Test and diagnosis on a secondary voltage loss fault of a 220kV capacitor voltage transformer

A 220 kV Capacitor Voltage Transformer (CVT) quitted operation because of losing secondary voltage. After the accident, power-off diagnostic testing was performed and it was found that several data were abnormal such as the transformation ratio, the secondary winding's DC resistance and the capacitance values, showing various types of fault characteristics. However, according to the CVT's internal structure, further tests in different conditions revealed these abnormal data were not caused by multiple independent faults, but showing a relationship of mutual support. The common pointed cause of the failure was a short circuit between the metal shell and the initial terminal of the intermediate transformer. And the judgment of the fault was fully confirmed through the inspection and test under disassembled condition. Moreover, the reasons of the fault were analyzed. In addition, strengthening the operation monitoring and improving the structure of CVT were suggested to prevent such risk, and test methods for the similar fault were recommended.