Effects of surface partial discharges on the dielectric strength of ester oil-impregnated pressboard insulation in power transformers
Date
2021
Authors
Ndlovu, Bakhekile A
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Abstract
The importance of the smooth operation of power transformers to ensure continuity
of supply is indisputable. One of the main transformer failure modes is insulation
deterioration because the transformer insulation system constitutes approximately
60% of the transformer material. The aim of the present investigation was to
determine the effects of surface partial discharges (also referred to as creepage
discharges) on the AC breakdown voltage of ester oil-impregnated pressboard
compared to the same effects on mineral oil-impregnated pressboard. This was done
by exposing the impregnated pressboard to surface discharges for 0, 3 and 7 hours,
followed by subjecting the respective pressboard samples to AC breakdown
voltage. The phenomena observed on both the mineral oil and ester oil-impregnated
pressboard under exposure to surface discharge activity include bubbles that
formed, tree-like white marks propagating from the needle-point to the ground
plane and carbon-tracks emanating from the needle point. Some of the mineral oil-impregnated pressboard samples exhibited burn marks as a result of high electric
stresses which resulted in further insulator deterioration. The subsequent
breakdown voltage results showed a general decline in the breakdown voltage of
both the mineral oil and ester fluid cases as the time exposure surface discharges
increased from 0 hours to 7 hours. However, the ester oil-impregnated pressboard
breakdown voltage values were consistently higher than the mineral oil case by an
average of 20%. The observed discharged phenomena and obtained breakdown
voltage results are explained using various surface discharge mechanism theories.
Exposure of pressboard to surface discharges deteriorates its dielectric strength.
The surface discharge mechanisms include insulation deterioration due to cracking,
oxidation, high electric field stress and the space charge effect. The present
investigation demonstrates how surface discharges affect the breakdown voltage of
ester fluid and mineral oil-impregnated pressboard and concludes that ester fluid is
a viable possible alternative to mineral oil with regards to the ability to withstand
the effects of surface PD on solid insulation in oil filled power transformers
Description
A dissertation submitted to the Faculty of Engineering and the Built Environment,
University of Witwatersrand, Johannesburg, in fulfilment of the requirements for
the degree of Master of Science in Engineering, 2021