Effect of contaminant on breakdown characteristics of mineral oil and commercial natural ester

This research paper presents the results of an investigation into the effect produced by the addition of various contaminant particles at various temperatures on the 50 Hz AC breakdown voltage of mineral oil and natural ester. The test cell apparatus employed mush room-shaped electrodes according to IEC 60156 and the various liquid samples which were prepared for the tests contained various combinations of the following: a) three different contaminant particles (cellulose, spherical iron and copper) of approximately 20 µm diameter, b) three different concentrations of contaminant particles (0.001% m/V, 0.003% m/V and 0.005% m/V) and c) four different temperatures for the test cell liquids (25°C, 50°C, 70°C, and 90°C). The test results show that the presence of the added particles in the tested dielectric liquids resulted in a reduction of breakdown voltage, with the 4 major determinants of the reduction being a) change in particle type, b) change in particle concentration, c) change in liquid type, and d) change in liquid temperature. The most important findings from the testing and analysis conducted are as follows: 1) higher contaminant concentrations resulted in lower breakdown voltages, 2) the addition of copper particles had the most significant effect on reducing breakdown strength, and 3) that the largest decrease in breakdown strength was recorded during testing of the contaminated mineral oil test samples and the FR 3mineral oil test samples at 70°C.