Search

In this article, the impact of the waveform switching frequency (SWF), overshoot (OS), and rise time on the time-to-failure and the endurance of Type II machine insulation is investigated. Reference life curves at 1 and 4 kHz SWF are derived utilizing a unipolar repetitive square-impulse of 15% OS and 300 ns rise time. The effects of the impulse OS and rise time in the ranges of 0%–30% and 400–600 ns are also evaluated and reflected on the established reference life curves. Based on the endurance test results and the reported time-to-failure data, it is evident that the turn insulation is subjected to additional stress factors when energized by an inverter supply as opposed to the stresses exhibited by sinusoidal waves. This indicates an incomparable aging rate between the two types of waveforms, even at similar peak voltages applied, resulting in inadequate use of sinusoidal supply to qualify inverter-fed machines. Moreover, the OS component of the waveform is found to have a substantial impact on the endurance of turn insulation apart from the jump voltage impact, and therefore, is identified as a significant factor influencing the life of turn insulation independent of other factors. Moreover, the reported results of the time-to-failure analysis advocate a nonlinear relationship between the SWF and the life of turn insulation, whereas the waveform rise time is found to have a negligible impact within the considered range in this study. [ABSTRACT FROM AUTHOR]

In this article, the effect of repetitive square wave impulse voltage, switching frequency, rise time, and overshoot (OS), on partial discharges (PDs) within Type II turn-to-turn insulation is reported. Two types of samples are used in this study: a back-to-back turn insulation sample and a single cavity layered insulation. This work aims to analyze the influence of the square wave impulse voltage parameters on the characteristics of PDs during aging while relating their effect on the life of turn insulation. The experimental setup used in this study separates the effect of the waveform rise time from the OS and highlights the role of the OS rather than the jump voltage in aging. The results, within the range of parameters considered in the study, indicate that the number and magnitude of PDs are lower at a higher switching frequency while shorter rise time triggers fewer discharges of larger magnitudes. It is also found that PDs, both occurrences and magnitudes, are in favor of waveforms of higher OSs even when the waveforms have a similar jump voltage. This highlights the role of the OS, rather than the jump voltage, as a main and additional factor affecting the life of turn insulation which is contrary to what has been described in the International Electrotechnical Commission (IEC) 60034-18-42. Based on the average magnitude of PDs per unit time, the results show, in the reported range of parameters, that the switching frequency and OS of the voltage waveform are among the main parameters affecting turn insulation life, whereas the effect of the rise time is found to be minimal. [ABSTRACT FROM AUTHOR]

The paper presents the results of surface charge measurements on insulation film samples with different surface resistivities. Polytetrafluoroethylene (PTFE) and Nylon 6 (PA6) as well as mica paper subjected to unipolar and bipolar square waves at different switching frequencies. The experimental data are presented to explain the differences observed in the times-to-failure of turn insulation when aged using unipolar and bipolar square waves. Time-to-failure experiments and COMSOL? simulations are used to estimate the accumulated charge on the surface of insulation films with different surface resistivities. [ABSTRACT FROM AUTHOR]

The paper examines the time-to-failure of enameled form-wound machine turn insulation subjected to unipolar and bipolar square waves at different switching frequencies. Partial discharge under the same square wave parameters are also reported using a point-to-plane geometry from which the measured PD features are related to the time-to-failure results of the turn insulation. The results show that the life of enameled turn insulation is considerably longer under bipolar square waves than for unipolar waves and that turn insulation lifetime is related to the power of the partial discharges. In addition, a frequency threshold in the time-to-failure results is observed corresponding to a decrease in PD power and the development of surface space charge that affects the PD features is discussed. [ABSTRACT FROM AUTHOR]

The lifespans of enamelled turn insulation samples are studied when stressed with square waves characterized by different rise times, duty cycles, switching frequencies, and applied voltages. Partial discharge activities under these test conditions have also been recorded using an RF antenna in an effort to correlate the time to breakdown with material degradation under fast repetitive pulse aging. The aging and PD features were independent of rise time for a 50 % duty cycle; however, slightly faster times to failure were observed at a 15 % duty cycle for the faster rise times, for the considered range. The higher duty cycle and the higher number of applied cycles accelerated the aging. [ABSTRACT FROM AUTHOR]

This paper reviews the aging mechanisms and stresses that affect toughened glass-suspension-type insulators. Procedures and criteria for determining the overall condition of inservice toughened glass-suspension insulators are described so that a decision can be made on their suitability for continued service or to replace the insulators. [ABSTRACT FROM AUTHOR]

The paper presents the results of the study on the influence of insulator profile on the aging performance of silicone rubber (SIR) insulators in salt-fog. Experiments have been conducted on various two-shed arrangements at 35 V/mm average stress and 0.25 S/m salt-fog conductivity level. The work is also extended to include commercial 15 kV class insulators with different profiles. Shed spacing, shed diameter, alternate shed design and shed shape are the parameters investigated in this study. The low frequency harmonics of the leakage current, early aging period (EAP), and equivalent salt deposit density (ESDD), are used to evaluate the aging performance of different designs. Insulator profile is shown to greatly influence the aging performance of SIR insulators. Shed shape proves to be the most important parameter to be considered in designing non-ceramic insulators profiles. Also, as the shed spacing decreases, the performance of SIR insulators improves. Simulation results using FEMLAB© show that the electric field on insulators is below the corona onset at both dry and wet conditions. Dry band arcing is therefore the main electrical cause for aging in distribution class insulators and it is possible to improve the pollution performance of SIR insulators using appropriate profiles as suggested in this work. [ABSTRACT FROM AUTHOR]