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Title: Characteristics of creepage discharges along ester-pressboard interfaces under AC stress
Author: Yi, Xiao
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2012
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Ester liquids including natural ester and synthetic ester are considered as potential substitutes for mineral oil, due to their good biodegradability and high fire points. Although these liquids have been widely used in distribution and traction transformers, research efforts are required for the purposes of design and manufacture of high voltage and large power transformers which are filled by esters. Indeed, it would be risky to apply esters in large power transformers without thorough understandings of their behaviours in large gaps and/or when combined with pressboard insulation. Therefore, investigations of creepage discharges along the surface of pressboard in esters are vitally important and their behaviours should be compared with those of mineral oils. This thesis is aimed to investigate the creepage discharges along pressboard in esters and mineral oil under ac divergent electric field. Apparent charges, current signals and images of streamer channels were obtained synchronously to identify whether and how the introduction of pressboard surface would influence the inception and propagation of discharges as compared to tests in open gap. When over-stressed by higher voltages, the surface tracking along the pressboard-ester interface, triggered by sustaining creepage discharges, was studied and the evolutions of accompanying creepage discharge patterns were investigated. In these experiments, both esters and mineral oil impregnated pressboards were comparatively studied. The test results indicated that at the inception stage, the presence of pressboard or any other solid types in different liquids under test do not influence the PD inception voltages; in the propagation stage, solid surface tends to promote the development of discharges, especially those occurring in negative half cycles, and shifts more discharges towards the zero-crossing phase angles. This discharge promotion effect is much more evident in esters than in mineral oil, probably because of higher discharge intensity in esters and higher viscosity of esters. The space charge effect and the residual low density channel effect are proved as the mechanisms best explaining the influences of solids on creepage discharges. Under higher voltages, it was found that the impregnated pressboard is susceptible to discharge erosion characterized by “white and carbonized tree-shaped marks”, due to intense discharges occurring on or near the pressboard surface. The “white mark” appears at a lower voltage and propagates more easily on ester impregnated pressboard. The gaseous “white mark” channels will attract the subsequent discharges to follow the same discharge routes; the accumulative energy dissipation in these channels will then result in the carbonization of the channels. Once formed, the surface tree-shaped mark can continue to grow even under reduced voltage levels until it bridges the gap and causes the final flashover.
Supervisor: Wang, Zhongdong Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: creepage discharge ; ester insulating liquid ; surface tracking ; pressboard ; partial discharge