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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]

The results of erosion resistance, tensile strength, elongation at break, hardness, and thermal stability measurements are presented for silicone nanocomposites prepared using various nano 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[TM], 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. Index Terms--Surfactant, Triton[TM], erosion resistance, mechanical properties, nanofiller, dispersion, dielectrics, silicone, nanocomposites.

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]

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 [degrees]C, 120 [degrees]C, and 155 [degrees]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. Index Terms--Adjustable-speed drives, converters, dielectric heating, groundwall insulation, induction motor, partial discharge (PD), pulse voltage, solid-state switches.

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. Index Terms--Composite insulator, corona ring, E-field, electric field, grading ring, nonceramic insulator (NCI), polymer insulator.

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. Index Terms--RTV silicone rubber coating, HVIC, accelerated aging, depletion of fluid, hydrophobicity, de-polymerization, Soxhlet extraction, lifetime, gas chromatography, LMW fluid.

This paper reports on the heat erosion resistance of various silicone rubber compositions under infrared laser heating that simulates the dry band arcing phenomenon on outdoor insulation. The composites are made using a two-part addition cure silicone rubber, with various amounts of alumina, natural silica, or fumed silica, nano- or micron size, and different amounts of Triton[TM] X-100 surfactant. It is shown that the surfactant greatly improves the dispersion of nanosized particles, yielding composites that are more homogeneous, with improved resistance to heat ablation. Significantly improved dispersion in the presence of surfactant is confirmed by SEM analysis for some nanofillers, while very little effect is observed for others. The surfactant is beneficial up to the point where the filler surface is saturated with the additive. Fumed silica is shown to impart greater heat ablation resistance than natural silica and alumina. Furthermore, a distinct protection mechanism appears to be operative for fumed silica and natural silica as opposed to alumina. Index Terms--Surfactant, Triton[TM] X-100, erosion, nanofiller, dispersion, dielectrics, silicone, composites.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This second part of the paper describes the strength of typical line insulation in these conditions and then deals with selection and mitigation methods based on the in-service stresses described in Part I. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulation coordination, line insulators, snow.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This first part of the paper describes the environmental and electrical stresses encountered in service that influence the risk of flashover on line insulators. Part II of the paper describes the strength of typical line insulation in these conditions and deals with selection and mitigation methods. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulation coordination, line insulators, snow.

This letter describes a high-resolution technique called thermally stimulated depolarization current (TSDC) that can be used for the measurement of space charge accumulation in the medium voltage magnet wires operating on pulsewidth modulated (PWM) inverters. Index Terms--Fast rise time pulses, induction motors, inverterfed drive motors, magnet wires, space charge injection.

The paper presents the experimental results on accelerated aging of enameled wires with and without nanofiller in the coating under partial discharge (PD) activity. The residual life of the wires, and the surface roughness as measured by a scanning electron microscope (SEM), are used to evaluate the effects of the nanofiller. The surface erosion in the specimens without nanofiller is considerably greater, which is 1120-1270 nm, under pulse aging; whereas, in specimens with ~1%, by wt, fumed silica (Si[O.sub.2]) nanofiller it was measures to be 230-250 nm. An evaluation of the residual life of the wires, aged under high frequency ac waveforms, reveal that the wires with Si[O.sub.2] achieved a life that is twice the life of conventional wires. The two-parameter Weibuil distribution of dielectric strength data show that the value of the shape parameter, [beta], increases from 2.6 to 11.4 for the coatings with nanoffilers indicating that the material becomes more homogeneous and exhibits fewer defects. Finally, the paper discusses possible reasons for the reduction in surface erosion and improvements in breakdown strength of enameled wires filled with various types and concentrations of nanofillers. Index Terms--Adjustable speed drives, dielectric strength, fast rise time pulses, inter-turn insulation, nanofilled enameled wires, residual life, standard deviation.

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[C] 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. Index Terms--Leakage current, silicone rubber insulators, aging.

The paper describes the investigation of the failure mechanisms of medium voltage inter-turn insulation, as a consequence of pulse aging. To study the performance and failure behaviour of enameled wires, models of inter-turn insulation are prepared with enameled wires having polyimide and nano-filled coatings. The aging of such bar samples is attained by sinusoidal (60Hz), fast repetitive unipolar voltage pulses, and high frequency ac waveforms. To establish the residual strength of the bar samples, dc breakdown voltages are determined before and after the aging. In addition, the thermally stimulated depolarization current (TSDC) is used to provide information about the relaxation processes, such as the disorientation of the dipoles and the release of charges from the trapping sites in enameled wires. The various trapping levels, which have discrete energy differences with distinct peaks in the thermograms, are discussed in terms of space charge polarization. Index Terms--Form-wound stator coils, inter-turn insulation, nano-filled enameled wires, partial discharge inception voltage, residual strength, space charge injection, charge trapping levels.

The standard methods available to test tracking and erosion resistance of filled silicone rubber do not allow to delineate the fundamentals of the thermal degradation, although the heat from the dry band arcing is the main degradation factor. In this work a thermal imaging camera is used to investigate the performance of filled silicone rubber in an inclined plane test, and a scheme is established to relate the temperature and the electrical discharge energy. Further, an infrared laser technique, based on constant energy approach, is developed to study the material performance under laser heating. The technique consists of applying a certain amount of infrared laser energy during a set time period, in order to produce erosion in the silicone rubber samples. Through correlation studies the applicability of the proposed laser technique to rank silicone rubber samples with various fillers has been validated. The laser tests show identical ranking of samples when compared to samples ranked using inclined plane tests, yet offering significant advantages of being faster, simpler, and reproducible. Index Terms--Outdoor insulation, composites, erosion resistance, silicone rubber, dry band arcing, diagnostics, high voltage, fillers.

Silicone rubber samples having various concentrations and mean particle sizes of either alumina tri-hydrate or silica fillers, prepared by room temperature and heat cured under pressure (hot pressed), are tested for erosion resistance in the ASTM D2303 inclined plane tracking and erosion test. Their corresponding thermal conductivities are determined using a transient temperature technique in which an infrared laser is employed as the heat source and a thermal imaging camera as a temperature detection device. Scanning electron microscope observations show greater filler bonding to the silicone matrix in the hot pressed samples than in the room temperature vulcanized samples leading higher thermal conductivity and increased resistance to erosion, for both ATH and silica filled samples. The correlation study shows a strong relationship between the erosion resistance and the thermal conductivity of the tested samples, highlighting the importance of an outdoor insulating material to have high thermal conductivity in order to withstand dry band arcing. The results can be used to provide guidance on filler selection for silicone rubber compounding for outdoor insulation applications. Index Terms--Thermal conductivity of silicone rubber, erosion resistance of filled silicone rubber, inclined plane tracking and erosion test of filled silicone rubber.

Silicone rubber samples, filled with 30% alumina trihydrate, subjected to the ASTM inclined plane test, show good correlation between the measured harmonic power components of dry band arcing and the surface temperature recorded with a thermovision camera. The simultaneous study of the low frequency Fast Fourier Transform and variations in temperature shows that when the temperature rises above 100[degrees]C there is a well defined increase in the low frequency harmonic components of the measured power; however, this trend is not verified by the fundamental component. The eroded mass of the samples is measured independently by a laser approach and a model to rank the tested samples using the Inclined Plane Test is developed. The model is validated by applying it to both the recorded harmonic power components and temperature, resulting in the correct ranking of damage on the samples tested in the Inclined Plane test. Index Terms--Inclined plane test, dry band arcing, surface temperature, erosion resistance, thermal degradation, filled silicone rubber, erosion resistance.

The paper presents the results of a study on the effect of shed design on the aging performance of silicone rubber (SIR) insulators in salt-fog. The experiments on a single shed design at 35 V/mm average stress and 0.25 S/m salt-fog conductivity level are reported. Shed diameter, location and inclination angle are the parameters investigated in the study. The low frequency harmonics of the leakage current, early aging period, equivalent salt deposit density (ESDD), and physical damage are used to evaluate the aging performance of various designs. Insulators having sheds with 100 mm diameter show a better performance than those having 80, 60 and 40 mm diameter sheds. The inclination angle of the sheds is shown to have a significant effect in reducing the leakage current to a very low value but simulation results using FEMLAB[R] show that a steep inclination angle results in field enhancement beneath the shed that may lead to corona damage of the shed. A cup shaped design for the shed is shown to have a low leakage current level and a !ow field enhancement beneath the shed. Index Terms--Silicone rubber, ageing, electric field calculation, polymeric insulators, non-ceramic insulators, outdoor insulation, leakage current, harmonics, erosion.

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]

This letter describes a neural network approach to the prediction of the leakage current (LC) on silicone rubber insulators exposed to salt-fog. The validity of the approach was examined by testing several insulators in a salt-fog chamber. Feed-forward back propagation was found as the best method among several training methods evaluated for the prediction of the LC. The predicted LC with this method has less than 12% error for the tested cases. Index Terms--Leakage current, neural network, polymeric insulator, salt-fog test.

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]