Volt-time characteristics of transformer oil under impulse voltage with different waveforms

Abstract

This paper conducts an experimental investigation about the breakdown in transformer oil under the action of impulse voltages with different waveforms for the insulation of the transformer in field may be subjected to lightning and switching overvoltage. In the paper, an impulse voltage generator is built based on the solid state switch and it can produce different kinds of impulse voltage. Then the breakdown tests are carried out under the standard lightning, the standard switching and the nonstandard impulse voltage. Specifically, the planeplane electrodes immersed in the bulk of the transformer oil are used to form a quasi-uniform electric field gap. The clean oil samples (oil samples were filtered, degassed and dehydrated) are used to avoid the effect of particles and moisture on the breakdown strength of transformer oil. The electric field strength of the parallel-plate electrodes is taken into consideration in order to determine the properties of breakdown. The impulse voltage is increased step by step following standard IEC 60060 with a step increment of 2 kV for positive polarity (15 shot/step) until the oil gap breakdown takes place every shot. The description of discharge and breakdown in the oil gap is obtained by the measurement of impulse voltage. Meanwhile, the breakdown voltage and breakdown time for impulse voltages with different waveform parameters are obtained and then mathematically analyzed the obtained data. The influence of wave front/tail time on the Volttime characteristics of oil insulation is experimentally studied. The results show that the breakdown voltage is closely related to the front time of the impulse voltage. The factors that influence the gap breakdown voltage are finally analyzed and discussed.