Through the neutral point of the main transformer is breakdown gap under the lightning over-voltage, and the main transformer is tripped by the zero-sequence protection, the discharge dispersion of gap is large is indicated; When 220kV neutral point is ungrounded ,the protection must be sharing after arrester parallel gap is proposed. The two-phase or three-phase wave is the main reason of the protection device action are pointed by discussing the wave process in the transformer; The most simple and effective way is placing special transparent insulating over the gap is proposed.