Case Studies of PD Measurement Experiences on Site Maintenance and in Workshop Fault Investigations of Oil-filled Power Transformers and Shunt Reactors

It is very difficult to initiate any form of partial or full electrical discharge in well-designed, properly dried and impregnated oil-filled transformer and reactor insulation, manufactured with sufficient quality workmanship. However, partial discharges do occur in oil-filled transformers and reactors after successful field commissioning or during field operation, as proven by analysis of dissolved gas in the oil and, rarely, by off-line conventional field tests. In order to initiate a discharge, there must be a defect in the insulation system, such as moisture, voids, bubbles, delamination, free metal or fixed metal particles or fragments, poor contact or connection of electrostatic shielding, surface tracking and static electricity.