A pumped storage unit has the functions of peak load, frequency, and phase regulation as well as emergency standby. Thus it plays an important role in the safe and stable operation of a power grid. Because of the large capacity, complex structure and expensive cost of a pumped storage unit, it is impossible to carry out a large number of fault and abnormal operation tests on site and physically model them. Based on RTDS, a simulation model of a pumped storage unit is built to simulate internal and external faults as well as the abnormal operation of the unit such as excitation-loss under various working conditions and load levels. It is verified that the existing stator impedance judgment criteria of excitation-loss protection is defective and the unit excitation cannot be switched in time when excitation-loss occurs under the condition of pump phase regulation. This is not conducive to the stable operation of the system. Based on the simulation analysis, a new KP impedance circle criterion is proposed to improve excitation-loss protection, so as to realize the excitation-loss protection without a dead zone in the whole working condition of the unit. The criterion can be used as a reference for relevant relay protection practitioners. This work is supported by the National Natural Science Foundation of China (No. U1804252).