Partial discharges studied by dielectric response method

Doktorsavhandling, 2015

The increasing demand of integrating various renewable energy recourses in power system requires extensive use of power electronic solutions, which allows energy conversion between different frequencies and stabilizes the system. Consequently, other than the traditional 50/60 Hz sinusoidal voltage stresses act on the high voltage insulation systems. Therefore a need for elaborating fast and accurate characterization methods arises for facilitating studies of the different types of voltage waveforms on the behaviour of insulation materials and systems. Two commonly applied non-destructive insulation characterization techniques, dielectric response and partial discharge (PD) measurements, are addressed in the project. Several methods based on the so called Arbitrary Waveform Impedance Spectroscopy (AWIS) technique have been developed to enable fast and accurate characterization of dielectric material frequency response. This approach was further adopted to study the behaviour of PDs in various types of test objects, including needle-plate electrode arrangement, twisted pair enamel wires and dielectrically insulated cavities, by simultaneously applying the dielectric response measurements and the stochastic PD detection. Various experiments, involving occasionally changing voltage level, circulating air around a specimen, and modifying conductivity of cavity walls, were performed and allowed identifying additional PD current components in the total current response, which are in the following named as excess currents. It is shown among others, by comparing the excess currents with simultaneously detected PD pulses, that contributions from weak discharges lying below the conventional PD detection threshold as well as slow contributions to the current caused by charge movements within the partial discharge area can be identified and evaluated. An important component of the excess current is a non-PD excess current that repeatedly appears in all studied types of objects and causes a decay or even disappearance of PD activity with time. At longer exposures of the dielectrically isolated cavities, it also yields oscillating interchanges between PD activity and the excess current.