Bushing End Screen Failure - Case Study

摘要

Paper deals with two 400 kV bushing failures which happened in a relatively short period of time, after about ten years of transformer service. Failure appearance was practically identical in both cases - breakdown between bushing end screen and turret wall followed by a small oil leak through mechanically damaged bushings. Failure happened during medium load, temperature and voltage ordinary conditions. Analysis of the periodic (off-line) and continuous (on-line) diagnostic results revealed that up to the moment of failure no any signs of possible problem had been recorded: no bushings capacitance change, no DGA rise and no any significant overvoltage. All measured values correspond to new equipment. It seems that observed bushing failures were totally unpredictable despite of applied on-line and off-line diagnostic methods. Several end screens were taken from service for further research in HV laboratory. End screen electrode is casted with a thick layer of resin. El. field inside and around end screen electrode was calculated and based on the results special test setup was developed which simulates dielectric stresses as in service. PD measurement revealed that some end screens suffered from harmful PD even on (simulated) service voltage. It was found that PD inside bushing end screen thick epoxy coating can be hidden despite of bushing and transformer testing according to relevant standards.