Detection of Partial Discharges at Fast Rising Voltages

The present demand for higher efficiency and flexibility in the energy sector has ledto an increased use of power electronic generated waveforms as these allow energyconversion between different frequencies including DC. The generated waveformsare usually synthesized by so called Pulse Width Modulation (PWM) techniques,where the desired waveform is approximated by a number of square shaped pulseswith a short rise time. Applications such as variable speed drives and reactive powercompensation are saving vast amount of energy, while HVDC transmission wouldnot be possible without them. This makes it important to understand how rapidlyrising voltages affect insulation systems.Despite the obvious advantages of a synthesized waveform the effect of fast risingvoltages on an insulation system are not at all as well studied as for conventionalsinusoidal waveforms at power frequency. In particular this applies to partialdischarges (PDs), where the knowledge on their properties as well as on the detectionmethods need to be improved to facilitate the design of equipment resistant to thesynthesized waveforms. PDs, which are considered as being a sign of weakness canaffect the life time of insulation considerably. This thesis presents a continuation ofearlier investigations regarding the different behavior of PDs for different voltagescharacterized by steep rise times. A method for electrical detection of PDs in a systemsubjected 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 highresolutiondigitizers is their lower sample rate. To enable use of faster digitizers withless resolution to facilitate the study of the rise time of the PD, the filters in the PDdecoupler must be optimized for voltage slew rate and PD magnitude.Thus the design of a versatile PD coupler is presented in this thesis. Although entirelypassive, the filter suppression can be made to change two orders of magnitude in halfa decade of frequency. Thus, a low-resolution fast digitizer can be employed withwhich variations down to nanoseconds in the PD rise time can be studied. Only a fewparameters need to be set to optimize the filter. Examples are presented, whichillustrate the advantages of this modified PD decoupler on actual measured PDscompared to the previous approaches and to traditional methods. It is alsodemonstrated how an accurate model of the transfer function can be utilized torecreate both the shape of the applied voltage as well as the PD signal. This allows formeasurement of PD characteristics and the applied voltage using only one channel.The presented PD decoupler is employed with voltages of different rise times, whichresulted in significant differences in the PD behavior. This indicates that the effect onthe insulation system is indeed dependent on the voltage wave shape. Applyingsquare-like voltages to in particular cavities with dielectrically insulated electrodessignificantly affects the discharge amplitude, its rise time, the inception voltage andiithe distribution shape. The investigation shows that PD amplitude increases while therise time of the PDs decreases for shorter voltage rise times, these being indicationsof a possible change in the discharge mechanism. It is further examined how cavitydimensions affect the PD characteristics, again providing more evidence of a possiblechange in the discharge mechanism. This in turn can yield a faster deterioration andreduction of insulation service life. To illustrate the degradation process, microscopicimages show how the rise times affect the cavity surface deterioration, theseobservations are consistent with the others and these effects need to be consideredwhen designing insulation systems exposed to fast transients.Keywords: Partial discharges, square like voltages, measurements, cavities, repetitivevoltages, short rise time, high dV/dt