Abstract: This paper investigates an incident at a photovoltaic power station in which misoperation of grounding transformer protection causes a complete outage of the 35 kV Section II busbar. Through accident investigation and short-circuit current analysis, it reveals the cause of the protection misoperation and the mechanism of setting-value coordination failure. To prevent misoperation of grounding transformer protection from happening again, the grounding transformer protection is systematically optimized in accordance with national standards for new energy grid-connected relay protection and staged protection technical requirements, from three aspects: setting configuration, time-delay graded coordination, and protection configuration integrity. Verification confirms that the optimized scheme satisfies both the time-delay grading requirement of ≥0.3 s and the timing coordination criterion, effectively preventing protection misoperation. This paper provides a technical reference for the safe and stable operation of photovoltaic power stations.