Particle Effect on Breakdown Voltage of Mineral and Ester Based Transformer Oils

ac Breakdown voltage tests are usually used for quality check of mineral oils during transformer operation. When studying ester-based oils, previous experimental research including the ones carried out at University of Manchester laboratories, were based on `as-received\´ oil samples and particle effects were not quantified. Similarly, water effects on oils determined previously were also ambiguous since water can be combined with particles to reduce breakdown voltages. This paper used mineral and ester-based transformer oils, i.e. 10 Gemini X as mineral oil, Midel7131 as synthetic ester-based oil and FR3 as nature ester-based oil, in the forms of "as-received" and `processed\´, and their AC breakdown strengths were comparatively studied. Weibull distributions of ac breakdown voltages of three oils were discussed. It can be seen from the results that particle effects on ac oil breakdown voltages are dominant for `as-received\´ mineral oil and the `processed\´ mineral oil has a distinctly different breakdown characteristic from `as-received\´ mineral oil in terms of the effect of water contents.