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Frequency response analysis (FRA) is increasingly being accepted as an effective technique to diagnose transformer faults. Transformer electric parameters are affected by such faults in a complex manner, and there is yet no standard approach for interpretation of FRA results. Most studies have focused on diagnosing winding and core deformations, but subtle defects in the winding insulation and turn-to-turn short circuits can develop into a more serious fault, and their early diagnosis is equally important. Furthermore, there are several test configurations which have different sensitivities to different faults. This study reports a study where the winding input impedance is measured to diagnose turn-to-turn short circuits using different measurement configurations. A comparison is made between the sensitivities of each measurement configuration to faults of increasing severity. It is found that this fault is detected in the low- and mid-frequency regions as significant reduction in impedance and a shift in resonance peaks towards high frequencies. The results, analysed using different statistical parameters, indicate differences in sensitivities with different levels of short circuits. Marginal variations were found between the sensitivities of statistical parameters in different frequency regions. The study provides useful contribution into interpretation of FRA signatures for turn-to-turn short-circuit diagnosis of transformers.

This study presents both numerical and experimental results regarding the effects of pollution on the flashover performance of glass suspension-type insulators. The finite element method (FEM) was used for simulating polluted insulators both at power frequency and at lightning impulse voltages. Partially conductive epoxy resin with graphite powder was used for modeling pollution on the insulators, which enabled the measurement of the electrical potentials on the polluted insulators. Numerical studies demonstrated the variations in the field distributions caused by the location and severity of the pollution as well as by the type of voltage applied. The simulations, which were verified to a degree through measurement, showed that pollution conductivities greater than 10-6 S/m and 10-3 S/m alter field distributions for power frequency and lightning impulse voltages, respectively. Unlike the results obtained for partially polluted insulators, the voltage distribution was almost linear for fully polluted insulators. Experimental studies revealed that the location of the partial pollution plays an important role in flashover performance for lightning impulse voltages but has a negligible effect for power frequency voltages.

Polymeric cable thermal ageing is a key end-of-life consideration for medium voltage (MV) cable, low voltage (LV) power cable, and LV instrumentation and control (I&C) cables. This is particularly true for generation plant cables. For example, in wind generation plants, low thermal design margins and component compatibility issues have been known to cause internal over-heating of conductors and accessory connectors, leading to premature cable system failures. In nuclear generation plants, cable proximity to external heat sources can often result in thermal stress being a dominant insulation ageing mechanism, a fact which is reflected in environmental qualification procedures for nuclear power plant cables. As such, numerous applied studies on electrical diagnostics for cable thermal ageing have been led by organizations such as the Electric Power Research Institute (EPRI) [1,2], the US Nuclear Regulatory Council (NRC) and Department of Energy (DOE) [3], the European Union [4].

The use of pulsed electric field (PEF) is considered as one of the most promising nonthermal food treatment methods. One of the design challenges in applying PEF technology is the design of the treatment chamber. In this paper, the effect of using multiple treatment zones on both the pulse shape and killing efficiency of PEF technology is addressed. With the increase in treatment zones, the loading effects and, hence, the circuit impedance changes result in changing both the pulsewidth and pulse shape. The changes have resulted in generating simple exponentially decaying pulses to oscillatory pulses, with significant reduction in pulsewidths. On the other hand, the increase in treatment zones also results in increased number of pulses applied as the liquid flows through different zones. It is therefore important to investigate the effects of treatment chamber impedance characteristics on PEF treatment performance, considering different liquids with a wide range of conductivity.

This paper presents the results of an examination of the erosion resistance of two-part room temperature vulcanizing (RTV) silicone rubber (SiR) nanocomposites. The SiR composites were filled with 7 nm size fumed silica and 1.5 μm 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.

In this paper, the effect of pulsed electric fields (PEFs) on naturally contaminated orange juice was investigated. A 2-log reduction in a colony count for the naturally grown microorganisms was achieved after application of 120 pulses/mL at a PEF level of 46 kV/cm. Also, the effect of the PEF on microorganisms inoculated in orange juice was studied. Compared to the naturally grown microorganisms, about 1-log more reduction was observed with inoculated microorganisms that were subjected to the same PEF conditions. The results suggest that other factors like the increased temperature and antimicrobial additives might be required to enhance the killing efficiency. A 20 degC increase in temperature prior to the application of the PEF resulted in about 1-log more reduction in microorganisms

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.

Many industrial applications like high-field food sterilization require short-duration fast-rise-time high-voltage pulses. Measurement of such narrow steep-front high-voltage pulses are always difficult, considering the potential levels and the time scales involved. In addition, by nature, liquid foods are highly conductive, hence, any small change in circuit inductance has substantial influence on the measured pulse. Further, the oscillations introduced by the circuit parameters influence the voltage magnitude and the effective treatment time. Hence, these points must be taken into account while computing the total treatment time and the energy actually spent, as these parameters influence the kinetics of food processing. In this paper, the effects of load impedance, the circuit inductance, and the location of the probe position that influence the shape of the measured pulse with liquid foods as load are discussed. It has also been shown that knowing the low-voltage conductivity of the food to be treated, and with a realistic estimation of circuit inductance, it is possible to predict the shape of the pulse under different test conditions using PSPICE circuit models.

Pores occur naturally in cell membranes. Application of an electric field generates a force which tends to open the pore, a mechanism known as electroporation. The force generated depends on the electric field in the pore and the difference in dielectric constant between the fluid in the pore and the membrane. In this paper, we use transient finite-element analysis to investigate the field as a function of time in a system which consists of an electrolyte, an ellipsoidal bacterial cell membrane, and cytoplasm within the cell membrane. For this shape of cell, the field in the cell membrane is caused largely by the relaxation of the field in the cytoplasm. In addition, we compute the force on the pore wall, both in an analytic approximation and numerically as a function of time during a variety of applied voltage waveforms, using transient finite-element analysis. The numerical data are in good agreement with the analytic approximation and provide a basis for judging the efficacy of waveforms for sterilization based on a "figure of merit" which is suggested in the paper.

In this paper a parametric study of the dry-band arcing characteristics in the inclined plane tracking and erosion test is conducted under AC and DC voltages. The standard inclined plane testing apparatus is modified to promote stable and intense dry-band arcing, thereby analyzing the characteristics of the eroding dry-band arcing. The behavior of the liquid contaminant, which may bias the dry-band arcing current under equal AC and DC voltages, is investigated for different flow rates to determine suitable testing conditions. The highest leakage current magnitude is obtained under +DC followed by AC and -DC voltages, and reduced resistivity for the dry-band arcing column is shown under the +DC and AC as compared to the -DC voltages. The reduction in resistivity is speculated to be the result of metallic ions from electrode corrosion under +DC as compared to -DC, with less significant difference between +DC and AC. In addition, the inception behavior of the discharge influencing the length of the dry-band arc is found to be dependent on the voltage type. Reduced dry- band arcing length is measured for DC as compared to AC, with further reduction shown under +DC as compared to -DC. Thermal analysis is accordingly conducted, showing higher hot spot temperatures with dry-band arcing of a shorter length.

High particle collection efficiency in terms of particle weight volume mg/m/sup 3/ is well achieved by a conventional electrostatic precipitator (ESP). However, the collection efficiencies in terms of number density for the ultrafine (particle size between 0.01 to 0.1 /spl mu/m) or submicron particles by a conventional ESP are still relatively low. Therefore, it is necessary to improve the collection efficiency for ultrafine particles. In this work, attempts have been made to improve the ultrafine particle collection efficiency using the short pulse energizations. The present version of ESP consists of three sets of wire plate type electrodes. For ESP under DC operation modes, experimental results show that the collection efficiency for DC applied voltage decreases with increasing dust loading when particle density is larger than 2.5/spl times/10/sup 10/ part/m/sup 3/. For ESP under pulse operating modes, the particle collection efficiency increases with increasing pulse peak voltage until 25 kV then decreases with increasing pulse peak voltage. The ultrafine particle collection efficiency based on particle density by DC energizations is much higher compared with pulse energizations without DC bias may be due to the reentrainments of ultrafine particles.

The results of an experimental study of the effects of streaming electrification on the heat transfer of transformer oil (Voltesso 35) in Plexiglas and paper/pressboard pipe models are reported. The magnitude and the polarity of the charge in the oil entering the test section was varied by using two different filters (Millipore cellulose and Sartorius fiber glass) in the upstream of the oil flow. An enhancement in heat transfer was observed with both types of filters in the Plexiglas model and with cellulose filter in the paper/pressboard model; whereas a reduction in heat transfer was observed with fiber glass filter in paper/pressboard model under similar flow rates and temperatures. With uncharged oil, heat transfer was either inhibited or no enhancement was noticed in both models. The results have been discussed considering the temperature gradients, the flow conditions and the electrical forces due to static charge in the liquid.

The performance of an electrostatic precipitator (ESP) to collect moderately high resistivity flyash has been tested under pulse/precharger energization using the semipilot scale integrated electrostatics combustion flue gas (IECFG) cleaning system at Ontario Hydro's 640 MJ/h Combustion Research Facility Center. The pulse energization enhanced the performance of the existing de-energized wire-plate electrostatic precipitator in collecting moderately high resistivity (/spl rho//spl sim/10/sup 10/ /spl Omega//spl middot/cm) flyash. A 26% improvement in particle collection efficiency and 30% energy saving were obtained with pulse energization, with moderately high resistivity flyash generated by burning high-sulfur (3.7%) Nova Scotia coal with limestone conditioning. The cold precharger specially designed to suit the three-stage wire-plate ESP has also been tested for its ability to improve the performance of ESP in reducing the back corona. With an additional 30% of energy supplied to the precharger it was possible to obtain up to 40% enhancement in collection efficiency. In general an enhancement factor of 1.8-2.8 was obtained with dust loading /spl sim/2 g/m/sup 3/. A thyratron switched pulse power supply was used for the ESP.

In this paper the leakage current signature in the DC inclined plane test is examined for statistical indicators of the incipient erosion of silicone rubber. The mean, skewness and kurtosis are examined as useful indicators of the dry-band arcing current causing erosion. The moving window method is employed during the test to obtain an enhanced resolution in time. Simultaneous thermogravimetric-differential scanning calorimetry analysis is used to predetermine the temperature at which erosion begins, and temperature measurements taken during the inclined plane test are correlated to the leakage current leading to erosion. During erosion, more stable discharges as well as a recognizable change in the statistical distribution of the dry-band arcing current is recognizable through the analysis employed.

In this paper, an electro-thermal model is proposed to determine the distribution of the temperature rise inside a non-geometry resistive cable termination. The model is used to calculate the temperature distribution along the termination due to the application of both high frequency and pulse voltage waveforms. The model is solved by finite element technique by considering the modes of heat transfer such as conduction, convection and radiation. The simulation results show good agreement with the experimental results.

This paper reports the influence of an anionic surfactant, dodecyl benzenesulfonic acid on charge injection and convection assisted conduction in a naphthenic transformer oil, and three paraffinic liquids subjected to a step voltage. When impurities are present in a liquid they scavenge the surfactant and reduce its effective conduction; hence the surfactant has less influence on conduction in naphtenic oil than in the paraffinic liquids. The transient currents in test liquids containing surfactant were similar to injection currents with electrohydrodynamic (EHD) motion rather than to currents due to ionic dissociation. Injection in nonpolar liquids containing an anionic surfactant was controlled by the charge density at the interface due to adsorption of surfactant molecules and not by true charge injection. Based on the relation between the delay time t/sub d/ and field E a simple expression of the form C/sub n/=kE/sup -(2+/spl beta/)/ has been derived to describe the injection strength in nonpolar liquids with weak injection. The results show that the /spl beta/ values for nonpolar liquids with weak injection range between -0.90 and -1.32 which are significantly different from the /spl beta/=-2 value for polar liquids with strong injection. Also, the steady state current I/sub ss/ varied as I/sub ss/=k'E/sup /spl alpha// with /spl alpha/ >

The paper investigates the repetitive fast pulse tests using IEC-TS 60034–18–42 as applied to 13.8 kV bar samples with two different stress grading systems: nonlinear resistive and capacitive. Insulated copper bars were furnished with a capacitive stress grading system consisting of aluminum foils embedded into the groundwall insulation forming a capacitively graded system. The effectiveness of the capacitive stress grading is shown by passing all of the IEC/TS 60034–18–42 qualification tests showing considerably lower temperature rise than resistive stress grading systems.

A simple computational method for the calculation of natural frequencies and transient voltage distribution in the primary and the secondary of a single-phase transformer windings was developed. The equivalent circuits developed for different test conditions for transient analysis accounts for various inductive and capacitive couplings between various sections of the windings. The validity of the computational method was verified by comparison between measurements and calculations on a model transformer. Influence of secondary winding terminations on the measured natural frequencies in case of three-phase winding model was studied. The differences between the natural frequencies measured for similar excitation of the primary, and different terminal conditions of the secondary were as high as 300%.

Electrospinning is a straightforward and inexpensive process that produces continuous nanofibres from polymer solutions. The characteristics of the electrospinning process are mainly governed by the surface charge of the polymer solution which ultimately determines the fibre morphology and diameter. The charge density at the fluid surface is greatly influenced by the ionic carriers that present in the polymer solution. Therefore, in this paper, we discuss the effects of ionic carriers on the electrospinning process and hence the fibre morphology by changing the type of ions and their concentration using polyethylene oxide and polyacrylic acid solutions. The type of ions in the polymer solutions is altered by using different salts that include LiCl, NaCl, NaF, NaHCO 3 , KCl, and CsCl.

Nanofibres are useful in many areas due to their outstanding characteristics: small size and high surface-to-volume ratio. Recently, drug delivery systems using polymer nanofibres have gained significant attention. For these systems, a chopped nanofibre is required because the amount of drug released depends on the length of the fibre. Electrospinning is the simplest method of fabricating polymer nanofibres. In the process, a high voltage creates an electrified jet which eventually becomes a nanofibre. The jet ejects when a high voltage is applied and vice versa. It is therefore possible to fabricate and chop nanofibres by controlling the value of the voltages applied. In this research, an IGBT-based pulsed power supply has been designed and built. The IGBTs are connected in series to increase a withstand voltage. The power supply can produce square pulses with a width of a few hundred microseconds to DC and amplitudes up to 10 kV. The technique of non-continuous-mode electrospinning using the pulsed power supply was able to fabricate and chop nanofibres with PEO. The minimum pulse width that can initiate a jet is approximately 80 ms in this study. A jet is always ejected during the pulse-on voltage when the duty ratio is more than 40%.

The electro-spinning process and morphology of electro spun nanofibers depend on many processing parameters. These parameters can be divided into three main groups: a) solution properties, b) processing conditions, c) ambient conditions. In this paper, we report the results of a comprehensive investigation of the effects of changing the conductivity of poly ethylene oxide (PEO) water solution on electro-spinning process and fiber morphology. The effects of the conductivity of PEO solution on jet current and jet path are discussed. Furthermore, fiber diameter and fiber uniformity are investigated by using scanning electron microscopy (SEM) techniques. It was found that the fiber diameter decreases with increasing solution conductivity at first, reaches a minimum value and it increases again.

Applications of solid state driver for medium or high voltage motor are widespread. However, due to high switching frequency as well as high dV/dt, increased dielectric stresses and thermal stresses are applied on the insulation system; thus leading to the insulation failure of the motor. In order to test the insulation performance of the motor under the above stress conditions, a motor insulation testing facility which can generate high voltage and high frequency square waveform is developed. In the paper, low voltage solid state switches are used to generate high voltage square wave. The generator consists of several cascaded stages and each stage is made of IGBT stacks, capacitor, and wave shaping resistors. With specially designed charging and discharging circuits, each stage can generate square wave independently. Several stages can be cascaded to generate higher output voltage; the rise time can be as short as 200 ns, and the pulse width can be up to several milliseconds. Cascade connection methods are discussed and experimental results are presented in the paper. It has been shown that the cascade connection can make it possible to generate high voltage output by using low voltage solid state switches without any problem related to voltage division and synchronism of the cascaded stages.

Stress grading coatings of cable and coil terminations are considerably affected by the high dV/dt present in PWM voltage source converters. As repetitive steep transients can result in the development of hot spots and enhanced electric fields in the stress grading (SG) system, premature failures of power apparatus can occur. Stress grading systems need to be improved to work under such conditions and to obtain the desired stress relief. In this work, a stress grading system that can help to control the electric stress as well as the hot spots under fast rise time pulses is studied. It is important to design and test these systems considering the real PWM waveform in which both the fast pulses and the fundamental low frequency are included. With that purpose a fast single-phase low power two-level PWM generator was used to evaluate the stress grading system. Also this generator was used to measure some of the dielectric properties of the material.

The accelerated degradation under high frequency AC voltages of enameled wires that are nano-filled is compared to enameled wires without nano-fillers. The surface roughness and erosion depth of wire specimens, aged in the presence of partial discharge (PD), are studied using a scanning electron microscope (SEM). Changes in the chemical bonding behavior of wire insulation due to PD erosion, are also assessed by Fourier transform infrared spectroscopy (FTIR). A comparative evaluation of the residual life of the wires, aged under high frequency AC waveforms reveals that the newly developed nano-filled enameled wires, especially those with fumed silica, attain a life that is twice that of conventional wires, which is a significant improvement.

This paper presents a new prediction method for the level of the fundamental component of leakage current in the early aging period. Several silicone rubber (SIR) insulators were tested in salt-fog chamber and the LC was continuously recorded, A neural network has been used to predict the level of LC in the early stage of aging of the SIR insulators. Initial value of LC and its increasing slope in the first hour are used as the input of the network and the value of LC after 10 hours is the output of the network. It was found that a 2-layer feedforward back propagation with a biased output is a suitable network to predict the LC hours based on its initial values with a maximum of 15 % error.

A novel laser technique is used to study the degradation of silicone rubber composites, along with inclined plane test for 18 different composites, using ATH and silica as fillers, with various concentrations and mean particle sizes. The results from the constant temperature approach are compared with the results from inclined plane test by means of a statistical correlation tool. The two empirical relationships observed in the experiments have shown that it is possible not only to detect erosion on a silicone rubber surface by knowing the third harmonic component of the discharge energy, but also quantify such damages.

The paper presents the results of a study on the use of high dielectric constant silicone housing compositions on composite bushings to shape the electric stress in the vicinity of the mounting flange, thereby eliminating the need for separate grading rings or capacitor foils. The materials studied are various mixtures of barium titanate and aluminum powder in a two-part RTV silicone yielding a dielectric constant of 15, and sufficiently low in concentration as not to affect the molding properties of the silicone.

The paper discusses the experimental results in an effort to understand the enhanced erosion resistance of silicone composites with fumed silica (12 nm) as fillers. A laser based method has been used as a source of heat to treat the filled and unfilled samples of silicone rubber. Both the thermal and chemical bonding behaviour have been analysed using atomic force microscopy (AFM), thermo-gravimetric analysis (TGA), infrared microscopy (FT-IR) spectra and thermal conductivity.

In this study, the influence of nano-size silica filler is compared with micro-size filler in silicone rubber using the ASTM 2303 inclined plane tracking and erosion test. Silicone rubber samples filled with 5 /spl mu/m mean size ground silica are compared with silicone rubber filled with 12 nm average size fumed silica. Both the low frequency components of leakage current and eroded volume are used to evaluate the performance of the two types of composites. While the fundamental component of leakage current does not correlate with the erosion, the third harmonic component of the leakage current shows good correlation to the erosion, in terms of volume, and for both composites. Based on the results of the eroded volume, it is observed that 10% by weight of nano-size filler in the silicone rubber gives a performance that is similar to that obtained with 50% by weight of micro-size filler. The paper discusses the merits of nano-size filler in silicone composites.

The paper presents two different methods to compute the current density along contaminated and wet silicone rubber insulator surface. The first method, based on field theory approach, uses the commercial software FEMLAB/spl reg/ to compute the current density. The conductivity of the contamination layer used in the calculations was extracted from the measured equivalent salt deposit density (ESDD) separately for different regions of the insulator surfaces. The second method is based on the circuit theory approach, and, the insulator surface was divided into different sections for resistance calculations to account for different contamination levels. Rankings based on the calculated current densities based on segmentation of the insulator surface for ESDD measurements match with those extracted from measured leakage currents.

This paper presents the results of using the low frequency harmonics of leakage current as a measure of dry band arcing in salt-fog. Sleeved high temperature vulcanized (HTV) silicone rubber rods were tested under 0.25, 0.40 and 0.60 kV/cm in salt-fog at two different salinities, i.e. 1000 and 2500 /spl mu/S/cm. The leakage current was measured and analyzed using fast Fourier transform (FFT). The results show that as the electric field increases, the fundamental component of the leakage current decreases and both the third and fifth harmonic components increase. Also, increasing the salinity of the fog increases both the third and fifth harmonic components with no significant change in the fundamental component. In general, harmonic components increased only when arcing occurred; thus providing a signature for the dry band arcing activities.

In this work, the influence of particle size and concentration of fillers used in silicone rubber compounds for outdoor insulation is investigated. Alumina tri-hydrate and silica with mean particle sizes of 5 and 10 /spl mu/m are used as fillers with concentrations of 10, 30 and 50% by weight in a two-part room temperature vulcanized silicone rubber base polymer. The results from inclined plane tests show that samples with high concentrations of filler and/or smaller particle size have better tracking and erosion resistance than samples with lower concentration and/or larger particle size.

In this study the effect of filler particle size and concentration on the tracking and erosion resistance of silicone rubber loaded with silica and alumina tri-hydrate (ATH) as fillers has been investigated. Samples with different filler particle size and concentrations were tested in an inclined plane tracking and erosion resistance test apparatus. The loss of material is linked with the power dissipated by the fundamental and harmonic components, due to dry band arcing. The results show that smaller particle sizes and higher percentages of fillers, up to 50%, improve the tracking and erosion resistance of silicone rubber.

This work explores the thermal behavior of silicone rubber filled at different concentrations with alumina trihydrate (ATH) and silica. Three particle sizes, 1.5 /spl mu/m, 5 /spl mu/m and 10 /spl mu/m, were employed in the sample preparation. A near-infrared laser was used to heat the samples. The concentration of the filler plays a major role on improving thermal conductivity when compared to particle size. Nevertheless, better thermal conductivity could be distinguished at 5 /spl mu/m for ATH and 1.5 /spl mu/m for silica, at all concentrations.

In this study the effect of shed diameter and position on the aging performance of silicone rubber test samples in salt-fog has been studied. Tests have been conducted at 0.35 kV/cm electric field and 2500 /spl mu/S/cm conductivity. The low frequency components of leakage current, equivalent salt deposit density (ESDD) and surface damage are used to evaluate the performance of the different designs. The test results have shown that both the shed diameter and position influence the aging performance of silicone rubber insulators.

Removal of trace element, mercury, from gas stream using a wire-plate electrostatic precipitator has been investigated under pulse voltage application. Realizing the measurement problems with the mercury detector (Jerome 431-X) under the influence of charged air, the atomic absorption spectrometry technique was chosen to analyze mercury concentrations in the air flow. Samples were collected from both upstream and downstream of ESP using special sampling tube which has irreversible affinity for mercury (OSHA-140 method). The actual mercury concentration was measured using cold vapor atomic absorption spectrometer. Collection efficiency of ESP depended on the pulse voltage, pulse frequency and the initial concentration of mercury in the gas flow. As a function of concentration efficiency was higher at higher concentration of mercury; however, as a function of frequency the efficiency had a maximum around 150 Hz.

The objective of the work is to investigate the use of partial discharge (PD) measurements from water droplets as a possible means to study the pre-leakage current development on silicone rubber insulating surfaces. The experimental set-up, which allows PD measurements from water droplets on the insulating surface under controlled atmospheric conditions, is described. An optical system is also used for simultaneous observations of the surface conditions. Experiments with single droplets and water condensation on the insulator surface are reported and the corresponding PD patterns are presented. The study shows that it is possible to detect PD at the very early stage of leakage current development and correlate it to the changes in the field distribution due to water droplets.

The paper presents the results of a study in which the third harmonic frequency component of leakage current is used to study various design parameters of weathersheds that affect the ageing of polymer insulators in salt-fog. Salt-fog was applied to model insulators for 120 hours at two different conductivity levels, i.e. 1000 /spl mu/S/cm and 2000 /spl mu/S/cm as an aging test. The effect of the number of sheds and shed inclination angle on the aging performance of polymer insulators was studied. Model insulators having 2-sheds showed better performance than 4-sheds. Also, the shed inclination angle improved the performance of the insulators in salt-fog.

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.

Applications of pulsed electric fields (PEF) are dependent on the output pulse shape and pulse parameters. Both the design of the supply and the sample being tested can influence the pulse shape and its parameters. In this paper the output pulses of two pulsed power supplies, one using a thyratron switch and the other a MOSFET, are examined. The load used in the experiments consists of two circular coaxial electrodes separated by conductive water. The water conductivity is changed from 2.2 /spl times/ 10/sup -4/ S/m to 5000 /spl times/ 10/sup -4/ S/m and the effect on the output pulse amplitude, rise-time and fall-time are investigated and explained.

This work represents the continuation of the work presented by Meyer et al. (IEEE CEIDP, p.848-52, 2002), where thermal conductivity of room temperature vulcanized samples was tested. The thermal behavior of hot pressed silicone rubber filled at 30% and 50% concentrations with alumina trihydrate (ATH) and silica is now explored. A near-infrared laser and a thermovision camera were used to calculate thermal conductivities based on a thermal transient model. In addition to thermal conductivity results, the measurement of temperature in a modified inclined plane test highlights the importance of the thermal conductivity on tracking and erosion resistance.