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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 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]

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 TritonTM X-1OO 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. [ABSTRACT FROM AUTHOR]

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 nanofihler. 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 (SiO2) 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 SiO2 achieved a life that is twice the life of conventional wires. The two-parameter Weibull distribution of dielectric strength data show that the value of the shape parameter, β, increases from 2.6 to 11.4 for the coatings with nanofillers 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. [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]

The first article in this issue is "A Critical Review of Partial-Discharge Models" by Eberhard Lemke of Doble Lemke GmbH, Germany. The article contrasts the classical capacitive equivalent circuit model, commonly used to describe partial-discharge (PD) phenomenon, to a dipole model that was first introduced by A. Pedersen in the mid 80s. The article argues that the capacitive equivalent circuit is not capable of reflecting the physics of discharges in gaseous cavities embedded in bulk dielectrics. A better approach is to model the discharge phenomenon by a dipole moment in a uniform field between solid dielectric layers. Consequently, the electromagnetic transients detectable at the electrodes are not caused by discharging of an imaginary cavity capacitance but rather by a dipole moment established in the cavity as a consequence of gas discharges. The article reveals that the apparent charge detectable at the terminals of the test object is well correlated with the dipole moment established at the PD site. [ABSTRACT FROM PUBLISHER]