Diagnosis of partial discharges at pulse voltage

Abstract

The continuous installation of renewable energy systems and the ongoing trend towards electric vehicles are fundamental to a more sustainable and environmentally friendly use of electrical energy. However, this progress also brings new challenges: Modern insulating systems are subjected to various forms of pulse voltage excitation, mainly due to the switching of modern power electronics that use pulse width modulation. Components exposed to such repetitive pulsed voltage often show significantly reduced lifetimes. The main reason for accelerated ageing is the inception of partial discharges. Partial discharges can occur within an insulating system due to flaws in the material and can cause continuous deterioration until a complete breakdown occurs. The recent introduction of wide bandgap semiconductors increases the demands on insulation systems by enabling higher operating voltages and steeper switching transients than silicon-based semiconductors. For the development of reliable components, it is necessary to understand the effects that occur in insulation systems exposed to fast transient voltages and the partial discharge behavior at said voltage. Partial discharge measurement has become an established and sensitive diagnostic method for quality assurance and maintenance of equipment at sinusoidal and direct voltage. The further development of this method for use with arbitrary waveforms such as fast voltage pulses is of crucial importance for the development, testing, and maintenance of modern electrical devices. In this dissertation, a partial discharge measurement circuit is developed to perform research on four different common sources of partial discharges at fast front double exponential voltage pulses. The method relies on the conducted signal of a partial discharge and can hence be calibrated and access all quantities known from traditional partial discharge measurements. The thesis walks through the development of the test circuit and the signal processing necessary to access and measure individual PD pulses. Measurements at impulse voltage are evaluated for their discharge behavior, for characteristics enabling failure identification, and compared to tests at sinusoidal voltage. The method presented in this dissertation can be applied to various voltage waveforms, enabling research towards the next generation of electrical components and their testing.