Characteristics of partial discharge during its evolution in transformer winding

Following the investigation of two recent transformer failures in winding insulation, researchers have carried out experiments to identify the cause of the failure, which was found to be the partial discharge initiation between transformer winding turns under harsh environment and its development to breakdown under normal operation condition over a long period. Further studies were carried out to simulate the entire `life´ of inter-turn partial discharges, i.e., from the discharge initiation to the final breakdown of oil-paper insulation. A lab test platform was built to simulate the evolution of such discharge, and to record all the necessary information during the course. This platform is equipped with the sensors and instruments to record the traditional IEC discharge current, the ultra wide band discharge current, the ultra high frequency signal, and the ultrasonic signal, plus the dissolved gas components. Experimental data were analyzed along the partial discharge development. It was found out that such course could be divided into several stages of different severity according to the combination of discharge characteristics. The characteristics discussed in this paper include the discharge magnitude, phase, repetition rate, and the frequency spectrum of discharge waveform. Possible application and associated limits are discussed for field investigation.