Abstract: In recent years, transformer explosion and combustion accidents have occurred frequently; however, the existing transformer differential protection and other main protection are passive defense after insulation breakdown, and cannot defend actively against transformer explosion and combustion. The discharge signal can react to the fault quickly, but it is easy to be disturbed and difficult to be applied. To solve this problem, based on the transformer on-line monitoring technology and relay protection technology, this paper studies the active defense technology based on the transformer discharge signal to protect the transformer before the insulation breakdown, and puts forward the protection principle of tripping in the high energy discharge stage. The discharge experiments of 500 kV true transformer, laboratory equivalent model discharge experiment and discharge simulation study were carried out, and the development law of transformer discharge characterized by discharge signal was obtained. On this basis, the transformer active defense technology system was constructed, the active defense strategy of "three signals, three parameters, seven modules" was established, the active defense device was developed, the threshold setting method of the active defense device was proposed, and a series of tests and tests were carried out. At present, 17 sets of devices are running normally, which verifies the correctness of the active defense technology and the reliability of the active defense device.