A mal-operation of line No.Ⅰ zero-sequence protection of 110 kV wind farm booster station is resulted from a out-area single-phase ground fault occurring on 110 kV regional power grid grounding system. The unearthed transformer neutral point protection gap strike and discharged in booster station is the fault cause， which is found by the recorded fault waveforms and scene investigation. In addition， it also points out the existing problems due to relay settings improper coordination in substation and problem of neutral point protection gap arrangement for main transformer. Through detailed theoretical analysis on the mal-operation according to the area power system situation， an improving measure to avoid protection malfunction is put forward. The application results on site of substation are verified effective.