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]

The aim of this article is to present a review of the development and application of superhydrophobic insulation for outdoor high-voltage applications. The methods of preparing insulation exhibiting superhydrophobic properties to mitigate the pollution problem under high voltages in the outdoors are explored. In addition, the existing evaluation techniques at the material development stage are reviewed and potential new modifications are discussed. The self-cleaning and anti-icing properties, the electrical performance, and prospects of superhydrophobic insulators are also addressed. Although superhydrophobic insulators may show promising behavior in small-scale laboratory conditions that do not use electrical stress, the effect of surface micro–nano structure responsible for the superhydrophobicity on the performance under the actual outdoor and weathering conditions under voltage remains to be fully understood and verified. Therefore, there is a need to verify the feasibility of superhydrophobic insulators for outdoor high-voltage applications with a solid evaluation protocol, while using material screening methods in the laboratory, and with reference to field experience. [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]

This paper investigates the tracking and erosion of silicone rubber and EPDM insulation in the DC inclined plane tracking and erosion test. Silicone rubber filled with alumina tri-hydrate fails due to combustion in the gas phase or erosion paths, but with no evidence of tracking. EPDM fails due to tracking and surface capturing fire, from dry-band arcing leaving a black residue while impinging the surface. Carbonaceous residue on EPDM gives rise to leakage current magnitude, whereas fused silica-based residue on silicone rubber is shown to promote combustion. A high amount of alumina tri-hydrate replacing the polymeric fuel in the composite is shown as a primary factor preventing failure and changing the nature of the residue in the DC inclined plane tracking and erosion test. Depolymerization is the main degradation mechanism reported for silicone rubber and EPDM, with notable degrees of crosslinking reported only for silicone rubber. Crosslinking reduces the amount of silicone rubber undergoing depolymerization, thus, the amount of fuel prone to combustion is reported to promote a coherent residue shield against the eroding dry-band arcing. [ABSTRACT FROM AUTHOR]

Temperature and electric field along the stress grading system of a form wound coil under repetitive impulse voltage are studied using 2D axisymmetric and 3D models in COMSOL¯ 5.3a. The validity of the 3D model is shown by comparing the results with the measured temperature profile along the stress grading system. The evaluation of both temperature and electric field shows that using a stress grading system based on a micro-varistor characteristic reduces the maximum temperature; however, with an increase in the electric field, which can be reduced to an acceptable level. An optimization on the initial conductivity of the micro-varistor characteristic confirms that desired electric field and temperature rise are achievable by selecting an optimized conductivity characteristic. 2D and 3D simulations show that a proposed stress grading system, which is a combination of an optimized stress grading tape conductivity and minimum conductive armor tape length, has a good temperature and electrical performances under repetitive impulse voltage. [ABSTRACT FROM AUTHOR]

Evaluation of thermal and electrical characteristics of a 13.8 kV rotating machine stress grading system shows that a reduced length of conductive armor tape (CAT) from the slot exit reduces the electric field and temperature rise under pulsed voltage. Although, the electric field in stress grading tape (SGT) increased slightly, with a shorter length of CAT during the rise time of the pulse voltage, it decreased during the DC portion of the pulse voltage. The temperature profile of the stress grading system under pulsed voltage at room, elevated room, and near typical operating temperatures are measured and simulated for several CAT lengths; results show lower temperatures for shorter CAT length. The partial discharge inception voltage is unchanged for shorter CAT lengths. A lower temperature rise is desirable as this leads to a longer life in the absence of partial discharges. [ABSTRACT FROM AUTHOR]

The thermal and electrical characteristics of a stress grading system are studied by simulations and measurements of the electric field and temperature distributions in the overhang region of a form wound medium voltage motor coil. Simulations show that a highly conductive stress grading tape (SGT) reduces the maximum electric field in this region but increases heat production and temperature. In contrast, a highly conductive armor tape (CAT) decreases the maximum electric field in this region and also reduces the heat production and temperature in the stress grading system but also produces a slight increase in the electric field in the SGT region. For the simulation studies, accurate measurements of the electrical and thermal conductivities of materials are made, taking into account the effects of the vacuum pressure impregnation (VPI) process, temperature, and tape builds. The electrical conductivity of single and double half-lap layers of CAT and SGT are measured from 30 to 100 °C. For the evaluation of the simulation model, the temperature profile along the stress grading system was measured and simulated under pulsed voltage at room and near typical operating temperatures. [ABSTRACT FROM AUTHOR]

The proven reliability of ceramic cap-and-pin insulators for high voltage (HV) and extra-high voltage (EHV) overhead lines suffers in the presence of harsh industrial or strong salt pollution [1]. In these cases, apart from the countermeasures such as increasing creepage distance, washing, and applying greases, insulators with a silicone rubber surface have demonstrated better performance [2]. The two alternatives for having insulators with a polymeric surface are (1) composite insulators, which consist of an inner fiberglass rod and an outer silicone rubber housing, and (2) precoated toughened glass cap-and-pin insulators with room temperature vulcanized (RTV) silicone rubber coating, hereafter referred to as RTV insulators. [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]

This paper describes the suppression of dry-band arcing erosion of silicone rubber by alumina tri-hydrate and silica fillers in the DC inclined plane test, employing the wavelet based multiresolution analysis of leakage current. The third detail component of the leakage current as decomposed by the wavelet-based multiresolution analysis is shown to be an indicator of the effectiveness of the filler type in suppressing erosion by dry-band arcing. The addition of alumina tri-hydrate or silica filler to silicone rubber increases the thermal conductivity of the composites, retarding the development of the eroding temperature, and thus the evolution of the third detail. Additional effect is also obtained for the dehydration enthalpy, of alumina tri-hydrate in silicone rubber at a filler level of 30 wt%, in impeding the development of hot spots on the tested surface. A reduction in the magnitude of the third detail is evident with filler level, indicating that the increasing volume of silica or alumina tri-hydrate reduces the temperature of the dry-band arcing plasma. Comparable levels of the leakage current third detail is found between 30 wt% alumina tri-hydrate and silica filled composites which suggests the water of hydration plays a minor role in diluting the SiR but at 58 wt% an internal oxidation mechanism that produces gases diluting the arcing phase appears to suppress erosion. [ABSTRACT FROM AUTHOR]

This paper describes an investigation into the dry-band arcing mechanism leading to erosion of filled silicone rubber in the DC inclined plane test. Good correlation is obtained between the formation of surface residue, hotspots and the eroding dry-band arcing. The hotspots detected by infrared camera correlates well with the temperature of degradation determined by thermogravimetric analysis and differential scanning calorimetry. To characterize the physical mechanism of dry-band arcing, wavelet-based multiresolution analysis up to seven levels of resolution is applied to the leakage current waveforms. The analysis shows that both stable and intense discharges characterize the eroding dry-band arcing. A time-to-eroding parameter is proposed to determine the relative resistance to the formation of residue. Faster accumulation of residue is evident under -DC than +DC. Earlier inception of erosion is obtained for the silica filled- as compared to the alumina tri-hydrate filled- silicone, in which the release of the water of hydration is postulated to suppress the residue formation. [ABSTRACT FROM AUTHOR]

"Polymer" is used in this article, but often "nonceramic" or "composite" is used in place of "polymer." So, polymer insulators are synonymous with nonceramic or composite insulators. This article focuses on the development of suspension-type polymer insulators, used on overhead distribution and transmission lines, and on the present-day issues that limit their greater use. [ABSTRACT FROM AUTHOR]

This paper presents the results of an examination of the erosion resistance of twopart room temperature vulcanizing (RTV) silicone rubber (SiR) nanocomposites. The SiR composites were filled with 7 nm size fumed silica and 1.5 ?m size micro-silica. Separate samples were prepared using conventional mechanical mixing and an electrospinning technique, and their erosion resistances were compared. The electrospinning technique allows for a higher volume fraction of nanofiller and micro-filler in the composites as compared to the conventional samples. The erosion resistances were tested using both an infrared-laser-based source and the ASTM D2303 standard inclined plane tracking and erosion method (IPT). The experimental results for the eroded mass, tracking voltage, and tracking time show that the erosion resistances of electrospun samples are much greater than those of conventional samples. Scanning electron microscope (SEM) images and variable tracking patterns of the conventional samples after the IPT tests suggest an improvement in the dispersion of the fillers within the electrospun samples compared to the conventional samples. Power analysis during the IPT tests also shows that more power is required to track and erode electrospun samples before sample failure occurs than is needed for conventional samples, which suggests improved bonding between the fillers and the silicone rubber matrix. These findings also are supported by the results of thermo-gravimetric analysis. [ABSTRACT FROM AUTHOR]

The paper addresses the ac modeling and anisotropic dielectric properties of stress grading and conductive armor tapes that are two crucial components in form-wound motor coils. The limitations of the conventional dc conductivity modeling and the necessities of using ac conductivity modeling and anisotropic properties of these materials at high frequencies and high electric field are discussed. The method of measurement of the dielectric parameters at high frequency and high electric field for 2D modeling is also presented. The transient FEM simulations from conventional modeling and the proposed ac modeling are compared to experimentally obtained results. Good agreement is only obtained from the latter modeling. [ABSTRACT FROM AUTHOR]

The paper discusses the mechanism by which nanofillers improve the erosion resistance of silicone rubber nanocomposites under simulated conditions of dry band arcing. Various micro-nano composites are studied, including microfiller, nanofiller, combinations of both, and with a commercial surfactant to improve the dispersion of the nanofiller. The thermal conductivity of the composites, measured using a standard method, shows higher thermal conductivity when the nanofiller is well dispersed. A thermal model is developed and solved with finite element method (FEM) to examine the temperature profile in the modelled nanocomposites under laser heating, which simulates the hot spot from dry band arcing. The hot spot is compared to the measured temperature profile by an infrared camera. SEM, EDAX, and XRD techniques are used to analyze the thermally decomposed silicone residue after laser heating to elucidate the protective mechanism in the silicone rubber nanocomposites. [ABSTRACT FROM AUTHOR]

The paper addresses the temperature and electric field dependence on the conductivity of the conductive armor and stress grading tapes on form-wound motor coils. The temperature and electric field dependence is measured in the laboratory, and the nonlinear dependency is used in transient FEM simulations of the potential distribution on the form-wound coils. The surface potential distribution, for various temperatures and applied voltages, is measured in the laboratory from which the surface electric stress is calculated. Good agreement between the simulated and measured stresses is obtained. A temperature of 155 °C is shown to have little influence on the stress grading, but the electric stress dependence on conductivity is shown to have a major influence on the stress grading of the motor coil examined in this study. [ABSTRACT FROM AUTHOR]

This paper explores how the addition of inorganic nanofiller and microfiller to silicone rubber can impart resistance to erosion for overhead insulation applications. Various micro-nano composites were prepared for this study, namely microfiller, nanofiller, and the combination of microfiller and nanofiller with a commercial surfactant to improve the dispersion of the nanofiller. The effect of the surfactant on the surface wettability of the composites was analyzed with contact angle measurements. Salt-fog, inclined plane and laser ablation tests were conducted to evaluate the erosion resistance of the micro-nano filled composites. The results in all three tests showed that the combination of microfiller and nanofiller with surfactant resulted in composites with improved erosion resistance to dry band arcing, with the exception of the case where calcinated filler was used in the formulation. [ABSTRACT FROM AUTHOR]

The results of erosion resistance, tensile strength, elongation at break, hardness, and thermal stability measurements are presented for silicone nanocomposites prepared using various nãno and micro silica and alumina fillers in a two-part silicone rubber (SiR) matrix. The fillers are used to improve the erosion resistance of SiR, which is necessary for outdoor insulation housing applications. Good dispersion of the fillers is achieved using Triton™, a common surfactant. An optimal surfactant concentration imparts good erosion resistance to the nanocomposites in laser heating tests without adverse effects, but excess surfactant has a negative impact on the mechanical properties of the silicone. Thermal gravimetric analysis demonstrated that nano fumed silica imparts better heat resistance to silicone than natural nano silica or nano alumina fillers. Fourier transform infrared spectroscopy analysis of the nanofillers indicated a significantly higher concentration of silanol groups in the nano fumed silica filler than in micro silica. The influence of the increased number of silanol groups on the erosion resistance of the nanocomposites and their mechanical properties is discussed. [ABSTRACT FROM AUTHOR]

In this paper, the degradation processes in stator-bar groundwall insulation that is subjected to steep-front voltage pulses, at different temperatures, are studied. The test conditions include pulse voltage amplitudes up to 6 kVp and pulse switching frequencies up to 3 kHz in ambient temperatures of 23°C, 120 °C, and 155 ° C. During the aging process, the surface temperature is monitored with an infrared camera to determine the regions with highest temperatures and the partial discharge (PD) activity. The visual examination of failed bars confirmed that the groundwall insulation in these regions is degraded. [ABSTRACT FROM AUTHOR]

The paper reports on tests done on four commercial room temperature vulcanized (RTV) silicone rubber high voltage insulator coatings aimed at achieving information on the relative performance of the coatings with respect to aging. The initial characterization of the coatings includes the identification of the primary filler type, concentration of filler, the amount of free fluid, and the molecular species of the free fluid. Soxhlet extraction and gas chromatography tests show considerable depletion of fluid after accelerated aging tests in acid-water and the associated changes to the physical properties is assessed by standard mechanical tests on coupon size specimens. The effect of acid-water aging on the hydrophobicity is assessed by measurements of the static contact angle and the onset of leakage current in an incline plane test. Several field aged coatings are assessed for residual LMW fluid which is compared to accelerated aging tests. [ABSTRACT FROM AUTHOR]

This paper provides an overview of the electric field (E-field) distribution on transmission line composite insulators applied in alternating current applications. Factors that affect the E-field distribution are discussed as well as the influence of the E-field distribution on the short and long term performance. Modeling and measurement methods are reported and examples of calculated E-field magnitudes determined are presented together with corona ring application information. This paper was developed by the IEEE Task Force on Electric Fields and Composite Insulators. [ABSTRACT FROM AUTHOR]