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Partial discharge (PD) detection is a fundamental test for the design and diagnosis of insulation systems, particularly organic materials which must operate throughout their whole life without PD. The measurement of Partial Discharge Inception Voltage (PDIV) and Repetitive Partial Discharge Inception Voltage (RPDIV), the latter defined for power electronic-type supply, is therefore a key issue for the design and qualification tests of Type I insulation system of rotating machines fed by power converters. While experience and technology for PD inception measurements under sinusoidal voltage supply is well established, the same does not hold under repetitive-pulse voltage waveforms, such as those provided by inverters. In addition, electromagnetic noise generated by electronic switch commutation can overlap with PD pulses signals and this is even more cumbersome when fast rise-time components are employed, such as GaN or SiC semiconductors. Moreover, this is also the condition which is the most stressful for stator wire insulation. This work investigates methods to detect PD pulses on enameled wires, rejecting noise efficiently, under repetitive impulsive voltages with different rise times, from 60 ns to 1000 ns. The proposed techniques can be implemented to achieve un-supervised noise rejection, which would be an important goal for both off-line and on-line PD testing.

This paper highlights the effects voltage supply waveform can have on partial discharge, PD, characteristics in insulation systems of rotating machines fed by power electronics drives. While standards allow both sinusoidal and repetitive voltage impulse waveforms to be used for insulation design and type tests, the dependence of PD on the voltage waveform implies that the machine design and performance can be affected significantly by the waveform. Partial discharge inception voltage, partial discharge patterns and insulation life are, in fact, dependent on the type of test waveform. This can affect significantly the design of machine insulation system.

The partial discharge inception voltage (PDIV) and the repetitive partial discharge inception voltage (RPDIV) are key parameters for the characterization of the insulation system of rotating machines fed by power converters. In particular, these quantities have a fundamental role in the design and qualification of Type I insulation which has to last all life without partial discharges (PD). This paper analyzes the two definitions, taken from the IEC 60034-18-41 and IEC 60034-18-42 standards, and shows potential discrepancies which can affect significantly insulation system design, especially going from two-level inverter to sinusoidal waveforms.

The reliability of low-voltage inverter-fed motors is highly dependent on the inception of partial discharges. The effect of impulsive waveform parameters must be investigated to predict the repetitive partial discharge inception voltage (RPDIV) and define test procedures that can indicate properly the behavior in service of the insulation system. This paper focuses on the RPDIV of magnet wires using twisted pairs subjected to repetitive unipolar impulsive voltage waveforms. The effects of supply frequency (5 to 200 kHz) at pressure levels that are typical for aircraft (20 to 100 kPa) is examined. Results show that RPDIV steadily decreases with frequency up to 100 kHz where it reaches a plateau. This behavior is explained as an effect of the oscillations that inevitably exist in the applied voltage waveform. Therefore, a conservative estimate of the RPDIV could be achieved by raising the supply frequency well above the operation frequency. In the experiments, the RPDIV is decreasing linearly with pressure. If this behavior could be confirmed for other insulation systems, the design of systems working at pressures typical of aircraft would result relatively easy.

Nowadays the massive adoption of electronic converters to supply electrical motors leads to critical problems during the normal operation. In fact when the motors are supplied by electronic converters, the stator insulating system could be subjected to over voltages due to reflection and resonance phenomena. Frequently this over voltages exceed the partial discharge inception voltage and partial discharge activity incepts. The presence of partial discharge activity implies the degradation of the dielectric characteristics of the type I insulating system and in the short-medium time, leads to a total breakdown discharge. Three different types of type I insulating system used in rotating electrical machines fed from inverters have been considered and studied. The PDIV and RPDIV at different temperatures have been measured and compared. During this research study the stators of asyncronous motors have been subjected to sinusoidal waveform and unipolar positive voltage pulses at different temperatures. In order to perform the RPDIV measurements a modified surge tester has been used so to apply unipolar positive voltage pulses at the terminals of the windings. The correlation between the repetitive partial discharge inception voltage and the partial discharges inception voltage measured at different temperatures has been studied.