Recommended Protection to Prevent Safety-Related Motors from Losing All AC Power When an Open Phase Fault Occurs in the Auxiliary Transformer of a Nuclear Power Plant

The primary objective of this study is to address the issue of complete AC power loss of safety class motors within nuclear power plants in the event of an open phase fault occurring in the auxiliary transformer. The analysis commences by examining the mechanism responsible for inducing voltage on the primary side following an open phase fault in transformers featuring d-type windings on the secondary side. It employs positive sequence and negative sequence equivalent circuit models for motors to assess voltage and current behaviors during this fault. The study then shifts its focus to how the voltage of the auxiliary transformer and the safety-related motor respond to variations in grounding resistance and auxiliary transformer load ratio. The analysis reveals that when the grounding resistance falls within the range of 500 to 2500 Ω, the safety-related motor in the unit may lose its entire AC power supply. Moreover, if the grounding resistance exceeds 2500 Ω and the auxiliary transformer bears even a minimal load, the safety-related motor will lose all AC power due to a significant imbalance. In conclusion, the chapter proposes a solution by introducing a negative sequence phase voltage imbalance protection relay. This approach ensures that, in the event of an open-phase fault, there will be no adverse impact on the safety-related motor. In other words, it will prevent equipment damage or false tripping and guarantee that the safety-related motor does not lose its entire AC power supply, thus contributing to the overall safety and stability of nuclear power plant operations.