Breakdown Characteristics of Transformer Oil Insulation under Impulse Voltage with Different Waveform Parameters

This paper conducts an experimental investigation about the breakdown in transformer oil under the action of impulse voltages with different waveform parameters for the insulation of the transformer in field may be subjected to the 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. Five different impulse voltage waveforms are used: 1.4/50 mu s, 11/210 mu s, 60/700 mu s, 130/1700 mu s and 250/2600 mu s. Then the breakdown tests are carried out under these impulse voltages. Specifically, a needle plate electrodes immersed in the bulk of the transformer oil is used to form a nonuniform electric field gap. The different radii of curvature and different gap distances of the needle plate electrodes are also taken into consideration in order to determine the properties of breakdown electric field strength. The impulse voltage is increased step by step with a step increment of 2 kV for positive polarity (1 shot/step) until the oil gap breakdown takes place. The description of discharge and breakdown in the oil gap is obtained by the measurement of impulse voltage, transient currents and the emitted light. Meanwhile, the breakdown voltage and breakdown time for impulse voltages with different waveform parameters are obtained. And the average velocity of streamer propagation is deduced. The results show that the breakdown voltage is closely related to the front time of the impulse voltage, the gap distance and the radii of curvature of the needle electrode. The propagation time of the streamer increases with the increase of the impulse front time, also the breakdown time delay has the same characteristic. The factors that influence the gap breakdown voltage are finally analyzed and discussed.