Abnormal External Insulation Flashover Analysis of $\pm 500\text{kV}$ Power Supply Transformer for DC Circuit Breaker Based on Electric Field Simulation

The power supply transformer is designed to transmit LV AC power to the HV components and isolate pole-to-ground voltages of the DC circuit breaker. An abnormal external insulation flashover occurred during switching impulse withstand voltage tests of one ± 500kV power supply transformer. In order to obtain the cause and solution of the problem, the insulation structure of the power supply transformer was analyzed and the test object status after flashover was evaluated. Considering the preliminary analysis of the start point of flashover, the 3-D finite-element method model for electric field simulation was built according to the test object structure. The electric-field calculation results show that the serious electric field distortion occurred at the joints of two LV shielding half-rings, where small gaps were found caused by the test object installation. The maximum electric field strength at the joint was 40.2kV/cm under the breakdown voltage which exceeded the critical strength value in air, and it increased with the increase of the gap length and eccentric distance of the two half-rings. Based on the calculation results, it could be predicted that the start point of flashover was at the joints, which corresponded with the observation and analysis of the actual test object status. Then the solution was put forward that the LV shielding ring should be full ring type or two half-rings with hemisphere terminals. After optimization, the electric field strength at the surface of the power supply transformer was lower than the critical value 30kV/cm under the switching impulse withstand voltage. The research could provide references for the development of power supply transformer for DC circuit breaker.