Investigating the Effects of Creepage Discharges on the Breakdown Voltage of Natural Ester Oil Impregnated Pressboard in Power Transformers

Power transformers are the most expensive and important equipment in the transmission network. It is therefore crucial that they are well protected to avoid damage of equipment and possibly loss of life. The common transformer failure modes are due to dielectric failures. The aim of this project is to subject pressboard to creepage partial discharges and investigate the effects on the AC breakdown voltage. Furthermore, since ester oils are considered to have superior characteristics to mineral oil, the effects of creepage discharges on the respective breakdown voltages are investigated and reported. This is done by using an inclined needle-plane electrode configuration, with an electrode gap of 25 mm. Creepage discharges are generated for various times and the pressboard samples undergo breakdown voltage tests. Ester oil was found to withstand breakdown voltages up to 20 % higher than mineral oil. Generally, a downward trend was observed as the exposure to creepage discharges increased for both oils. The Weibull distribution model reveals that a brand new mineral oil-impregnated pressboard sample can withstand breakdown at the same level as an ester oil-impregnated pressboard sample that has been exposed to creepage discharges for 7 hours. Ester oils can therefore improve the lifetime of pressboard (and thus the transformer itself) regardless of exposure to insulation deterioration through creepage discharges.