Investigation on Transformer Oil Impedance during Breakdown under Impulse Voltage

This paper presents an experimental investigation on transformer oil subject to impulse voltage. The dynamic impedance of the mineral oil samples was determined from the instantaneous ratio of voltage and current characteristics at the instant of breakdown. A dedicated test cell was constructed to contain the insulating oil. A sphere-sphere electrodes gap of variable length was used for the experiments. Series of test have been conducted for 1 mm inter-electrodes gap, without the use of non-inductive resistor, and through different non-inductive resistors: 50 Omega, 4,8 k Omega and 9,6 k Omega connected in series with the test cell. Other series of tests have been conducted through a non-inductive resistor of 50 Omega, connected in series with the test cell for three different inter-electrodes gaps; 1mm, 2,5 mm and 5mm. The results revealed that the oil impedance wave shapes showed similar variation for all gap lengths. However, the gap length and the existence of non-inductive resistor in series with the test cell have a significant influence on both the impedance magnitude and the decay time taken to reach the minimum impedance.