Unrepeated of Test Tap Capacitance New RIP Bushings

Abstract

Capacitance testing is the most important for the service bushing condition assessment. However, for the new Resin Impregnated Paper (RIP) bushing used in EGAT, the repeatable test result quite deviates. The aim of this paper is to investigate the root cause of the unrepeated testing values of the test tap capacitance for the installed new RIP bushings compared with factory values. The measurement results of test tap capacitance were collected from ten installed 123 kV bushings. Those results deviated from the factory values of approximately 33%. A new RIP bushing was used to set up the laboratory test experiment. The laboratory test used a portable insulation power test set to measure the test tap capacitance. The aluminum foil and the sheet metal were used to wrap around the bottom part of the bushing for simulation of the ground electrode and leakage current guarding. Onsite measurement was performed with an installed bushing with BCT (bushing current transformer) by grounding BCT and guarding of the leakage current of BCT. It was found that the test tap capacitance was decreased with increasing the diameter of the simulated ground electrode and decreased with the application of the leakage current guarding. The test results showed that the bottom part of the RIP bushings without the ground sleeve and conditions of BTC such as the diameter of bushing pocket, installation with or without BCT mainly impacted the unrepeated test tap capacitance.