Lab investigation of PD development in transformer winding

Two winding insulation breakdown failures in Extra High Voltage transformers are investigated. The discovered cause, partial discharge development between winding turns, which is normally considered rare, is studied through lab simulation. Simplified winding samples are tested under a combination of various environmental conditions, including the electric field, temperature and humidity. Studies revealed that the deterioration of oil-paper insulation in transformer winding can be defined in four stages from the initiation of partial discharge to the final breakdown according to the detected partial discharge phenomena. Each stage is identifiable by the trending of partial discharge pulse magnitude, the repetition rate, the power spectrum, the phase-count in 2-D models, and the accumulation of the dissolved gas components, etc. The above partial discharge deterioration stage can be identified by all signals including the IEC current pulse signal, the ultra wide band current pulse signal, the ultra high frequency signal, though each with a different sensitivity. Such results, although not applied in field investigation yet, has the potential to be employed in early warning of partial discharge development and the local insulation degradation.