Research into rotating machine turn insulation life under repetitive impulse voltages from power converters

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The design of rotating machine coil or bar insulation under power frequency sinusoidal voltage is well understood and testing procedures have been standardized which have served the industry well. However, today, medium voltage (13.8 kV) rotating machines, controlled by power electronics, are becoming common place in industrial plants and in renewable generation. The power electronics of an adjustable speed drive (ASD) synthesizes a sinusoidal waveform of variable frequency by means of a pulse width modulator (PWM) from a direct voltage supply. The overvoltage, electric field distribution, and partial discharge (PD) experienced by the insulation when fed by power electronics presents new challenges in the insulation of rotating machines. The ASD technology requires new test procedures and the purpose of this study is to provide rotating machine insulation, controlled by power electronics, the same level of reliability as conventional, power frequency supplied machines, which, at the present time, is not the case. From both laboratory studies and field monitoring of PD, if PD is incepted involving significant energy, they will increase, sometimes alternating between increasing and extinction periods, until they cause insulation failure, or breakdown. The IEC 600341842 standard considers that the aging factor is the peak-to-peak voltage, and therefore, the aging stress is the electric field resulting from the applied voltage and independent of the voltage waveform. This implies that PD, which is the aging driver, would not change from sinusoidal to repetitive impulse waveforms and consequently the life time is the same for any voltage waveform provided that the frequency of the supply voltage and the peak-to-peak voltage are the same. Recent publications investigating the life characteristics of rotating machine turn insulation has shown that both PD and life time under sinusoidal and repetitive impulse voltages differ significantly. In fact, the PD magnitude, repetition rate, and also time behavior, depend on the shape of the supply waveform and more importantly, on the rise time of the voltage waveform.The influence on accelerated aging from repetitive impulses, compared to AC sinusoidal voltage, has been clearly singled out and depending on the rise-time of the impulses, the energy involved in the PD process can be significantly higher than under AC. This reflects into shorter life times, and such behavior can be suitably modelled. This research proposes to investigate accelerated life tests on turn insulation specimens using sinusoidal and power electronic waveforms while monitoring PD during aging, up to breakdown, in order to explore the PD time behavior under different waveforms. By investigating the failure time behavior under sinusoidal and impulse waveforms, the type of life model that can be applied to power electronics fed insulation aging can be inferred.