Partial Discharge Behaviours and Breakdown Mechanisms of Ester Transformer Liquids under ac Stress

Mineral oil has been widely used in liquid insulation of power transformers. However, it is poorly biodegradable and could cause serious contamination to the environment if a spill occurs. With increasingly strict environmental rules and regulations, there is considerable interest from the Utilities to apply esters in power transformers as substitutions to mineral oil. In order to use esters in large power transformers, their dielectric properties should be thoroughly investigated. This PhD thesis covers the experimental studies on the dielectric properties of a type of synthetic ester (Midel 7131) and a type of natural ester (FR3) in both uniform and divergent electric fields, using a mineral oil (Gemini X) as the benchmark.The dielectric properties of transformer liquids in uniform fields were investigated using breakdown voltage tests. The breakdown voltages of esters should be at least similar to that of mineral oil to allow a replacement in transformers. To obtain a fair comparison, the AC breakdown voltages of well-processed transformer liquids were tested, and their distributions were statistically analyzed. Since the breakdowns of transformer liquids in uniform fields are caused by the weakest-links, conditions representative of in-service transformer liquids were also considered by testing the effects of extraneous factors, such as particles, water and electrode area.The divergent fields were produced by sharp needle electrodes with tip curvatures of a few micrometers. The dielectric properties of transformer liquids in such fields were studied using several methods. A traditional PD detector was used to study the partial discharge characteristics of insulating liquids, such as the inception voltages and the repetition rates. A high speed camera was utilized to identify the streamer generation, propagation and breakdown phenomena. An oscilloscope was used to investigate the current signals associated with these phenomena. The breakdown voltages of transformer liquids were also determined in the divergent field at various gaps. Furthermore, the fault gases in transformer liquids under partial discharge faults were determined and analyzed.The following findings and conclusions can be made from the research in this thesis:* The AC dielectric strengths of esters in uniform fields are similar to that of mineral oil when they are in a well-processed condition. When practical liquid conditions are considered, the AC dielectric strengths of esters are higher than that of mineral oil. * The partial discharge behaviours at overstressed voltages can be used to differentiate various transformer liquids. Esters are relatively inferior to mineral oil in terms of higher discharge amplitude, higher discharge repetition rate and more negative partial discharges. * Mineral oil possesses a higher ability to suppress the propagation of negative streamers. Thus, the AC dielectric strength of mineral oil in the divergent field is relatively higher than those of esters. * Esters generate the same types of fault gases due to electrical discharge as mineral oil, but in relatively larger amounts.