Abstract
Steep voltage transients from power converters and vacuum circuit breakers result in increased electric stress and risk of insulation degradation in transformers. This paper presents a time-to-breakdown study for impregnated pressboard using a plane-to-plane electrode set with circular razor-sharp upper electrode on a thin pressboard sheet. The partial discharge repetition rate was measured for each sample by stepwise increasing the voltage at 60 second intervals with sinusoidal or switched voltages, respectively. The PD repetition rate for switched voltage was much higher compared to sinusoidal voltage. Voltage endurance tests were then performed at several voltage amplitudes for both sinusoidal and switched bipolar voltage. The result shows that the time-to-breakdown for switched voltage compared sinusoidal voltage was reduced by a factor of more than 105 with equal peak voltage and frequency. This difference is explained by the creation of a significant amount of space charges around the sharp electrode that act as homocharge and reducing the field for sinusoidal voltage, while it acts as heterocharge and strongly enhance the field for polarity reversal at switched voltages. These experiments show that care should be taken when introducing fast voltage changes to transformer insulation systems such as impregnated pressboard.