A comparison of breakdown properties of natural and synthetic esters at high voltage

Ester-based insulating liquids are considered today for electrical insulation in medium and high voltage transformers. The main advantages expected are their non-toxicity, excellent biodegradability, and fire resistance. Esters can be either of natural origin or synthetic. In this work, we present a study of prebreakdown and breakdown properties at high voltage and large gaps in several natural and synthetic esters, and a comparison with the well-know mineral transformer oil. This study is focused on the study of streamer propagation in large gaps up to 10cm. Compared to mineral oil, the transition to fast streamers in esters occurs at much lower voltage. Measurements also show no difference between esters, whatever their origin, viscosity, etc. From these results, it appears that the properties of esters are less favorable for very high voltage insulation than in mineral oil: once triggered, streamers are able to propagate very quickly to breakdown at lower voltage. In consequence, esters show a lower resistance to fast transients such as the standard lightning impulse.