The study of propagation characteristics of partial discharge in transformer

Partial discharge (PD) measurement is a widely accepted nondestructive technique to detect incipient insulation faults. The PD signal gets degraded in amplitude and shape while propagating through the insulation structure. Depending on the location of monitoring, the original PD signal may get degraded. A laboratory study is undertaken to study the propagation characteristics of injected PD signals at various locations of a 3 phase 1 kVA transformer. Three fast rise PD sources are developed using UJT and mercury switches, and the signal is injected across one turn of outer winding at different locations. The responses at different tap points and phases are recorded using a 500MHz/4 channel digital oscilloscope with a sampling rate of 5GS/s. The results are analyzed in both time and frequency domains. Analysis is made using wavelets and an experimentally determined transfer function. Decomposed high and low frequency signals with wavelets show different attenuation for the same injected signal at different locations. The transfer function analysis shows that it is very sensitive with respect to PD injected location and the phase terminal at which response is measured. Also, it is dependent on the shape of injected PD pulses indicating the dependence of propagation path on the frequency content of injected PD signals.