Since the early 1990s there has been growing application of composite-housed HV apparatus at substations. This has particularly been the case for instrument transformers where the safe failure mode of a silicone housing compared to porcelain is seen as a crucial benefit. Of course, expected performance and service life of the polymeric housings, compared to porcelain, is also a concern to users. That is why any evidence of premature ageing is important, as is what remedial measures are available should this be discovered.
Recently, inspection of composite-insulated instrument transformers at substations operated by CEPS – the Czech Transmission System Operator – revealed varying degrees of ageing of their housings. Degradation typically took the form of a hard surface layer that had become either hydrophilic or offered only poor hydrophobicity. There were also cases of more extreme degradation marked by spontaneous development of cracks on housing trunks as well as flaking of the surface. This article, contributed by Vaclav Sklenicka of EGU HV Laboratory in Prague as well as Karel Fiala of CEPS and Manfred Bruckner of Lapp Insulators, discusses the application of an RTV silicone coating as a remedial measure to restore the condition of degraded polymeric housings.
Instrument transformers equipped with composite housings have been installed at 245 kV and 420 kV substations in the Czech Republic since 1992. These transformers, all supplied by one manufacturer, have been operating in a temperate middle European climate where annual sun radiation ranges from 1300 to 1800 hours duration and ambient temperatures can drop to -30°C. All substations where these units have been put into service have low pollution exposure (according to IEC 60815-1) and pH values of local rain range between 4.2 and 4.9.
Regular inspection of polymeric housings on all instrument transformers installed at CEPS substations has revealed that degree of degradation does not always correlate with years in service. For example, it was found that some instrument transformers with only 5 years’ service suffered high housing degradation while similar transformers with a 20-year service history showed no degradation. Moreover, degree of degradation of housings at the same substation and with the same number of years of service differed from one transformer to the next. This suggests that the composition of the polymeric insulating material was not exactly the same for all these transformers.
In some cases, degradation along the length of the housing was uniform (e.g. distributed evenly along the core as well as on the top and bottom of sheds). In other cases, much degradation was evident on the top of sheds but little on the bottom; or degradation was higher on the south-facing side of the transformer than on the shaded side – suggesting some influence of UV. Finally, there was often no difference in surface degradation between the line and grounded ends of the transformer housing, suggesting that it was not caused by corona discharges.
Investigation of Degraded Housings
The problem of degradation of polymeric housings on instrument transformers is still under investigation. But there are already some results based on measuring thickness of the degraded layer using Fourier Transform Infrared Spectroscopy (FTIR) and thermal gravimetric analysis as well as by studying the influence of acidic solutions and UV radiation. Partial results include:
• Maximum thickness of the degraded surface layer is up to 250 μm;
• Hydrolytic degradation of siloxane chains takes place in the surface layer under typical service conditions. This process is connected with reduction in molecular weight and results in loss of physical properties such as strain and flexibility as well as formation of a non-flexible, brittle layer on the surface. While the degradation process starts and progresses from the surface, the inner portion of the housing remains unaffected to a certain depth;
•Thermal stability of degraded samples after years of service falls within acceptable limits;
• Acidic solutions and UV radiation seem to be factors linked to degradation;
• Degradation can occur along the entire surface of the housing. This suggests it could be influenced by the specific composition of individual batches of the silicone rubber material used in manufacturing the insulator or by slight variations in the production process.
Based on all the above, it is reasonable to assume that, whenever the degraded surface of the housing remains unbroken by cracks, degradation reaches a certain thickness that then the surface is protected against further degradation. The degraded housing either becomes hydrophilic or is only barely hydrophobic and such insulators have to be considered as offering no hydrophobicity transfer properties (i.e. no capability to reduce surface conductance and leakage current activity).
If a degraded insulator has sufficient creepage distance, even though hydrophilic, such insulators can still provide the required service performance. This assumption has been confirmed during inspection by the fact that no erosion or tracking was recorded on housings having degraded surfaces. Moreover, three transformer housings with degradation were wetted under voltage and corona camera inspection showed no partial or corona discharges along their surfaces.
A much different situation could arise, however, in the case of spontaneous development of cracks in the degraded layer on the housing’s trunk. Due to the internal tension in the polymeric material at this location, any cracks will remain open and degradation processes can continue. A similar situation can occur in the case of extensive flaking on the surface, where relatively large areas of virgin material become exposed to environmental conditions. The most dangerous situation would be if cracks reached all the way to the equipment’s FRP tube core, with resulting possible moisture ingress and contamination of its internal oil or gas insulation. In such cases, prompt measures to prevent further degradation would have to be considered.