The failure analysis of SF6 gas load switch for railway

초록

The failure accident was occurred in SP and PP, it’s cause was ground fault. In order to check the accident cause of ground fault, the examination was performed such as liquid penetrant examination(PT), thermal photography in field, insulation resistance test, outside overlook and internal expertise for accident load switch. The PT test was able to check the crack on the surface and welding side. From this test, the crack was not detected. Thermal photography was used to check the temperature in installed condition of field. From this test, we were able to examine the arc or discharge in load switch tank. There was no abnormal temperature and the maximum temperature was 42 centigrade at bushing near to gas tank. It was moderate results from insulation resistance test between bushing and tank surface. The analysis of outside overall look was followings. The surface of tank presented some hot points from the internal arcing but there were no holes. The flaps on the frame were not opened. The membrane was tore apart the internal high pressure during the arcing time. The powder around are the small grain of SF6 desiccant agent. The tank has been opened and the cover was removed to check the internal examination. The arc evidences were shown between the top bushing’s conductor and the tank, the low bushing’s conductor and the tank and the top connection and the screw dedicated to maintain desiccant agent.From those tests and examination, the cause of failure of accident was following. There was no evidence of any leakage. There was no evidence of low pressure before incident occurred. Low pressure indication may have been driven by the first arc and bursting disc explosion. The most likely event was an overvoltage which has exceeded the capability of the load switch