Effect of repetitive impulse voltage on models of EHV transformer windings to determine ageing of paper insulation

Impulse testing of transformer involves a defined sequence of impulse applications at the specified voltage levels. However, in case of failure/damage during these tests, the winding gets subjected to repeated impulse shots, after repair. In this paper the effect of such repetitive impulses on transformer insulation has been investigated. Typical thicknesses of insulating paper as well as disc-to-disc winding models have been subjected to repetitive impulse stresses as encountered in actual service. The performance has been monitored at regular intervals by measurement of tensile strength and degree of polymerization (DP) for paper and partial discharge (PD) parameters of winding models.