Behavior of air pockets in an oil-filled insulation system

Failures of high-voltage oil-polypropylene insulated capacitors during operation at 20 kHz were experienced after the capacitors had been filled with new transformer oil. Examination indicated that air pockets trapped in the insulation system during the oil fill had ionized with subsequent damage to the polypropylene. An investigation of the behavior of ionizing air pockets was carried out by means of a model capacitor. A conducting glass electrode permitted visual observation of the conditions in the air pockets. Radically different behavior was noted depending on the type of transformer oil used.