Abstract: During the commissioning of a 220 kV booster station in a wind farm, the phase-A differential protection of the outgoing line bay tripped, causing the phase-A circuit breaker to open. Several milliseconds later, the phase-B and phase-C differential protections operated, tripping phases B and C while blocking the reclosing function, with no other protection devices activated in the station. Comparative analysis revealed that the three-phase currents at the local line bay were in phase with those at the opposite grid side but had half the magnitude. By examining the current magnitude relationship and protection operation characteristics, it was confirmed that reversed polarity marking of the current transformers in the line bay caused the maloperation. The analysis demonstrates that incorrect polarity during equipment installation and wiring adversely affects system operation. Therefore, targeted measures should be implemented in equipment installation, factory testing, on-site commissioning, and fault resolution to prevent human errors from compromising operational reliability.