Improvement of the test method for 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 is raised up linear from zero to the breakdown value usually by 0,5 ... 3 kV/s. After filling the test cell with the test oil there is a pause between 3 to 10 minutes until the start of the first breakdown test 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, 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. Further repeated tests lead frequently to contradictory results. So the standard oil test is very common but the results are sometimes not satisfied. Therefore we have performed breakdown tests with changing test parameters like pause times, different voltage raising up velocities and with and without stirring. Further we have tried to use additional methods for filling the test cell with test oil or better cleaning of the electrodes after each breakdown. And last but not least we have also created a new test cell. This paper will show the results of our efforts