AC capacitor failure mechanisms

Capacitors with a cylindrical feedthrough geometry are often used to protect sensitive electronic equipment from extraneous power line transients and conducted electromagnetic interference. It is important that such capacitors have high reliability, long life and stable electrical parameters when subjected to moderate temperature changes. In an effort to determine the reasons behind recurring failures in power frequency rated capacitors, a series of tests was conducted on 300 volt AC rated capacitors at elevated temperatures. Many of the individual units were dissected to determine aging related failure mechanisms. At the beginning of the study it was assumed that most of the capacitors would show failures at the foil edges where maximum field stress is known to exist. The findings indicated that a few failures occurred at the edges as anticipated. The majority of failures occurred in the central portion of the dielectric. There was evidence of gas evolution in the dielectric system at elevated temperatures, as indicated by low partial discharge initiation voltage. In addition, practically all the failures occurred either at the start of the coil winding or at the foil end making up the final turns of the dielectric. The assumptions for dielectric failures in the central regions are given. Furthermore, a few nonfailed capacitors were unwound to determine the existence of prefailure mechanisms. Indications of similar incipient failure mechanisms in these nonfailed capacitors are discussed.