Improvement of overcurrent protection for control rod drive mechanism power system

Abstract

Control rod drive mechanism (CRDM) in nuclear reactors powered by two motor generator units running in parallel, an excitation fault in one of the generators will cause the terminal current to rise in another generator, which will cause the overcurrent protection to operate before the loss-of-excitation protection, resulting in nuclear reactor shutdown. To solve the problem of mismatch between loss-of-excitation protection and overcurrent protection, this paper applies the equivalent circuit and phase diagram analysis, obtains the conclusion that the generator terminal current phase after an excitation fault is significantly different from the current phase after a phase-to-phase short circuit fault, based on this, a generator overcurrent protection criterion of compound fundamental current phase difference is constructed, which can solve the maloperation of overcurrent protection under possible excitation fault in actual engineering. Simulation results of PSCAD/EMTDC verify the effectiveness of the proposed methods under different operating modes and fault conditions.