Creepage Discharge Investigations with Ester-based Dielectric Liquids Under Switching Impulse

The use of ester-based dielectric liquids is growing rapidly and more projects are now being specified with transmission voltage level transformers containing these liquids. It has been acknowledged through previous works that there are some differences in electrical behavior between ester-based liquids and mineral oil, which has traditionally been used for transformers. To optimize designs for ester liquids the behavior of the liquids under various test waveforms need to be understood. This work aims to evaluate the creepage discharge strength of four different ester-based liquids (three natural, one synthetic) in comparison to mineral oil, when subjected to high voltage with standard switching impulse wave-shapes (250/2500µs). The study of creepage discharge behavior was carried out at Budapest University of Technology and Economics using an electrode arrangement that was based upon the Weidmann creepage arrangement [1] which has been used in the past to derive design curves for transformer windings. The applied waveform adhered to the standard requirement for switching impulse of 250/2500µs and the conditions were carefully controlled to provide a direct comparison between the liquids. The results of the investigation showed that the trend of breakdown voltage with increasing gap size under both positive and negative switching impulse was very similar for all five of the liquids tested using this test arrangement.