In 2009/2010, ESDD levels on post insulators at the Longquan Converter Station reached 0.08 mg/cm2 and in the case of the Yidu Conveter Station were between 0.11 and 0.13 mg/ cm2. Given the 22,789 mm total creepage of the three DC dividers, their pollution withstand voltage under moderate contamination was only about 485 kV – less than the normal operating voltage. The probability of flashover became high simply because there was no longer any insulation margin.
PTs used in the Long-Zheng, Jiang- Cheng and Yi-Hua DC projects were all supplied by a European-based multinational and investigation at Huaxin and Yidu converter stations revealed that even unused spares from this manufacturer had poor hydrophobicity on shed surfaces, i.e. only HC5 to HC6. Figure 4 shows examples of the hydrophobicity of these standby indoor DC voltage dividers.
Composite insulators with diminished surface hydrophobicity are more likely to experience arc burns as well as tracking and erosion caused by partial discharges or corona activity under service conditions marked by UV, temperature change, rain, fog,snow, etc. Loss of hydrophobicity by a composite insulator usually means unsatisfactory service performance since, even if it does not experience pollution flashover or serious discharge phenomena, it will still require greater scrutiny by maintenance staff.
Moreover, premature ageing of silicone rubber sheds and housings due to a variety of service stresses might cause performance to decrease as the insulator mcould experience discoloration, loss of gloss, hardening, deformation, cracking or increased brittleness.
In such cases, one solution would be to coat the unit with RTV material to restore hydrophobicity and increase pollution flashover voltage. At the same time, this experience has shown that it is a good idea to regularly monitor silicone insulators – even on spare equipment – for any possible decrease in hydrophobicity
c. Problems Due to Design Defects
Another cause of flashovers on ±500 kV voltage dividers at Jiangling and Longquan Converter Stations under conditions of fog or drizzle was discovered to be poorly designed grading rings and insulator sheds.
The external insulation distance of the DC voltage dividers at the two affected stations was 5600 mm. Their circular grading ring at the high voltage end had a diameter of 1580 mm while tube diameter was 240 mm. Given the grading ring’s shield depth of about 1100 mm, the air gap between it and the nearest bushing shed was only about 265 mm – clearly too short.
Basically, because some grading rings have such high shield depth, effective creepage distance of composite bushings can be shortened by as much as 20%. When the surface becomes wetted, the air gap between the grading ring and the nearest shed on the bushing has to withstand very high voltage, especially when the temperature near the top of the divider (i.e. within range of the grading ring) is relatively high. Once the air gap breaks down, all voltage is taken on by the lower portion of the insulation and voltage per unit of creepage for this section increases by 20% relative to that along the whole bushing. This can induce flashover.
In addition, spiral-shaped voltage divider housings with big-small alternating shed configurations do not seem ideal when it comes to inhibiting rapid increases in surface leakage currents. This is because, should hydrophobicity be reduced or lost, water on the sheds can more easily flow down along the spiral surface and accelerate formation of a full film of moisture.
The geometry of the composite bushings on faulted voltage dividers was basically the same as that of the porcelain bushings used on DC voltage dividers at the Gezhouba Station that never flashed over (even though they did experience minor discharge phenomena on their surface). The explanation here may be that while the composite housing has alternating sheds with spiral structure, the alternating big-small sheds on the porcelain bushing are concentric.
By contrast, the reason that the porcelain-housed DC voltage divider at the Tianshenqiao Station experienced flashover during rain was that the geometry of its sheds was poorly designed. In this case, the spacing between the large sheds was 65 mm while width of sheds was 65 mm to 70 mm, meaning a ratio of close to 1. Moreover, the difference in the widths of the small and big sheds was just 20 mm. A DC arc is more stable than an AC arc and therefore any arc-shortening phenomenon between sheds becomes more serious since it reduces effective utilization of the entire creepage.
It has been found that the pollution and rain flashover performance of any DC device that has relatively small shed separation distances on its external insulation can be improved by installing silicone rubber booster sheds. These not only control shorting of adjacent sheds by an arc but also improve the condition of the insulation when wetted. Utilization rate of the creepage distance is improved as well.
Classification of Alternative Flashover Countermeasures
China’s Electric Power Research Institute analyzed localized arc generation and its impact on various configurations and types of external insulation on DC devices experiencing different forms of discharge (i.e. severe, moderate and slight).
The goal was to facilitate evaluation of discharge phenomena under various wetting conditions such as fog, rain, snow and ice as well as to judge the comparative reliability of different insulation based on field and laboratory experience. At the same time, this analysis would assist classifying the most appropriate countermeasures to deal with discharges in each case (see Table 4).
Based on the existing operating situation at each converter station studied, discharges in the DC field during fog or rainy weather were either partial arcs along the surface of the external insulation or corona discharges at the insulator fittings. Different countermeasures are appropriate to deal with such problems. For example, equipment should be taken out of service and either coated with RTV silicone or have booster sheds added if experiencing level II or level III discharges. Moreover, if there are corona discharges, better grading should be installed to improve field distribution and reduce or eliminate such phenomena.
At the same time, there should be greater monitoring of silicone composite bushings as well as of the hydrophobicity on the surface of RTV coatings at a converter station. In particular, it is important to inspect for any sustained loss of hydrophobicity during several days of consecutive humid conditions as well as the rate of hydrophobicity recovery once the weather improves. It is also recommended to test for hydrophobicity changes due to the existing surface pollution layer and prevailing weather conditions, with measurements best carried out when there are several days of continuous sun. The ideal interval between recoating with RTV silicone should be determined according to the station’s specific operating environment. The surface of composite external insulation does not normally need to be cleaned or washed since this can result in short-term damage to the hydrophobicity of the silicone rubber material. The only exception is when the pollution layer has become so thick that the low molecular weight species in the bulk rubber that are responsible for transfer of hydrophobicity can no longer effectively migrate to the surface.
At the end of 2012, China had already built ten ±500 kV pointto- point DC transmission circuits, transporting a total capacity of 28,800MW. These ±500 kV transmission lines have now taken on an important role in interconnecting the country’s regional power grids.
Typical external insulation failures of DC dividers at ±500 kV converter stations include flashover of both composite and porcelain bushings, typically occurring under severe wet weather such as fog, sleet or heavy rain. The main causes of failures on potential transformers have been found to be inadequate external insulation configuration, reduction or loss of surface hydrophobicity of the composite housing and design defects in the equipment.
The main measures to improve the level of external insulation in such cases have been found to include adding a coating of RTV silicone to the porcelain or to the composite insulators as well as installing booster sheds. Periodic monitoring of surface hydrophobicity of the silicone housing or of the RTV coating should also be increased. Problems due to inappropriate grading ring design and/or shed geometry should be identified and solved as quickly as possible.
Discharges affecting equipment in the DC field of converter stations can fall into one of three classifications: severe, moderate or slight, and different measures are required to deal with each. Ideally, equipment should be coated with RTV material or have booster sheds added should either level II or level III discharges be present.