and 704 cm2 at the bottom), associated ESDD values can be calculated for each. Readings are then averaged out and charted over time for every disc sampled from the string, along with information on rainfall to allow a more complete picture of the real pollution scenario. Watson reports that ESDD readings for each disc typically vary by location in the string, with discs in certain positions more likely to accumulate pollution due to aerodynamic effects. Data is then sent to Eskom’s planning department

Photos: INMR ©

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Conductivity measurements made after fixed volume of de-mineralized water added to contents of DDG containers.

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Top disc: Dummy – never measured Second disc: Sampled every two years Third disc: Sampled once a year Fourth disc: Sampled at 6 month intervals Fifth disc: Sampled at 3 month intervals Sixth disc: Sampled monthly Bottom disc: Dummy

Table 1: Link Between Pollution Index & Pollution Class Pollution Index (µS/cm)

Pollution Class

0 – 75

I: Light

76 – 200

II: Medium

201 – 350

III: Heavy

> 350

IV: Very Heavy

as well as to the individuals responsible for each substation as well as their supervisors.

also equipped with a full-scale meterological station enclosed in a shipping container, provides a good example that unpredictable events One of the DDG and reference can sometimes occur, such as a string sites that form part of herd of passing cows knocking into Watson’s monthly ‘circuit’ is 400 kV and tipping the DDG. Aurora Substation, located several hours drive north of Cape Town. Already about 40 years in service, the site is surrounded by mostly barren land but still classified as polluted due to nearby steel mills as well as the coastline. Photos: INMR ©

Not every DDG and reference string site is at a substation. For example, these are also installed at a place called Haksteenpan, in the Kalahari Desert bordering Namibia. The area is mostly wilderness with no major substations nearby but is still considered important to monitor from a pollution standpoint since a new transmission line is planned to pass nearby. Watson points out that the specific location selected for the DDG in this case is on the crest of a hill which feels the full impact of dust blown from off a nearby dried salt lake bed.

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Steps in measuring ESDD level on different discs in reference string.

Another similar test location is outside the northern city of Uppington, where a new solar generation facility will be built using concentrated sunlight to drive turbines. The site, which was

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Reference string at cement plant shows impact of localized very heavy pollution.

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Watson remarks that, based on years of conducting these ESDD measurements, he has noticed surface contaminants become progressively more difficult to remove before replacing each washed disc for future measurement. He attributes this to a thin film of surface pollution that builds up and somehow becomes embedded in the glass.

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Watson checks rain gauge installed at test site (top photo).

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DDG test sites in locations where future network expansion planned (right and bottom photo).

Photos: INMR Š

Vosloo refers to all the data being gathered by Watson as well as to tests conducted for years at KIPTS and notes that both allow Eskom engineers to better understand pollution and how it impacts their network. Says Vosloo, “we believe that doing natural ageing is necessary to complement whatever testing is being done at laboratories. Only by monitoring insulators in the field do we get the true story of how they react to the natural environment. This 86 INMR Issue 100.indd 86

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400 kV Aurora Substation. AVERAGE NORMALIZED CONDUCTIVITY (DDG)

Pollution Measurements at Aurora Substation December 2000 ­– February 2013

ESDD ON DISC 1

RAINFALL

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Photos: INMR ©

(left) Sampled disc prior to washing to establish ESDD level. Discs after washing still show evidence of embedded contamination that cannot be removed.

allows us to adjust lab tests in such a way that they better simulate what is really happening in service.” Another element in this process of monitoring and reacting to pollution, he notes, is using test towers, such as the ones currently in operation in South Africa and nearby Namibia. These strain towers allow up to 9 test positions for different insulator designs, over and above the ones already installed on the line. An online monitoring system and weather station are also installed (as per CIGRE Guide 333) and together provide data at 10 minute intervals on leakage currents, wind speed and direction, humidity, solar radiation and precipitation.

Vosloo adds that the data being gathered and compiled by Watson also helps identify which substations are ideal for installation of sophisticated new Eskom-developed insulator pollution montoring apparatus (IPMA). This advanced equipment energizes naturally polluted test insulators every 10 minutes to measure resistance and also calculate ESDD. There is apparently also a leakage current recorder that identifies the worst performing insulator profiles at that location. All this, explains Vosloo, provides complete documentation on what is happening at the site from a pollution and climate perspective.

Finally, Vosloo points to the 765 kV Sterrekus Substation nearing completion north of Cape Town as a wonderful example of how pollution mapping using DDGs and reference strings has contributed toward Eskom’s design process. “The station’s 31 mm/kV insulation was basically decided on as a direct result of the work Watson carries out,” he says. “It may seem simple at first glance but ends up playing a significant role in the big picture.” 

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Insulation levels at 765 kV Sterrekus Substation were determined in large part by local DDG and reference string measurements.

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DDGs & Reference Insulators Help Utility Monitor Pollution Across South Africa