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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. [ABSTRACT FROM AUTHOR]

High voltage power transformers are the most challenging equipment for asset managers in that their failures are often due to thermal, electrical, ambient, and mechanical stresses that interact in a synergic way. An accurate failure mode analysis is often prevented by the compound nature of the insulation (partly solid, partly liquid, self-regenerative, and highly flammable) and by the high probability that failures involve explosions. Insulation failure is, by far, the costlier event with an average cost of about $150,000 (1999) per outage [1]. Therefore, it is worthwhile to understand better the symptoms that indicate an imminent failure. [ABSTRACT FROM AUTHOR]

Dealing with electrical rotating machines fed by voltage converters, understanding the effect of impulsive voltage parameters on partial discharge features is of great importance to select appropriate impulsive voltage to perform partial discharge inception voltage measurements. Similarly, the dependence of insulation endurance on voltage parameters might help to develop qualification tests providing more accurate evaluation of insulation performance. The effect of voltage duty cycle on partial discharge characteristics and endurance of inverter-fed motor insulation were investigated through a large number of tests performed on single-contact crossed enameled wires under bipolar repetitive impulse wave voltages. The partial discharge pattern, magnitude, repetition rate show that short impulsive voltage duration (i.e. small duty cycles), can induce asymmetric PD patterns and reduce the probability of PD inception at the falling flanks of impulsive voltages. Moreover, the endurance tests indicate that the duty cycle of the repetitive square wave voltage can affect the insulation endurance significantly. Repetitive square wave voltages with small duty cycles tend to overestimate endurance. Accordingly, when performing partial discharge inception voltage and endurance tests on insulation of inverter-fed motors, the influence of duty cycle should be carefully considered. [ABSTRACT FROM AUTHOR]

This paper reviews results of partial discharge measurements and accelerated life tests performed on models of random and form wound insulation systems subjected to impulse voltage waveforms. The goal is to highlight how the features of the supply voltage affect the partial discharge activity and how this, eventually, reflects on the endurance of the insulation system. The most salient results are that, for both types of insulation systems, short rise times trigger PD of large magnitude, which are able to erode the insulation system faster than the partial discharges incepted using waveforms with longer rise times. In particular, failure times estimated by tests under sinusoidal voltages can be overly optimistic in assessing the long-term behavior of the insulation system subjected to impulse voltage waveforms. Frequency also is very important, as it controls the interplay between physical and chemical degradation rates. As a result, above partial discharge inception voltage the frequency acceleration rule does not hold, and can lead therefore to excessively optimistic results. [ABSTRACT FROM AUTHOR]

In this work, we compare the dielectric withstand performance of nanofluids prepared using Shell Diala D as a base fluid, and magnetite, graphene oxide and silicone dioxide as nanoadditives. The ac withstand capability of the nanofluids was investigated as well as partial discharge inception voltage, partial discharge and repetition rate under ac, dc positive and dc negative voltages. The results indicate that, for all nanofluids, nanoparticle concentrations around 0.2 g/l enhance dielectric withstand properties under quasi uniform fields. Under divergent fields, partial discharge characteristics are improved under ac conditions. Under dc conditions silica nanofluid performs better than mineral oil, but the other two nanofluids do not perform well. [ABSTRACT FROM AUTHOR]

To address the reliability of rotating machine insulation, the International Electrotechnical Commission implemented new standards and technical specifications that are relevant to low-voltage and medium-voltage inverter-fed motors. Insulation evaluation under such power supply relies upon partial discharge (PD) measurements and accelerated life tests, which should be conducted (at least for the turn-to-turn insulation) under impulsive voltage waveform. The effect of impulsive voltage parameters on PD inception voltage and, in particular, endurance tests must be investigated to design test procedures that can indicate insulation quality and its behavior in service. This paper examines the effect of repetitive impulsive voltage waveform parameters on PD and the endurance (life) of organic insulation for rotating machine wires based on experiments performed on single-contact crossed pairs. Results show that the features of repetitive voltage impulse affect PD time and frequency characteristics, PD phase distribution, and endurance. Consequently, evaluation of insulation subjected to power electronics waveforms should take into account the appropriate choice of impulsive voltage parameter to fit to the type of PD phenomenology occurring in service conditions. [ABSTRACT FROM PUBLISHER]

The growing price of mineral oils (MOs) over the last 15 years, and environmental and safety concerns, have promoted research on alternative insulating fluids. Ester oils (EOs), which were used initially in transformer insulation [1], have again attracted attention; both synthetic and natural liquids have been investigated thoroughly in the search for suitable substitutes for MOs. EOs have interesting properties [1]. Their most attractive properties are those concerning flammability; depending on their chemical structure, their fire points are 100?C to 200?C higher than those of MOs, which ensures that fires are less likely in EO-filled transformers. They are also biodegradable, a property that has favored their use in environments where oil spills cannot be easily confined, e.g., traction and off-shore transformers. They are also much more hygroscopic than MOs, a property which favors drying of solid insulation, thus ensuring that depolymerization of cellulose is slowed down. This improves the long-term mechanical stability of transformer solid insulation. [ABSTRACT FROM AUTHOR]

This paper focuses on the influence of repetitive square wave voltage rise time on partial discharge pulse characteristics in the ultra-high frequency range. Paerial Discharge (PD) tests on single-point contact crossed pairs were carried out using repetitive square wave voltages with a wide range of rise times, from 200 ns to 400 μs, including also 50 Hz sinusoidal voltage waveforms. Experimental results show that rise time has a significant influence on magnitude and frequency spectrum of PD pulses. Voltages of shorter rise times generate larger PD pulses with a predominance of high frequency components in the energy spectrum. Interpretations are presented for this phenomenon. [ABSTRACT FROM AUTHOR]

Premature failures of random-wound induction motors, fed by adjustable-speed drives, have frequently occurred over the last two decades. Overvoltages at the motor terminals causing abnormal electrical stresses on the windings have been singled out as the most critical factor affecting winding insulation reliability [1]-[7]. These overvoltages are due to wave reflections associated with the mismatch between the impedance of the motor and that of the cable connecting the machine to the adjustable-speed drive. When the overvoltages are high enough to initiate partial discharges (PDs) within the winding, degradation of the wire enamel insulation progresses very rapidly, causing failure. To help circumvent premature failures, the International Electrotechnical Commission (IEC), in 2007, released a technical specification (TS) [8], currently under revision, supporting a quality-control procedure to check that new windings are free of PD when subjected to converter surges. The procedures consisting of both impulsive and sinusoidal offline tests are outlined in this article. [ABSTRACT FROM PUBLISHER]

This paper reports and discusses qualification and type test procedures for random wound motors as specified in IEC Technical Specification 60034-18-41. Since laboratory tests dealing with off-line partial discharges measurements followed by life tests showed that motors qualified as good can fail in very short times, an extensive investigation on the causes of such discrepancy and, in particular, on the differences existing between the stresses applied during off-line tests and in service is described. In addition, the results of partial discharge measurements performed on crossed pairs are presented, highlighting how the differences in the applied voltage waveform can influence the Repetitive Partial Discharge Inception Voltage. Suggestions to improve the off-line test procedure and related equipment are eventually discussed. [ABSTRACT FROM PUBLISHER]