Effect of Surface Discharges on Lightning Impulse Breakdown Voltage of Oil-Impregnated Pressboard in Power Transformers

Abstract

Power transformers inevitably experience impulse over-voltage stresses such as from lightning and/or switching events despite various protection systems that may be in place. At locations along oil/pressboard interfaces in the transformer, surface discharges may initiate due to various reasons. The surface discharges may or may not eventually lead to voltage flashover faults. This paper presents a study of how relatively small surface discharges affect the lightning voltage impulse (LI) strength of oil-impregnated pressboard insulation. It is found that surface discharges of magnitudes in the order of some hundreds of pC can reduce the LI withstand voltage of the oil-impregnated pressboard by up to 15%. Although the negative polarity LI withstand voltage of surface discharge-aged pressboard is higher than that of positive polarity, the reduction in the LI voltage strength is more pronounced for the negative LI impulse voltage. The findings suggest further scrutiny of the standardised acceptable levels of PD in power transformers.