Fault diagnosis and insulation structure improvement on 550kV resin-impregnated paper oil-SF6 bushings

A type of 550kV epoxy resin-impregnated paper oil-SF
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bushing, which used in main transformers equipped in a pumped storage power station, has gone wrong repeatedly and its capacitance and dielectric loss increased significantly. In order to clarify the cause of the batch of oil-SF
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bushing failure, the bushing has been tested to check its insulation characteristics, and has been dissected to study its internal parts. Additionally, the electric field is also being calculated to analyze the bushing fault reason. Based on these analyses, an improvement design was proposed for this application. The results demonstrated that the higher field strength as well as the poor plate edge treatment process leads to discharge and breakdown on the edge of the capacitor plates, which result in the rising of the bushing's capacitance and dielectric loss. The symptom is consistent with the theoretical calculation results. Considering the on-site installation conditions, the length of the gas side is suggested to be lengthened and the layer of the bushing's capacitance core plate is suggested to be increased. And the simulation results verified the feasibility of the improvement measures.