A fault of secondary current display of a CT in the filter site of a converter station is introduced.The possibility of the CT’s own fault is exclude through diagnosis experiment to all the primary equipments in the filter set where the CT locates

it is found that the breakdown of a capacitor in parallel connection with the CT’s electrical branch is the direct cause of the fault.After diagnosis and test of circuit breaker switching the filter as well as inspection and simulation analysis of phase discrepancy closing device

it is foud that the defect of the phase C of phase discrepancy closing device is the main cause of breakdown of capacitor due to overvoltage and overcurrent

thus leading to the fault of the CT’s secondary output display.