Before partial discharge, the electric field force, mechanical stress and particle force around the discharge point are in relative equilibrium. When partial discharge occurs, the rapid release or migration of the charge changes the electric field, breaks the equilibrium state, and causes the surrounding particles to oscillate mechanically, thus producing sound or vibration signals, as shown in FIG. 4-1. The ultrasonic method measures partial discharge signals by installing ultrasonic sensors on the external wall of the equipment chamber. The characteristic of this method is that the sensor has no connection with the electrical circuit of the geographical equipment, and is not disturbed by the electrical equipment, but is easily affected by the ambient noise or the mechanical vibration of the equipment when used in the field. Due to the large attenuation of ultrasonic signals in the insulation materials commonly used in power equipment, the detection range of ultrasonic detection method is limited, but it has the advantage of high positioning accuracy. The spectrum of sound waves produced by partial discharges is very wide, ranging from tens of Hz to a few MHz. Signals with frequencies below 20kHz can be heard by the human ear, while ultrasonic signals above this frequency must be picked up by ultrasonic sensors. By measuring the sound pressure of ultrasonic signal, the discharge intensity can be estimated. In addition, the ultrasonic method can also detect different types of faults, such as suspension, tip, along the surface, etc., online detection does not affect the normal operation of equipment, has a high sensitivity, good anti-electromagnetic interference ability and has the advantages of high positioning accuracy.