Abstract: A nuclear power plant is equipped with two external power sources: a 220 kV supply (powered by the start-up and reserve transformer) and a 500 kV supply (powered by the station transformer), both providing electricity to the plant's auxiliary power system. Parallel switching between these two external power sources can be achieved through a synchronization checking relay. During the period from 2021 to 2022, this relay triggered multiple out-of-synchronization alarms. To address the issue of frequent alarms from the synchronization checking relay, this paper systematically analyzes and verifies the problem by considering the plant's electrical main wiring configuration, the interlocking logic of circuit breakers for auxiliary power sources, and equipment parameters. The voltage vector difference and circulating current during parallel operation under different operating modes are calculated. Combined with the protection configurations and action characteristics of the station transformer and start-up reserve transformer, the impact of the alarms on equipment safety is assessed. Based on the overload protection setting of the transformer at 1.05 times I_tn (reliable return coefficient), a specific adjustment plan for the synchronization checking relay settings is proposed: maintaining the phase angle difference setting of 20° unchanged, and adjusting the voltage vector difference setting from the original limit corresponding to an amplitude difference of 26 V to 36 V. This solution eliminates the interference caused by frequent alarms during operation while ensuring equipment safety during parallel switching. It provides a technical reference for optimizing the synchronization system of external power sources in nuclear power plants.