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In this paper, the dependence of XLPE electrical breakdown on carrier mobility controlled by both electric field and temperature is evaluated by simulations and experiments. The molecular chain displacement model incorporating the thermoelectric effect of carrier mobility is examined to elucidate the experimental results. Trapping and de-trapping characteristics of space charge and mobility dynamics are modeled using hopping and Poole-Frenkel mechanisms. The experimental outcomes reveal that the DC electrical breakdown reduces with corresponding increment in temperature and thickness. Trapping parameters are estimated from experiments for simulations. The DC breakdown strength from experiments represents an inverse power relation with both the insulation thickness and temperature. The simulation results of molecular chain model with hopping mobility express more permanence with experimental results in comparison with Poole-Frenkel mobility. [ABSTRACT FROM AUTHOR]

This paper presents the surface modification and charge transportation mechanism of thin polyimide films. Here, we demonstrate that the surface modification of these films, by using the fluorine gas-phase, improves the charges mitigation and, thereafter, suppress the space charges. The experimental results indicate that the thermal step current for fluorine coated films reduce significantly in comparison to pure polyimide films. The thin layer of fluorine coating helps to release the surface charge, reduce the charge injection from the electrode, provide hindrance for charges to penetrate into the bulk of the films and the higher degree of charge disappearing helps to reduce the space charge growth. [ABSTRACT FROM AUTHOR]

Partial discharge inception voltage measurements under repetitive impulsive voltages play an essential role in evaluating the reliability of inverter-fed low voltage rotating machines. Using impulsive voltages, partial discharge (PD) signals and the disturbance induced by power electronics switches have often similar energy distribution in the frequency domain. Thus, PD test technologies used with sinusoidal voltages are no longer effective. Under repetitive impulsive voltages, ultra-high frequency detection is recommended to couple high-frequency spectral components from the PD and remove the low-frequency disturbance coming from power electronics. However, to obtain high signal to noise (S/N) ratio, the parameters of the antenna used for PD detection should be designed considering the PD and the disturbance energy distributions in the frequency domain. In this paper, we discuss the design and show the properties of an antenna with Archimedes plane spiral structure intended for PD detection under repetitive impulse voltages. The simulation shows that the antenna has low voltage standing wave ratio (<2) and improved directivity in the frequency range from 0.5 to 2 GHz. Moreover, PD tests performed using repetitive square waves with different rise times demonstrate that the designed antenna can receive PD signals efficiently with high S/N. [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]

To ensure insulation system reliability of rotating machines driven by power electronics, accelerated tests under repetitive impulsive voltages should be performed on turn-to-turn insulation according to the standards provided by the International Electrotechnical Commission. The parameters of repetitive impulsive voltage can significantly influence insulation lifetime, and thus, voltage parameters should be carefully controlled during the accelerated tests. To increase endurance test efficiency when testing many insulation specimens in parallel, the rise time of impulsive voltage should be controlled at certain constant values after the breakdown of one or more specimens to obtain objective endurance test results. This study provides a description of a novel square wave voltage generator used for the insulation evaluation of rotating machines driven by power electronics. A single-channel generator with variable rise times used for one specimen and a multi-channel generator with a constant rise time used to test five specimens in parallel are presented based on a push-pull solid-state switch. In the multi-channel generator, the RC parameter of the high-voltage circuit can be adjusted automatically by resorting to controlling units and a circuit structure after the breakdown of one or more samples when performing endurance tests on parallel specimens to keep the rise time of impulsive voltage at constant values. The multi-channel generator can significantly increase the endurance evaluation efficiency of the insulation systems of rotating machines fed by power electronics. [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]

Polyimide nano-composite film (Kapton 100CR) is a special type of engineering plastic film, which is widely used in turn insulation of form-wound inverter-fed motor. Partial discharge (PD) caused by high-frequency surge impulse is one of the key factors leading to premature failure of turn insulation of inverter-fed motors. Aging experiments on turn insulation samples under repetitive bipolar square impulse voltage were carried out in this study. Experimental results show that, in the presence of PD, the damage to the structure of the polyimide nano-composite film under impulse voltage is concentrated on the surface layer of the sample. Moreover, the results indicate that with the increase of aging time, the discharge phase distribution becomes broader. Therefore, the appearance of the discharge at higher phase would be the reflection that the insulation of the sample reaches its terminal stage. Surface microscopic images taken by the scanning electron microscope (SEM) show that, because of PD erosion, the polymer matrix was carbonized and volatilized, the residual nano-fillers remained on the surface and formed a screen layer to resist further damages caused by PDs. It is a symbol that faults expand rapidly when the floccules and voids appear on the surface layer, which indicates the insulation could be breakdown soon. [ABSTRACT FROM AUTHOR]

The dependence of endurance test results on repetitive square wave voltage waveform parameters is investigated in this paper focusing on crossed pair specimens made of polyamide-imide enameled wires. Endurance tests carried out on crossed pairs above partial discharge inception voltage using both sinusoidal and square wave voltages show that lifetimes can be influenced significantly by voltage waveform parameters (rise time and frequency). Partial discharge statistics, recorded from test beginning to breakdown, are used to explain the remarkable differences associated with voltage waveform parameters. According to this study, the influence of voltage waveform parameters should be carefully considered in order to obtain effective evaluation of inverter-fed motor insulation systems. [ABSTRACT FROM AUTHOR]

The insulation system of a dry-type transformer under the combined thermal, electrical, mechanical, and environmental stresses undergoes gradual deterioration, which ultimately leads to transformer failure. It is hence important to test specimen insulation of transformer coil so as to explore the aging law and the associated characteristic parameters related to the insulation condition. This paper proposes a method to evaluate reliability of the insulation system of a dry-type transformer based on the three models of degradation, regression and expert judgment. The three models are developed based on the quality parameters (QPs) from the lab tests. The method is illustrated by estimating the reliability of the insulation system of a mining flameproof dry-type transformer. The accelerated aging tests on the Nomex paper (i.e. the turn-to-turn insulation of the transformer) are conducted. The QPs of the Nomex samples of degree of polymerization, partial discharges, and photomicrograph were measured periodically during aging test. The effectiveness of the proposed method was proved through the lab experiments. The results show that the parameters from the aging test can be used to represent the condition of the insulation system. [ABSTRACT FROM PUBLISHER]

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]