Pre-energizing effect on breakdown voltage test for insulating liquids

One of the most commonly used tests for measuring insulating liquids is the determination of dielectric breakdown voltage at 50 or 60 Hz. It is accepted worldwide that the determination of the breakdown voltage at power frequency give very useful information mainly related with moisture and particle content. However, the actual set-up recommended by IEC and ASTM have some limitations in terms of statistical scatter of mean value of the breakdown voltage. The present paper shows the influence on the scatter of the breakdown voltages (IEC 60156 Standard) if a relative low electrical field is applied before carrying out the test (pre-energizing). The application of this charging field might be capable to release free electrons into the insulating liquid and might have the effect to trigger the breakdown in a more deterministic manner. An improvement on the scatter of the mean values of the breakdown voltage may also occur by increasing the number of shots.