Comparative evaluation of parameters of the dielectric breakdown test on transformer oil

The standard AC dielectric breakdown test result is generally used as one of the acceptance criteria for insulating oil and a maintenance tool for high-voltage power transformers in service. Using the standard electrode parameters and voltage application procedure, a small-oil-volume breakdown is obtained. Unfortunately however, these test parameters are far from field conditions. Therefore, oil breakdown results were systematically evaluated and compared using ASTM and IEC standard procedures and varying many of the test parameters such as the shape and dimensions of the electrodes, the oil circulation, the voltage application procedure, etc. Both clean and contaminated transformer oils were tested. The results reflect the surface and volume effects, giving lower breakdown voltages when a larger coaxial electrode arrangement replaces the standard shape. A 1-min step-by-step voltage application is also found to give lower breakdown voltages. Finally, the effect of water and solid particle contamination is evaluated and compared for different test methods