Partial Discharges at Fast Rising Voltages

Doctoral thesis, 2014

The present demand for higher efficiency and flexibility in the energy sector has led to an increased use of power electronic generated waveforms as these allow energy conversion between different frequencies including DC. The generated waveforms are usually synthesized by so called Pulse Width Modulation (PWM) techniques, where the desired waveform is approximated by a number of square shaped pulses with a short rise time. Applications such as variable speed drives and reactive power compensation are saving vast amount of energy. This makes it important to understand how rapidly rising voltages affect insulation systems. In particular this applies to partial discharges (PDs), which are considered as affecting the life time of insulation considerably. This thesis presents an investigation regarding the behavior of PDs for different voltage waveforms characterized by steep rise times. The analysis method is based on moderately sharp frequency filters in the PD decoupler, high-resolution digitizers and time-domain stochastic filtering. Although entirely passive, the PD decoupler filter suppression can be made to change two orders of magnitude in half a decade of frequency enabling studies down to nanoseconds in the PD rise time. Voltages of different rise times are employed, which resulted in significant differences in the PD behavior. Applying square-like voltages to cavities with dielectrically insulated electrodes significantly affects the discharge amplitude, its rise time, the extinction voltage and the distribution shape. The investigation shows that the amplitude of the PDs increases considerably while the rise time of the PD signal decreases for shorter voltage rise times. Thus continued investigations applying PWM waveform of different level of filtering (smoothness) is important. An approach is presented which accepts both timing jitter and a non-integer relation between carrier and modulating frequency. The modified method is first used on the cavity test object and the observations found for semi-square wave forms were confirmed also on other insulation system such as a motor stator and a twisted pair test object. For the phase resolved PD (PRPD) pattern to become similar to the normal AC pattern, it is required that the remains from PWM steps are lower than the extinction voltage. Thus limits for a sufficient smoothing level are found, which is of importance when designing insulation systems exposed to fast transients. To illustrate the degradation process, microscopic images show how the rise times affect the cavity surface deterioration, observations that are consistent with the electrical measurements. The same observations were found valid on motor insulation. Finally a simulation model was introduced with sufficient detail to reproduce the observations. A relation between over voltage level and PD quench voltage was found necessary to enable the reproduction of the measured results.. This also results in a lower extinction voltage for short rise times.