Investigation on transformer oil impedance during breakdown under AC voltage

In this paper, we present an experimental study on the properties of mineral oils under AC breakdown conditions. Breakdown tests were carried out on mineral oil samples using a sphere-sphere electrode test cell for various gap lengths 1 mm, 2.5 mm and 5 mm and at different voltage speed ramps: 0.5 kV/s, 2 kV/s and 3.5 kV/s. The dynamic impedance of the mineral oil samples was determined from the instantaneous ratio of voltage and current. Such dynamic impedance can be used to explore the properties of the arc and its development within the oil. The results of this investigation show that the gap length and the voltage speed ramp have a significant influence on both the impedance magnitude and the decay time taken to reach the minimum impedance.