An abnormal voltage fault during phase-checking process of pumped storage unit is analyzed. It is confirmed that the abnormal voltage is caused by the voltage displacement due to the sub-harmonic resonance excited by the electromagnetic potential transformer and the ground capacitance of generator circuit break. Simulation test indicates that resonance fault happens easily in the process of phase checking. The features of voltage displacement for various resonances are summarized. For the actual situation that the existing pumped storage power stations use power supply of grid system for phase checking

suggestions are offered to prevent the resonance from causing abnormal voltage

i.e. in possible condition removing the parallel ground capacitance of generator circuit breaker

selecting the potential transformer with high volt-ampere characteristics

ensuring three-phase closing synchronization of circuit breaker

and temporarily changing parameters for field handling. These suggestions may reduce the happening possibility of abnormal voltage effectively.