Surface discharge characteristics of impregnated pressboard under AC voltages

Three typical electrode systems immersed in transformer oil, i.e., needle-plate, cylinder-plate, and sphere-plate electrode systems, have been used to study the characteristics of impregnated pressboard under AC voltages. According to careful observations, the six stages of the discharges which occur when the applied voltages are increased can be distinguished. The inception electric field strengths of the surface discharge in the three electrode systems were computed. The results of experiments and calculations may be useful for analyzing the surface discharge failures of pressboard barriers in power transformers which have occurred in recent years in China