Partial Discharge and Streamer Characteristics of Transformer Liquids under AC Stress

UoM administered thesis: Phd

  • Authors:
  • Zhao Liu

Abstract

Pre-breakdown phenomena in transformer liquids have been extensively investigated. The published work can be broadly categorised into streamer and partial discharge (PD) studies, with the former focusing on physical nature and the latter being more relevant to industrial applications. Mineral oil, as the dominant candidate, has been used in power transformers for over a century. In the past decade, there has been an increasing interest in filling power transformers with alternative liquids, e.g. esters and gas-to-liquids (GTL) based oils. This work aims to correlate the PD and streamer characteristics of three transformer liquids under AC stress. The liquids include a conventional mineral oil (Gemini X), a GTL oil (Diala S4 ZX-I) and a synthetic ester (MIDEL 7131). A circuit arrangement in compliance with the IEC 60270 was used, which allowed PD measurements, wide-band current measurements and streamer shadowgraphs to be obtained simultaneously. To simulate the quasi-uniform electric fields in transformers, a plane-to-plane electrode system incorporating an adjustable needle protrusion (PNP) was employed. A needle-to-plane electrode system (NP), which is widely used in the field, was also employed to provide reference results. Based on the PD measurement results, the PD inception fields (PDIFs) of the three liquids were found to be independent of electrode geometry for the investigated tip radius. The PDIF of the synthetic ester is about 13% lower than that of the mineral oil or the GTL oil. Compared with the PD magnitude, the pulse repetition rate is more sensitive to liquid type. At the same voltage under both the NP and PNP configurations, the synthetic ester has the highest pulse repetition rate, followed by the GTL oil, and then the mineral oil. In divergent electric fields (provided by the NP electrode system), it was found that the streamers in the three liquids have a similar stopping length at the same voltage, even though the apparent charge readings are not the same. The correlations between PD and streamer characteristics indicated that the synthetic ester has the highest branching tendency, and has therefore the smallest stopping length per unit of apparent charge among the three liquids. In quasi-uniform electric fields (provided by the PNP electrode system), the streamer branching tendencies of the three liquids were largely suppressed. The change from a propagation-induced breakdown in divergent fields to an initiation-induced breakdown in quasi-uniform fields was explained. The correlations between PD and streamer characteristics revealed that the same apparent charge can indicate different levels of streamer development in the insulation, depending on the uniformity of the electric field. Overall, interpreting PD measurement results needs to take the electric field uniformity (PD location) as well as liquid type into consideration.

Details

Original languageEnglish
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Award date1 Aug 2017