Abstract: In order to detect and alarm in time before transformer (reactor) insulation breakdown, active protection of tripping before explosion burning, In addition to the traditional transformer power and non-power protection, research and development of comprehensive protection devices that can combine online monitoring status as a criterion, to effectively solve the explosion-proof and flame-proof problem of large transformers. State Grid North China Branch took the lead in organizing North China Electric Power Research Institute and Nari-Realys Electric Co.Ltd.to develop an embedded transformer active defense device that cooperates with the existing comprehensive monitoring device. Transformer active defense device receiving station online monitoring device transmission transformer high frequency, ultrasonic partial discharge signal and other online monitoring information, And analyze and calculate to realize transformer insulation fault early warning, alarm and active protection functions.