Evolution and discharge pattern of creeping discharge at aged oil/pressboard interface

The present paper investigates progressive creeping discharge at oil/pressboard interface in power transformer. Using a needle-to-plate electrode configuration, AC constant stress tests were carried out with differently aged pressboard samples. Two distinctly different discharge patterns have been observed, namely chronic creeping discharge and acute creeping discharge. Chronic creeping discharge happens with all pressboard samples, whereas acute creeping discharge happens only with massively-aged pressboard. The results of partial discharge detection, on-line temperature monitoring and gas analysis indicate that the chronic creeping discharge is closely correlated o oil phase, while the acute creeping discharge involves multiple discharge mechanisms. It is also found carbonization crack in pressboard interior is the principal cause of white mark and bubble generation during acute creeping discharge.