Abstract: This research addresses the issue of misoperations of grounding transformer protection leading to tripping of photovoltaic (PV) power plants when single-phase-to-ground faults occur on the grid side. A retrofit solution for zero-sequence directional current protection of the grounding transformer is proposed. By introducing the function of zero-sequence current direction judgment and combining it with the phase comparison of zero-sequence current from the feeder circuit breaker and zero-sequence voltage from the busbar, the solution achieves precise identification of the fault area, thereby reducing misoperations caused by external faults. Verified by MATLAB models and actual data from PV power plants, the results show that the solution can effectively distinguish between internal and external faults and significantly reduce the number of false trips.