Abstract: With the rapid development of China's economy, the scale of power grid continues to grow, and the growth of power demand is increasing day by day, the stable operation of power system is particularly important, and the correct action of relay protection device is closely related to it. Transformer is the main component, in which the main protection can protect itself, and the backup protection can be used as the backup protection of adjacent components, and the two cooperate with each other to improve the stability of the system; The following is a brief analysis of a trip accident caused by the malfunction of complex voltage and over current on the medium voltage side of the main transformer caused by a lightning strike line fault, and learn the line protection principle and corresponding treatment measures to add a layer of guarantee for the stable operation of the system.