Partial grounding is usually applied to 220 kV transformers. With increase of the capacity of power system， transformer faults caused by short circuit fault occurred now and then. The capability of the neutral point of 220 kV transformers enduring over voltage is analyzed. The limit of 220 kV transformer enduring short circuit fault and the over voltage of neutral point caused by single-phase fault are analyzed. According to the fault case， the effect of singe-phase fault and open phase to transformer neutral insulation is analyzed. The thermal resistance stability and dynamic resistance stability of transformers enduring short faults are evaluated. It is concluded that though neutral insulation usually cannot be broken down by single-phase fault， it is dangerous when there is other over voltage acting together. Given the short current of transformers with neutral point grounding directly is already close to its limitation， the current limiting measures are suggested or changing the grounding methods may take effect.