Resolving problems from poor insulator performance in desert environments by INMR

UTILITY PRACTICE & EXPERIENCE

Resolving Problems from Poor Insulator Performance in Desert Environments

UTILITY PRACTICE & EXPERIENCE

Resolving Problems from Poor Insulator Performance in Desert Environments

YEARS

INMR Issue 101.indd 64

Q3 2013

(Part 1 of 2)

2013-08-20 6:00 PM

ore than a decade ago, Brian Wareing, an insulation specialist, was invited to give a course on overhead lines in Oman. Among the first issues he faced was helping the local power company better understand the cause and remedy for pole top fires, one of the key problems confronting power network operators not only in that country but across the

Gulf region. Over the following years, the topic soon became the focus of his work there. This article reviews investigations he conducted into poorly performing line insulators and other insulation in the demanding Omani desert environment â&#x20AC;&#x201C; a leading contributor not only to pole fires but also to other problems that reduce system reliability.

Photo: INMR ÂŠ

INMR Issue 101.indd 65

2013-08-20 6:00 PM

Photo: Brian Wareing ©

Fig. 1: Initial stages of pole-top fire.

Fig. 2: Pole-top fire has died out, leaving crossarm bolt exposed.

Mechanism of Pole Top Fires

Since pollution levels are not uniform on each insulator, the cross-arm itself soon sees a rise in voltage as the current tries to reach earth by going down either the pole or the stay wire. Given the dry atmosphere, the pole’s cross-arm bolts tend to loosen due to pole shrinkage and thereby allow the arcing to occur between bolt and cross-arm strut. This can then ignite the pole’s ‘heartwood’ and a pole-top fire ensues (as in Fig. 1).

Photo: Brian Wareing ©

ole failures are generally triggered by excessive leakage currents on polluted insulators that have become wetted. The main pollution source in the case of deserts is windblown calcium and quartz particles found in sand. Dampness comes from an increase in overnight humidity – especially during summer months – leading to formation of dew. Around dawn, as surface moisture starts to evaporate, insulators are left with dry as well as wet patches along their surfaces. Given the service voltage, the wet patches can be at different potential so that surface currents start to flow between them, commonly visible at night as blue flashes and referred to as dry band arcing.

Fig. 4: Pole debris after fire.

Fig. 3: Pole completely burnt away.

Sometimes these fires die out, in which case the cross-arm bolt is often left exposed at the center of a hollow burnt-out space. However, as the day heats up (commonly to over 50°C during summer), the pole can smoulder all the way down to its base and leave the cross-arms hanging from the conductor, suspended in air.

One fault inspection report by a local utility, for example, revealed that pollution flashovers on the network usually occur in the early morning between the end of August and September when temperatures are circa 20°C and humidity is greater than 92%. Existing contamination levels on insulator sheds during these conditions have been known to yield ESDD values of as much as 0.1 mg/cm² (classified as very severe).

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Even if washing could remove 95% of the pollution layer on a porcelain insulator’s surface, the remaining 5% would be sufficient to provide a conductive path that could eventually lead to a flashover fault.

Photo: Brian Wareing ©

Fig. 5: Polluted glass anti-fog insulators.

Fig. 6: Polluted porcelain insulator.

Indeed, the pollution layer is typically key to the entire process. Insulators operating in hostile desert environments soon become covered by sand and salt pollution. Damage from UV and other degradation factors such as temperature, atmospheric pollutants (e.g. SO2), abrasion, etc. only make the situation worse by working to trap the pollutants on the surface. While during the day, leakage currents over polluted or degraded insulators will typically be less than 5 mA, at night, when there is dew, the wetted pollution layer results in much lower resistance values for insulators compared to when new. For example, tests in Egypt under such conditions showed resistance values of around 180 kΩ on polluted polymeric insulators, which would allow surface leakage currents of over 100 mA. It is for this reason that porcelain and glass insulators must be washed regularly to prevent problems with contamination flashovers. However, in the Omani desert, live washing is often not that efficient since the insulator glaze, which normally should be smooth enough to allow pollution to be washed off by the jets of water, is often damaged by sand abrasion. Instead, there is a rough surface that promotes adherence by pollutants. Indeed, even if washing could remove 95% of the pollution layer on a porcelain insulator’s surface, the remaining 5% would be sufficient to provide a conductive path that could eventually lead to a flashover fault.

Photo: Brian Wareing ©

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Fig. 8: Severe tracking on EPDM sheathed arrester in Oman.

Degradation Processes Affecting Polymeric Insulators Data from Israel indicate that surface leakage currents of 25 to 150 mA combined with a relative humidity (RH) of more than 75% would together result in a medium risk of flashover and phase-to-earth or phase-to-phase faults. Indeed, insulators recovered after such incidents were found to offer resistance values of circa 1MΩ and have surface leakage currents of 20 mA or less. At these levels, the currents have been known to etch the surface and release a white, chalky material that can be seen from the ground. When this occurs at a relatively insignificant level, the chalky layer can easily be wiped off by hand. More severe levels, however, result in the presence of light and dark bands under the sheds (as in Fig. 7) and leave the insulators more vulnerable to dry band arcing. Eventually, the surface can even degrade to the point that arcs travel in only one specific area rather than over the entire insulator surface. This leads to tracking, usually seen as black treelike marks, and is typically experienced on certain EPDM-housed arresters operating on desert networks (see Fig. 8). These marks can allow even higher currents and eventually lead to an ‘avalanche’ or runaway of electrons that becomes a full-blown arc of hundreds and even thousands of amps, with a subsequent line fault.

Since poorly performing polluted insulators are the main cause of pole fires, one of the affected power utilities conducted an inspection of insulators on its 33 kV and 132 kV lines. It was discovered that, despite frequent washing of glass and porcelain insulators, sometimes as often as once a month, pole problems were a continuing issue, especially in a southern area known as Nimr, where sea fog can lead to flashovers for the same reason as overnight dew formation. The utility in question looked to find three solutions – short, medium and long-term, the last of which involved use of spun concrete poles. But since it would still be some time before such a product could be tested and a local factory built (as eventually happened), the medium term solution was to replace all glass and porcelain insulators with polymeric types. The short-term solution, to be immediately implemented, involved targeted replacement of ceramic insulators, improved bonding of staywires and more focused washing in problem areas.

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While solving the problem of pole failures was the major concern, the assumption that specifying silicone insulators would remedy the situation was not regarded as necessarily true. One of the challenges as regards these short and medium-term solutions was that little was known about the ideal characteristics required of polymeric insulators in terms of material and optimal specific creepage distance. For example, the way such insulation reacts to pollution can vary a lot from one material to the next. While silicone relies on its superior hydrophobic properties, it is the actual pollution event, not the recovery, that is important in the case of insulators operating in a desert network. During periods of damp pollution, it was felt that silicone could lose hydrophobicity and allow high leakage currents that could trigger pole fires and flashovers. At the same time, there are numerous formulations of silicone insulation and each has different properties depending on chemical composition, vulcanization process, filler material

and additives. While the base polymer itself may not be subject to UV degradation, fillers such as ATH and CaCO3 are susceptible. These fillers determine tracking and erosion resistance, especially under high UV. As such, silicone insulators operating in this environment and containing poorly performing fillers could suffer severe dry band arcing as well as some degradation. Indeed, inspection of various lines through the desert found that performance of line components varied from material to material. For example, arresters with EPDM sheaths fared particularly badly (Fig. 8) whereas arresters and cable terminations with housings made from ethylene vinyl acetate (EVA) seemed to suffer less surface problems (Fig. 9). EVA is a co-polymer of ethylene material and not an elastomer as are EPDM and silicone – both of which have faced problems

in Oman. Indeed, it was developed specifically to have superior weathering properties as well as high resistance to erosion. While solving the problem of pole failures was the major concern, the blanket assumption that specifying silicone insulators would effectively remedy the situation was not regarded as necessarily true. Severe desert pollution, along with high temperatures (up to 60°C in summer), high UV and sulphur from diesel vehicles meant that not all silicone insulators would necessarily withstand the hostile environment for very long. 

Part 2 will appear in Q4, 2013 and discusses alternative techniques to deal with the cause of pole fires while also looking into problems with cable termination insulation in desert environments.

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Fig. 9: Simple hydrophobicity test on EVA 33 kV cable termination.

2013-08-20 6:00 PM