Detection of Partial Discharges at Fast Rising Voltages

Licentiatavhandling, 2011

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, while HVDC transmission would not be possible without them. This makes it important to understand how rapidly rising voltages affect insulation systems. Despite the obvious advantages of a synthesized waveform the effect of fast rising voltages on an insulation system are not at all as well studied as for conventional sinusoidal waveforms at power frequency. In particular this applies to partial discharges (PDs), where the knowledge on their properties as well as on the detection methods need to be improved to facilitate the design of equipment resistant to the synthesized waveforms. PDs, which are considered as being a sign of weakness can affect the life time of insulation considerably. This thesis presents a continuation of earlier investigations regarding the different behavior of PDs for different voltages characterized by steep rise times. A method for electrical detection of PDs in a system subjected to rapidly changing voltages has recently been developed and presented. The method is based on moderately sharp frequency filters in the PD decoupler, high-resolution digitizers and time-domain stochastic filtering. A limitation with highresolution digitizers is their lower sample rate. To enable use of faster digitizers with less resolution to facilitate the study of the rise time of the PD, the filters in the PD decoupler must be optimized for voltage slew rate and PD magnitude. Thus the design of a versatile PD coupler is presented in this thesis. Although entirely passive, the filter suppression can be made to change two orders of magnitude in half a decade of frequency. Thus, a low-resolution fast digitizer can be employed with which variations down to nanoseconds in the PD rise time can be studied. Only a few parameters need to be set to optimize the filter. Examples are presented, which illustrate the advantages of this modified PD decoupler on actual measured PDs compared to the previous approaches and to traditional methods. It is also demonstrated how an accurate model of the transfer function can be utilized to recreate both the shape of the applied voltage as well as the PD signal. This allows for measurement of PD characteristics and the applied voltage using only one channel. The presented PD decoupler is employed with voltages of different rise times, which resulted in significant differences in the PD behavior. This indicates that the effect on the insulation system is indeed dependent on the voltage wave shape. Applying square-like voltages to in particular cavities with dielectrically insulated electrodes significantly affects the discharge amplitude, its rise time, the inception voltage and ii the distribution shape. The investigation shows that PD amplitude increases while the rise time of the PDs decreases for shorter voltage rise times, these being indications of a possible change in the discharge mechanism. It is further examined how cavity dimensions affect the PD characteristics, again providing more evidence of a possible change in the discharge mechanism. This in turn can yield a faster deterioration and reduction of insulation service life. To illustrate the degradation process, microscopic images show how the rise times affect the cavity surface deterioration, these observations are consistent with the others and these effects need to be considered when designing insulation systems exposed to fast transients. Keywords: Partial discharges, square like voltages, measurements, cavities, repetitive voltages, short rise time, high dV/dt.