Partial Discharge Diagnostic Testing of Electrical Insulation Based on Very Low Frequency High Voltage Excitation

Electrical insulation plays an important role in the proper functioning of high voltage power system equipment/components. Examining the condition of insulation is crucial to keep the equipment safe and functioning efficiently. High voltage diagnostic tests, in particular partial discharge measurements, are very effective in detecting early signs of insulation damage. This type of diagnostic test is generally conducted at the power frequency to emulate normal operating condition. However, it is difficult to perform the test on-site due to the large reactive power required when testing high-capacitance objects such as cables. An alternative approach is to conduct the test at very low frequency excitation, commonly at 0.1 Hz, because the required power is proportional to the applied frequency and thus is significantly reduced. However, partial discharge behaviour varies with frequency and thus existing knowledge on interpretations of partial discharge at power frequency cannot be directly applied to test results measured at very low frequency for insulation diagnosis. The motivation of this research is to study partial discharge behaviours at very low frequency and search for physical explanations of such differences. Therefore, this thesis explains those differences in two types of partial discharge, corona discharge and internal discharge, based on extensive experimental measurements and computer simulation. Partial discharge patterns were obtained and analysed using the phase-resolved partial discharge technique. A comprehensive study of corona discharges at different applied voltage waveforms, such as sinusoidal wave and square wave, was carried out under the excitation of very low frequency. Experimental results showed that the inception voltage of corona discharges at very low frequency is dependent on applied voltage waveforms. Furthermore, effects of ambient air on corona discharges were investigated thoroughly at temperatures between 20C and 40C at very low frequency excitation and power frequency for comparison purposes. Measured corona discharge characteristics showed that the increase of ambient temperature results in larger discharge magnitude and causes corona discharges to occur earlier in the phase of the voltage cycle. This research also investigated internal discharge behaviour in a cavity at very low frequency using measurement and simulation. Measurement results showed that partial discharge characteristics are strongly dependent on applied frequency. A dynamic model for numerical computation was developed to study this dependence. The advantage of this model is that it has minimum adjustable parameters to simulate discharges in the cavity. These values were determined using a trial and error approach to fit the simulation results with measured data. Simulation results showed that charge decay has a significant contribution to discharge characteristics at very low frequency. Charge decay causes a reduction of the initial electron generation rate which results in lower discharge magnitude and repetition rate. Also, the statistical time lag of discharge activities was calculated and found exhibiting a great dependence on applied frequency. All in all, the major contribution of this thesis is the development of a dynamic model to characterise physical processes of partial discharge in a cavity. It enables determination of key parameters influencing partial discharge behaviour such as the statistical time lag and the charge decay time constant at different applied frequencies. Moreover, differences in partial discharge characteristics at very low frequency and power frequency as a function of cavity size, voltage waveforms and ambient temperatures are discussed and explained in detail. The findings from this research provide better understanding of discharge behaviours at very low frequency excitation.