Effect of temperature on the electrical breakdown strength of insulating oils

The aim of the paper is to analyse the effect of temperature on the electrical breakdown strength of transformer oils and to compare the behaviour of the mineral oil Mogul Trafo CZ-A with the synthetic oil Shell Diala S4 ZX-I. The measurements were carried out in the high voltage laboratory at the Department of Electric Power Engineering of the Technical University of Košice. The electrical breakdown strength was measured in the temperature range from 20 °C to 100 °C with a step of 20 °C, when applying alternating and direct voltage with both positive and negative polarity. The results show that with increasing temperature the electrical breakdown strength of both oils initially increases, reaching a maximum at a temperature of approximately 80 °C. With further increase in temperature, a slight decrease in strength is observed due to changes in the physical properties of the oils. In all measured cases, Shell Diala S4 ZX-I oil achieved higher electrical strength values than Mogul Trafo CZ-A mineral oil, which confirms its suitability for modern electrical power applications with higher temperature stress.