Resolving Problems From Poor Insulation Performance in Desert Environments (Part 2 of 3)
CABLE ACCESSORIES
Resolving Problems From Poor Insulation Performance in Desert Environments (Part 2 of 3)
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Oman, as in other countries across the Gulf Region, preventing pole top fires (as discussed in Part 1 of this series) have traditionally been one of the key issues confronting power network operators. However, another problem area has been cable terminations, whose polymeric material has in some cases been rapidly degraded by the combination of environmental stresses. This second in a 3 part article contributed by British specialist, Brian Wareing, reviews investigations he conducted into poorly performing insulation in
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demanding desert environments.
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Every specific formulation of silicone used in insulators or cable accessories is typically unique to the manufacturer and can feature different properties that play a key role in tracking and erosion resistance as well as preventing rapid ageing, especially under high pollution & UV.
All Silicones Not Equal While pole failures due to fire have been a major concern to power companies operating in desert environments, the assumption that specifying silicone insulators would immediately resolve the problem was not generally true. In fact, service experience showed that severe pollution levels along with high temperatures (up to 60°C during summer), high UV levels and sulphur from diesel vehicle emissions all combined to reduce lifetime of certain silicone insulators operating in this environment. The problem was that, although the silicone rubber material itself would last, the fillers and support matrix were not always of sufficient quality to perform well in desert climates. The term ‘silicone’ as used in most electrical applications, covers a family of polymeric materials based on polydimethylsiloxane (PDMS). But every specific formulation is typically unique to the insulating component manufacturer – whether line insulators, cable terminations, etc. – and can feature different properties that depend on factors such as chemical composition, vulcanization process, fillers and other additives. Insulating performance therefore does not depend solely on the base silicone rubber polymer. Fillers, for example, play a key role in tracking and erosion resistance and preventing ageing, especially under high UV. One cannot conclude that one polymeric material will always be superior to all others. One application where it proved possible to compare insulation properties of different polymeric materials involved 33 kV cables. In the typical Middle Eastern oil production environment, movement
Photos courtesy of Brian Wareing
Fig. 1: Slots in EVA (top) and silicone cable terminations operating near Nimr in Oman.
of tall drilling rings sometimes requires ground clearances under overhead conductors of as much as 17 m. Such tall structures can be very expensive and cables are therefore used widely as an alternative.
in terminations that were only 2 to 3 years in service. Investigation of these failures provided an opportunity to compare experience with performance of ethylene vinyl acetate (EVA) housings with those made using silicone rubber.
Problems with Insulation in Cable Terminations Cable termination failures have been relatively common in places such as Oman, sometimes even
Instances of holes and slits of various severities demonstrated that cable terminations can undergo a process of deterioration that eventually leads to failure. Fig. 1 91
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Close-up inspection of failed cable termination insulation revealed two different concerns: in the EVA samples, there appeared to be a design failure while in the silicone termination there was clearly a material problem.
1) windblown sand containing calcium and sodium salts; 2) high UV; 3) nitrous oxides from corona attack and discharges; 4) sulphur oxides from diesel exhaust fumes near roadways where most terminations were installed; 5) high ambient temperatures; and
Photo courtesy of Brian Wareing
6) high cable temperature because of poor heat dissipation to ambient.
Fig. 2: Early stage in the degradation of an EVA cable termination in Oman
shows a silicone and an EVA 33 kV termination that both seem to have tears in their sheaths. However, closer inspection revealed that these were actually slits caused by surface tracking currents that gradually eroded the material until it reached the cable screen cut-back, at which point failure occurred. The problems faced by the cable insulation in these cases was due basically to the impact of pollution, localized electric fields and the harsh desert environment with:
While all the above service conditions are perhaps impossible to avoid, it is possible to do something about how this pollution environment affects insulation properties. For example, there was noticeably more sand adhering to the relatively hydrophobic silicone termination than to the relatively hydrophilic EVA termination. Possibly, the presence of silicone oil on the surface provides a sticky substrate to which sand and salt adhere. Similarly, any UV damage to the insulation’s surface makes it rougher and therefore easier for fine salt particles to stick to. Heavier sand grains, by contrast, are either blown away or adhere only poorly. Several cable terminations were removed for closer inspection and it was found that some had holes or short slots (as in Fig. 2). This confirmed that the failure process was probably not an ‘instant phenomenon’ but rather due to ongoing degradation over a period of many months. As a result of the examination, the basic failure process was concluded to be as follows:
Photos courtesy of Brian Wareing
Fig. 3: Surface and cross-sectional view of EVA cable termination material from Omani desert network.
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1. The long-term stress on the termination is a puncture stress between the exterior, which is at a few kV, and the earthed interior where the busbar protection tube metric (BPTM) and stress control tubing (SCT) are adjacent. Any hole at that location only makes interior stresses more damaging to the insulation. 2. This allows surface leakage currents to flow from the phase lug, over the sheds and through the hole to this SCT/BPTM gap. It also allows moisture to penetrate into the termination, as evidenced by corrosion on the copper screen. 3. Under the influence of the internal electric field, the arc root gradually moves to the end of the SCT and towards the core screen cut back.
Photos courtesy of Brian Wareing
4. The surface leakage current flowing down along the exterior surface of the HV outside tubing (HVOT) and then through the hole and up along the inside of the HVOT gradually further erodes the original hole in an upward direction, thereby forming the slot. 5. As the arc root cuts into the SCT and the hole increasingly opens, moisture ingress and surface leakage current both increase. 6. Once the arc root reaches the end of the core screen, surface leakage currents will no longer increase but start to cause substantial damage to the cable dielectric at this point and eventually trigger breakdown of the HV conductor. The resultant fullblown arc destroys the polyethylene insulation and causes a phase-earth fault which burns the carbonloaded SCT and blackens the whole area. 7. The fault current then destroys the local area around the original hole.
Fig. 4: Surface and cross-sectional view of silicone cable termination material from Omani desert network.
Further laboratory examinations were conducted on EVA and silicone cable termination taken from the Omani desert networks. Figure 3 illustrates surface degradation seen on the EVA unit. While the EVA insulation shows evidence of surface crazing, there is only negligible penetration into the body of the material. The pollution degradation effects therefore seem quite low. The silicone material (see Fig. 4), by contrast, shows severe UV degradation with surface cracks that penetrate into the body – reducing insulation properties and enabling pollution to take an even firmer hold. While the silicone termination material is likely a complex formulation with screens and stabilizers, this formulation does not appear to have solved the demands of the Omani service environment, which is a combination of UV, chemical and electrochemical attack. For example, the silicone in the failed sample had cracks penetrating about 10% through the housing
whereas, in the EVA failed termination sample, crack penetration depth was less than 1%. This suggests that the silicone material was already ‘struggling’ with the environment after only 3 years of use. Of course, this particular case covers only one silicone termination supplier and one EVA termination supplier. Nevertheless, the close-up inspection revealed two different concerns: in the EVA samples, there appeared to be a design failure, while in the silicone termination there was clearly a material problem. The manufacturers involved were contacted with this information and, in the case of the EVA termination supplier, the product has been re-designed and now experiences a substantially lower failure rate.
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