Short circuit fault of rotor winding is a common fault type of variable speed pumped storage unit. However

the short-circuit fault protection in engineering usually relies on the integrated over-current protection of AC excitation system

which has low sensitivity to reflect the short-circuit fault of rotor winding. It is difficult to meet the safe operation requirements of the unit. Inspired by transformer differential protection

a different frequency differential protection method using both stator and rotor phase currents is proposed in this paper. In terms of working principle and structure

the stator and rotor windings of the variable speed pumped storage unit are similar to the high and low voltage side windings of the transformer. This shows that the conditions for differential protection exist. Furthermore

the characteristic variation law of the stator and rotor side current of the variable speed pumped storage unit before and after the rotor winding short circuit fault is analyzed by Clark transformation. On this basis

the criterion form of different frequency differential protection based on the characteristic component comprehensive effective value is constructed. The proposed protection method is simple in principle and easy to implement. Finally

the effectiveness of the proposed method is verified by dynamic simulation experiments.