Differential Protection Method of Stator and Rotor Current With Different Frequencies for Variable Speed Pumped Storage Units

Rotor winding short-circuit fault (RWSF) is a common fault type of variable speed pumped storage unit (VSPSU). However, the RWSF protection in engineering typically depends on the integrated over-current protection of the AC excitation system, which has poor sensitivity to reflect the RWSF and makes it difficult to fulfill the requirements of safe operation of VSPSU. Inspired by transformer differential protection, a different frequency differential protection method using both stator and rotor phase currents is proposed. In terms of working principle and structure, the stator and rotor windings of the VSPSU are similar to the primary and secondary windings of the transformer, and there are conditions for differential protection. Furthermore, the Clark transformation is used to analyze the characteristic changes of stator and rotor currents after the RWSF. On this basis, the criterion form of different frequency differential protection is constructed based on the trapezoidal area method. Finally, the effectiveness of the proposed method is verified by dynamic simulation experiments.