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.
Types of Degradation
Surface degradation can be divided into two categories in regard to its potential impact on performance and reliability of the apparatus:
• Type A Degradation
This is degradation without spontaneous development of cracks and where any cracks become visible only upon bending the insulation.
• Type B Degradation
Here, degradation is marked by spontaneous development of cracks (e.g. on the insulator housing’s trunk) and/ or extensive flaking of the degraded surface layer.
Type A Degradation
Examples of different degrees of Type A degradation are shown in Fig. 2. Type A degradation does not constitute an immediate threat to service life and reliability of the transformer and therefore it is sufficient to inspect any apparatus with this form of degradation every 4 years.
Type B Degradation
Examples of different degrees of Type B degradation are shown in Fig. 3.
Samples of silicone rubber were cut from insulator housings with cracks on the trunk and subjected to scanning electron microscope (SEM/EDX) analysis (see Fig. 4).
Type B degradation constitutes a potential risk to service performance and therefore apparatus with this type of degradation should be inspected every year. At the same time, remedial measures were investigated to improve the surface condition of housings with such degradation.
Results of Maintenance Inspections
Instrument transformers with polymeric insulator housings that are currently in service at CEPS substations were manufactured between 1991 and 2007. In total, 300 such units were subjected to inspection with some inspected repeatedly to monitor the progress of degradation. The proportion of insulator housings found to have experienced degradation is shown in Fig. 5.
The main years of manufacture of apparatus with Type B degradation are 1993, 1995, 1999 and 2000. Fig. 6 shows the incidence and distribution of degradation, by type, at the substation with the highest number of instrument transformers.
The dates of manufacture of the instrument transformers at this substation along with their respective types of degradation are outlined in Table 1.
Measures to Restore Service Performance
The degradation on transformer housings inspected up to now has not yet reached a level that immediately threatens the reliability of their operation. However, continuing Type B degradation, marked by development of deeper cracks on the trunk of the insulator housing, could become dangerous.
As one possible way to improve the surface condition of housings with Type B degradation, application of an RTV silicone coating was tested on a sample cut from the shed of a degraded transformer housing. Half the sample was coated with RTV silicone while the other half was left uncoated. The sample was then subjected to a 1000-hour accelerated ageing test in a xenon chamber with UV radiation of 0.47 W/m2, 340 nm and with dry and wet cycle duration of 102 min and 18 min respectively. Fig. 7 shows that no further degradation was apparent on the coated portion of the sample after the test.
Moreover, adhesion of the RTV silicone coating to the sample was the same as before the test. While the portion of sample with the RTV silicone coating has again become hydrophobic, the part without coating is hydrophilic.
Experimental Coating of Polymeric Housings in Service
Experimental coating with RTV silicone was performed first on a 220 kV combined (voltage & current) instrument transformer with degraded housing. For this first application, a procedure was used that allows for monitoring and evaluation of:
• effectiveness and durability of the coating;
• influence of electric field;
• progression of degradation on the surface, both with and without the coating; and
• necessity to apply a primer before coating.
The RTV silicone coating was therefore applied to only two-thirds of the housing surface: the upper one-third at the line end and the lower third at the ground end. The middle section was left without coating. Moreover, half the circumference of each coated section had a primer applied before the entire circumference was coated.
• Surface Cleaning
Before coating, the surface of housing was cleaned with a hard brush to remove any unstuck parts of the degraded surface layer. Dust was removed with a wet cloth. The surface was then washed with a water/soap solution sprayed onto the surface. After drying, the rest of the surface contamination was cleaned with acetone.
• Application of Primer
After the acetone evaporated, a primer was sprayed onto the selected parts of the surface. The goal was to find out if there is a difference in adhesion of an RTV coating to an aged housing surface with or without primer.
RTV silicone material was applied to the housing surface with airless spraying equipment. In total, there were 3 layers of silicone material such that between each layer application there was more than 30 minutes of drying time for solvent evaporation.
RTV Coating 220 kV Transformer Housing
RTV coating of an instrument transformer was performed in August 2014. The state of surface degradation of its housing before coating is shown in Fig. 8.
A hard degraded layer on the top and bottom surfaces of sheds was found along the entire insulator housing. On some parts, cracking and minor flaking of the degraded surface layer had occurred and the entire housing was hydrophilic.
Fig. 10 shows the difference between parts of the housing with and without coating, while Fig. 11 reveals the state of the surface and the hydrophobicity of the coated housing portion.
Inspection of Instrument Transformer with RTV Coating
The first inspection of the instrument transformer with RTV silicone coating was performed in June 2015 (i.e. 10 months after coating). The condition of the housing surface is shown in Fig. 12.
The results of inspection showed that the layer of RTV silicone coatings on both sections (upper and lower) is without any damages and traces of surface degradation. Surface hydrophobicity of the instrument transformer housing parts with coating is excelent (HC from 1 to 3 according to IEC/TS 62073, cl. 6. The surface of the middle section without coating is totally hydrophilic. The adhesion of the RTV silicone coating on the degraded housing surface is excelent and there is no detectable difference in adhesion on the surface with and without primer (see Fig. 13).
RTV Coating 400 kV Instrument Transformer Housing
Coating a degraded 400 kV instrument transformer housing was performed in July 2015. Here, the entire housing (upper and bottom parts of the sheds and trunk) had uniform degradation and was hydrophilic. Spontaneous development of cracks as well as flaking of the degraded surface layer were investigated and it was discovered that the flexibility of sheds had been reduced (e.g. cracks occurred even with minor bending). The extent of surface degradation is shown in Fig. 14.
Coating the housing was performed from a scaffold built around the transformer. In order to shield the work area from wind, the scaffold was protected using plastic tarps (see Fig. 15).
The coating procedure was basically the same as used for the 220 kV transformer but this time without any application of a primer. The state of the surface and degree of hydrophobicity of the coated housing are shown in Fig. 16.
Degradation of the surface of polymeric housings on instrument transformers had been observed in the Czech Republic during routine maintenance inspection. The degradation typically took the form of a hard surface layer up to 250 μm thick. The surface with this hard layer had become either hydrophilic or much less hydrophobic than when new.
Inspection results indicated that the degree of degradation was not dependent on duration of service. Moreover, degradation of the same type of apparatus from the same producer and the having the same length of service was different from one transformer to the next – even at the same substation with the same environmental conditions. This degradation had to be caused by some combination of environmental conditions (e.g. pollution, UV, etc.) and variations in the properties of the polymeric housing material (e.g. composition of different batches) or changes in the manufacturing process.
In most cases the hard surface layer cracks only when the sheds are subjected to bending. Moreover, whenever the thick, hard degraded surface layer occurs more or less uniformly along the entire top and bottom surfaces of sheds, they lose elasticity and even small bending leads to cracking.
If the housing has sufficient creepage distance, service performance and reliability of apparatus with surface degradation (but without spontaneous cracking) is not immediately affected. Spontaneous development of cracks (mostly on the housing trunk) and/or flaking of the degraded surface layer were observed on only a few transformer housings. Under such degradation, performance of the apparatus may decrease and measures to restore reliability should be considered.
RTV silicone coating of polymeric housings with a high degree of degradation (i.e. marked by spontaneous development of cracks on the insulator trunk) is one measure to increase service reliability. The coating can be sprayed directly onto the clean degraded housing, without previous application of a primer. The RTV material seems to fill the surface cracks and restores hydrophobicity of the previously degraded polymeric housing. It cannot, however, restore elasticity to the sheds