A Novel Third-Harmonic Voltage Protection Method for the Generator Stator Ground Fault of Nuclear Power Plant's Rotating Asynchronous Machine

The nuclear power plant's rotating asynchronous machine (RAM) is the sole power supply system of the control rod drive mechanism (CRDM). The current RAM's protection system consists of traditional discrete relays, which have low reliability and flexibility. The popularization of digital protection devices and new principles has been started in the engineering field. Based on the feature of the Zig-Zag star winding connection of the generator stator winding in RAM, this paper analyzes the after-ground fault third-harmonic voltage characteristics. It is found that when the fault resistance exceeds 5 kΩ, the application of the traditional third harmonic voltage criteria is difficult because of the low sensitivity. Thus, this paper proposes a novel protection method based on the variation in the third-harmonic voltage, effectively improving protection reliability by introducing restraining quantity. Simulation and dynamic model experiments show that, compared to the existing third-harmonic voltage criteria, the proposed method exhibits higher sensitivity and has an against fault resistance ability of at least 10 kΩ.