Preclusion of partial discharge from transformer insulation - a design perspective

Transformer operative life expectancy is governed by its insulation condition which is a complex arrangement of paper and pressboards submerged in mineral oil. It experiences persistent ageing due to steady physical and chemical deprivation being exposed to electrical and thermal stress in-service. The presence and or occurrences of even a minor defect in insulation can develop local concentration of electric field initiating partial discharges under normal operating voltages as well as under different electric excitation conditions. This Paper describes the different overstressed zones inside the transformer which need be taken care at design stage to reduce the chances of generation of partial discharges due to high electric field concentrations. Paper also discusses the methodology adopted by transformer designers to reduce the local concentration of electric field at particular zones and thereby reducing the chances of initiation of partial discharges. ELAX- 2D software based on Finite Element Method (FEM) was used to demonstrate the electrically overstressed zones inside the transformers and appropriateness of different methodologies adopted to preclude the partial discharges from transformer insulation. Partial Discharge test was conducted on analysed transformers and results were discussed to prove efficacy of the methodologies adopted to manufacture Partial Discharge free transformers.