By hvhipot 
Partial discharge is mainly caused by the winding or transformer oil moisture, between the winding, between the winding and the core or box wall.
Insulation distance is reduced, the winding internal welding or poor contact. Transformer partial discharge time is long, will make the insulation by slow erosion, once the accumulation of serious, eventually lead to transformer winding insulation breakdown therefore, the strength of the local discharge determines the length of transformer insulation life.
Partial discharge testing is limited by the following points: Pd background noise levels are determined by interference, and serious interference signals make PD measurements impossible. Pd measurements are not yet available to combine quantitative and positional measurements. This method requires specialized testing instruments. Such as ultrasonic positioning test device is usually by the ultrasonic sensor, partial discharge detector, multi-channel transient waveform storage and signal processor Partial Discharge Measurement method is divided into several parts such as electrical measuring method and electric measurement method are two kinds of electrical measuring method is widely applied pulse current method and the method of wireless interfering voltage, mainly used for the quantitative measurement of partial discharge, It can only roughly determine the electrical position of the power supply, and can not exactly point out the space position of the power supply, which is its deficiency non-electric measurement method, mainly sound measurement method, light measurement method, the method can not be used for quantitative measurement, but can determine the space position of the power supply.