Electrical Performance of Ester Insulating Liquids for Power Transformers

Ester insulating liquids, including both synthetic and natural esters, have been intensively studied in recent years, because the good biodegradability and high fire safety of these liquids bring the potential for manufacturers to produce and utilities to operate transformers which have lower intrinsic environmental risks. Depending on the application, transformers filled with ester liquids either directly reduce the environmental risk or allow reductions in the costs associated with the provision of additional environmental protection or leak mitigation. A collective research effort has been undertaken by the UK electric power industry on ester liquids to determine their suitability for use in higher voltage power transformers. This paper focuses on the electrical performance of ester liquids. Experimental tests investigating discharge initiation, propagation and breakdown under both AC and lightning impulse voltages in non-uniform fields were carried out on a synthetic ester and a natural ester, using mineral oil as the benchmark for comparison. The results indicate similar discharge inception levels for all three types of liquid, however discharge propagation characteristics (discharge amplitude and number, streamer length and velocity) were found to be different and somewhat worse for esters compared with mineral oil. The tests also showed that ester liquids have relatively lower breakdown strengths than mineral oil. In addition, it was found that a pressboard surface does not influence streamer propagation and breakdown under lightning impulse voltage, but it does promote propagation and breakdown under AC voltage stress, particularly on discharges occurring in the negative half cycle. These differences in the behaviour of ester liquids would probably need to be taken into account in the design of transformers at higher voltages.