Abstract: AC-DC Interference can cause maloperation of relay protection. Based on the experimental power model of AC-DC Interference, this paper analyzes and deals with a relay maloperation accident. The results show that the dampness in switch cabinet leads to the reduction of internal insulation of handcart aviation plug, which results in mutual discharge between wires, and causing the AC-DC Interference. AC current forms a loop through the distributed capacitance of long-distance control cable of the device for transformer overload cutoff, in addition, the output relay in PSL-621C protection device does not have the ability to resist AC voltage interference, then together induce the maloperation of relay. Through scheme comparison, a rectification scheme is proposed that the trip circuit of transformer overload cutoff is started by the UEG-F, and based on proposed power model, the safety of rectified circuit is verified.