New test method for dielectric breakdown voltage of insulating oils

Standard dielectric breakdown tests for insulating oils consist of five or six single breakdowns in a glass or a plastic vessel with about 0.5 litter test oil content. For each breakdown the voltage gets raised up linear from zero to the breakdown value usually with 0.5 ... 3 kV/s. After filling the test oil in the test cell until the start of the first breakdown test there is a pause between 3 to 10 minutes. Between each breakdown there are breaks about 1 ... 2 minutes. Optional a stirrer produces an oil circulation in the test oil. The form of electrodes are different, according VDE (sphere formed) or ASTM (flat formed). With these single breakdown values the mean value, which height is a parameter for the oil quality, and the coefficient of variation were calculated. A weak point of the standard dielectric breakdown tests is that it is possible to reach values for the coefficient of variation up to 30 % and higher. In this paper it is shown that it is possible to influence the coefficient of variation by changing of different test parameters and also the test procedure.