The 110 kV substation often equips with automatic transfer switch(ATS) for backup power supply in order to ensure the reliability of power supply. If the circuit breaker rejects operation when ATS operates, it may cause the power supply failure of the whole station. The power supply failure of 110 kV substation has been analyzed. Operation logic shortcomings of normal ATS are pointed out, and an improved scheme of incoming line ATS is proposed.