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Collecting the fiber mats with uniform characteristics is a current technological bottleneck to industrial application (e.g. filter media, nano-sensors, separators of Li-ion batteries etc.). This paper provides a promising industrial method for collecting uniform fiber mats in large area. The simulation results demonstrated that different collection targets had different surface electric field distribution. The experiments gave a result that thinner and more uniform fibers were collected when using a specific spherical-hat target uniform surface electric field distribution, and the thickness of fiber mats was more uniform as well. The paper presents that the collector's surface electric field has great effect on the electrospinning fibers arrived at the collector, and higher electric field intensity can guide the orientation of the deposited electrospinning fibers. Index Terms--Electrospinning, fiber mats, collection target, surface electric field, electrostatic.

Recent experiments demonstrated that the essential electrospinning mechanism is a rapidly whipping dielectric liquid jet in an electric field. There are generally two main methods to control the jet, one is to control the solution and the other is to control the flight of the electrospinning jet through manipulation of the electric field. In this paper, a modified electric field distribution, providing a stagnation surface, is discussed in order to know the effect on the electrospinning jet. The results illustrate that if the bending starting point happened below the stagnation point, the jet movement could be random to any direction as no electric force on the jet at the stagnation point and if the bending starting point happened before the jet reached the stagnation point, the jet movement could be bending as the modified jet path. The results also show that the modified electric field distributions could change the jet path and control the fiber morphology. Index Terms--Electrospinning, stagnation point, electric field distribution, PEO.

Stator winding vibration is the most common cause of unexpected failure of high-voltage (HV) generators. Stator bars are generally fixed in slots by using slot wedges and a side packing system. Loose slot wedges could result in excessive vibration of the stator bars, which would accelerate the deterioration of the ground wall insulation. Loose slot wedge testing is consequently one of the most important general examination procedures to make sure the condition of HV generators. The common method used for slot wedge testing is manual measurement, whereby the slot wedge is tapped by hand using a small hammer. A judgment on looseness is made based on the sound from it. A digital signal processor (DSP)-based loose slot wedge tester is described in detail in this paper, including both hardware and software design. It was developed in our laboratory based on the study of slot wedge vibration characteristics in its loosening process. Since embedded software system of the DSP is oriented to a specific industrial field while PC system is of a general purpose, the device has many advantages compared to traditional manual measurement or other slot wedge testing equipment, such as portability, high intelligence, low power consumption, etc. The tester was applied for laboratory loose slot wedge testing of a slot model. The test results soundly verified the good performance of the device. Index Terms--Digital signal processor (DSP), ground wall insulation, high-voltage (HV) generators, slot wedge, stator bar, tightness testing, winding vibration.

When large generators become operational, the stator bars in the slots are subjected to the electromagnetic force during normal operation. After the generators have been operating for a long time, the electromagnetic force may cause the stator bars to vibrate due to the existence of loose stator slot wedges in large generators. This leads to mechanical abrasion of the groundwall insulation and accelerates the deterioration of the generator, and can even cause generators to fail if the slot wedges are not treated. In order to ensure large generators continue working reliably, it is essential to detect loose slot wedges in the stator slots. This paper describes the modeling of slot wedges with different fixing styles in large generators to study the process by which slot wedges become loose, and also presents an analysis of the looseness of slot wedges based on their vibration characteristics corresponding to the loosening process of the stator slot wedge. Moreover, a series of tests were performed with a nondestructive detection system developed based on this analysis to check the tightness of slot wedges in the laboratory. The results showed that loose stator slot wedges of large generators could be detected either by their vibration frequency or by their displacement amplitude. This verified the correctness of the slot wedge loosening process and the validity of the detection system. Index Terms--Large generator, stator, slot wedge, looseness detection, vibration.

This paper presents an analysis of the arcing and flashover performance of contaminated dc insulators of various suspension patterns at high altitudes (1970 m). The experiments tested two types of dc insulators, a bell-type insulator and a tri-shed suspension insulator. The experimental results show that dc arcs play an important role in the flashover performance since they levitate more easily from the porcelain surfaces at high altitudes than at sea level. For heavily contaminated insulators, the relationship between the insulator length and the flashover voltage per insulator unit is affected by the arc development path, and is lower for shorter insulator strings (10 to 20 insulator units). With a non-uniform contamination distribution between the top and bottom insulator surfaces, the arcs propagate close to the insulator surface, resulting in a higher flashover voltage than with uniform contamination. The Y-string configuration appears to lead to much lower flashover voltages than in I- and V-string insulators. With the latter, the arc propagation path is greatly influenced by the angle between the two insulator strings with the flashover voltages for both bell-type and tri-shed insulators increasing with the angle but apparently due to different mechanisms. Experimental results indicate that the bell-type insulator has better flashover characteristics than the tri-shed insulator. Index Terms--Porcelain insulators, flashover, suspension pattern, arc.

The actual stator bars of an 18 kV/300 MW generator were used for accelerated multistress aging tests to investigate the degradation of ground insulation. An aging system with combined stresses including thermal, electrical, mechanical vibration, and thermomechanical factors was used to test the aging of stator bar specimens. The nondestructive parameters including partial discharge (PD) quantities and dissipation factor parameters were measured during aging test in order to evaluate the degradation of stator ground wall insulation. At the end of the aging test, an ac breakdown test was performed on the bars in order to obtain the remaining breakdown voltage (BDV) after their nondestructive data had been measured. The changes of nondestructive parameters indicated that the degradation of ground wall insulation. For example, the changes of skewness of [H.sub.qn] ([gamma]) indicated the shape of voids in ground insulation changes from flat to narrow, and even exhibits treeing, which is dangerous to insulation because it shortens the effective distance of insulation between high voltage and the ground. The results obtained are summarized as follows. The BDV data fit a Weibull distribution. The correlation between the dissipation factor, the PD parameters, and the breakdown voltage were analyzed, among which the skewness of the average discharge had the most correlation with breakdown voltage. Index Terms--Aging, breakdown voltage (BDV), correlation, discharge, dissipation factor, generator, ground wall insulation, multi-stress, nondestructive, partial discharge (PD) quantity.

According to hydrophobicity and hydrophobicity transfer studies on silicone rubber surfaces, although the silicone rubber can transfer its hydrophobicity to various contaminants deposited on its surface, the hydrophobic states of water droplets on clean silicone rubber surfaces and on various contaminated silicone rubber surfaces were found to be different based on evaluation by contact angle measurement. Furthermore, surface discharge and aging of materials decreases the hydrophobicity of the surface. Focused on the difference of hydrophobicity and the effect of water droplets on the hydrophobic contaminated surfaces, some contrastive computations and tests of water droplets on various surfaces were carried out in this study. The results showed that the water droplets existing on the surface of silicone rubber materials might change the electric field and voltage distribution along the surface, and even reduced the initial voltage of the corona discharge, which was verified with a comparison test on a high voltage side of polymer insulator monitored by a photomultiplier tube (PMT) system. Since the leakage current provides much useful information to diagnose the state of outdoor insulation, this study investigates the time and frequency distributions of surface discharges on ceramic insulators, new room temperature vulcanized (RTV) silicone rubber coated insulators and aging RTV insulators with various hydrophobic states with the short time Fourier transform and Gabor transform methods. Then a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces. Index Terms--Hydrophobicity, silicone rubber, discharge, leakage current, electric field distribution

The dynamics of positive streamer in air under different pressure and humidity conditions was studied. The widely used three-electrode system was used to obtain the streamer propagation probability and average propagation velocity as a function of background field. A simulation was performed based on the 1.5D fluid model. Simulated intrinsic stability field for streamer propagation and corresponding average propagation velocity were in agreement with those deriving from experimental results. The intrinsic stability fields obtained with the empirical equations proposed by the author and other investigators were compared. It was analyzed that the effect of the amplitude and duration of voltage pulse applied on the needle. The statistical distribution of propagation field was explained by the dispersion of discharge time delay. The decrease of intrinsic stability field with the reduction of pressure and/or humidity was due to the increase of the conductivity of the streamer channel as a result of the weakening of attachment and/or electron-ion recombination effect. Index Terms--Fluid model, humidity, pressure, propagation field, propagation velocity, streamer.

The control of fiber deposition in electrospinning, a technological bottleneck, has received a wide attention. This paper describes the use of polyvinyl chloride (PVC) insulating tubes to modify the electric field distribution and to produce a more uniform and concentrated electric field. The electric field distribution modified by the PVC tube plays an important role in controlling the swinging straight part of the jet and the spiral angle of the bending process. The fiber morphology was affected by the polarity of the applied voltage and finer fibers were produced by negative voltages than by positive voltage due to the ability of the charges caught and kept on the insulating material. Fibers without beads were produced in an 18 mm diameter area using a 7 cm diameter, 15 cm long PVC tube with 17.5 kV applied to the needle, 30 cm from the collector. The production rate was 13.89 g/h.[m.sup.2]. Index Terms--Electrospinning, nanofiber, insulator, electric field distribution.

Ice accumulation on high voltage insulators can cause accidents by flashover across the ice-covered insulators. The ice accumulation can be reduced by changing the insulator surface properties by application of a semiconducting RTV silicone coating. This method is based on the hydrophobicity and semiconductivity properties of the coating. The mechanism for reducing ice accretion is analyzed theoretically. Experimental results are presented to validate the effectiveness of the semiconducting RTV coating at reducing ice on insulator strings. The results show that the coating on the insulators delays ice formation and reduces the amount of ice. Index Terms--Ice, insulator, semiconducting, coating, RTV, flashover, hydrophobicity.

Synergy study is of great importance to optimize the two-stage plasma-facilitated catalysis system and also necessary to gain a mechanistic understanding of the overall reaction process. In this paper, the effects of the long-lived chemical products of plasma pretreatment on the subsequent selective catalytic reduction (SCR) of N[O.sub.x] at fixed and changed energy density of dielectric-barrier-discharge plasma reactor were investigated, respectively, for the aim of the synergy study of plasma-facilitated [C.sub.2][H.sub.4]-SCR (PF-SCR) of N[O.sub.x] over Ag/[gamma]-[Al.sub.2][O.sub.3] catalyst at lower temperature. For comparison, [C.sub.2][H.sub.4]-SCR of N[O.sub.x] over Ag/[gamma]-[Al.sub.2][O.sub.3] catalyst alone was also considered. Results showed that PF-SCR for N[O.sub.x] removal exhibited obvious synergistic effect at 240 [degrees]C, and the N[O.sub.x] removal efficiency for plasma alone, [C.sub.2][H.sub.4]-SCR alone, and PF-SCR were 43.6%, 6.7%, and 90.1%, respectively. In the NO/N[O.sub.2]/[C.sub.2][H.sub.4]/[O.sub.2]/[N.sub.2] system, the long-lived chemical products of plasma pretreatment for the synergistic link between plasma and catalysis included N[O.sub.2], aldehyde (formaldehyde and acetaldehyde), and nitrogen-contained organic compounds (methyl nitrate, nitro methyl, and ethyl nitrate). Among the nitrogen-contained chemical products of plasma pretreatment, the major synergistic link between plasma and catalysis was N[O.sub.2] at lower input energy density, transferred to the mutual action of N[O.sub.2] and nitrogen-contained organic compounds over catalyst at higher input energy density. Index Terms--Long-lived chemical products, nitrogen-contained organic compounds, N[O.sub.x] removal, plasma-facilitated catalysis, synergy.

The characteristics of field-aged room temperature vulcanized (RTV) silicone rubber coatings were investigated for use in heavily contaminated areas with emphases on the antipollution flashover performance of aged RTV-coated insulators. An atomic absorption spectrophotometer and ion chromatography (IC) were employed to study the composition of contamination deposited on the insulators, with scanning electron microscopy used to analyze the configuration of the contamination and the aged coatings. The results showed that the sample insulators were heavily polluted with significant changes of the coatings due to field aging compared to new RTV coatings. Flashover did not occur in voltage withstand tests even with high voltage. In addition, the wetting process of the lower surface was not in phase with that of the upper surface during the pollution tests, which increased the pollution flashover voltage on the sample insulators more than the reduced influence on flashover voltage caused by highly soluble salts deposited on the surfaces of the samples. All of the results showed that RTV coatings provided long available term of protection against the pollution flashover, even in heavily contaminated areas. Index Terms--Field aging, hydrophobicity, leakage current, pollution, pollution flashover, room temperature vulcanized (RTV) coating, wetting.

Room temperature vulcanized (RTV) silicone rubber coatings are used increasingly on outdoor high voltage (HV) insulators subjected to heavy pollution. An important property of the RTV coatings is the ability to restore the surface hydrophobicity after a pollutant layer has built up on the surface, which can suppress the development of leakage currents, dry band arcing and flashover. The present study analyzes the hydrophobicity and the hydrophobicity transfer of RTV coatings and the factors that affected the hydrophobicity. A modification of absorption and cohesion theory was used to analyze the hydrophobicity transfer mechanisms and the influences of temperature, polydimethylsiloxane (PDMS) molecular weight, degree of cross-linking, extent of soluble and non-soluble components, and the pollutant properties. The hydrophobicity transfer of RTV coatings was also investigated with various conditions of contaminations and various factors, which affect the hydrophobicity transfer, including the composition of RTV coatings and the outside conditions. Results show that the hydrophobicity transfer of RTV coatings is not only dependent on compositions but also on factors of circumstance. The analysis also explains the obvious difference between the hydrophobicity transfer of RTV coatings to kieselguhr and kaolin based on a particle and surface analysis using the Brunauer, Emmett and Teller (BET) method, which explains the relationship between a BET surface area and the hydrophobicity transfer of the RTV coatings. Index Terms--RTV silicone rubber, hydrophobicity, absorption and cohesion.

The effect of pulsed electric fields (PEFs) applied on the enzymes horseradish peroxidase (HRP) and pectinesterase (PE), which widely exist in vegetable and fruit juices, was investigated using chambers with paralleled plane electrode and coaxial cylinder electrode, respectively. The experimental results show that the relative activity of HRP and PE decreased after PEF treatment. Inactivation effect of HRP and PE increased with the increase of the applied electric strength and the number pulses. HRP activity decreased 14.4 % and 16.7 % in the parallel-plane chamber and the coaxial cylinder chamber, respectively, at 207 pulses of 25 kV/cm, while a 17% (coaxial) and 16.46% (plane) reduction, respectively, was attained for PE under the same experimental condition. Inactivation effect of HRP and PE in the chamber with coaxial-cylinder-electrode configuration was better than the parallel-plane electrode. The circular-dichroism analysis showed that PEF treatment caused a loss of [alpha]-helix and change of the secondary-conformation structure of HRP enzyme. The fluorescence intensity of both enzymes also changed after PEF treatment, indicating that conformation changes occur in the local structure of the enzyme molecule. Index Terms--Circular-dichroism (CD) analysis, fluorescence spectroscopy analysis, horseradish peroxidase (HRP), pectinesterase (PE), pulsed electric field (PEF).

Inactivation of Escherichia coli by bipolar pulsed discharge in an air-liquid-solid three-phase discharge reactor has been investigated. The effect of several operating parameters on the inactivation efficiency has also been studied. Experimental results showed that bipolar pulsed discharge in the three-phase discharge plasma reactor had good performance in the inactivation of E. coli. Power intensity and gas flow rate had a positive effect on the inactivation, while the initial solution conductivity had little effect. An alkaline solution condition was favorable for the inactivation of E. coli in the reactor. In addition, possible mechanisms of the inactivation were also discussed. Index Terms--Bipolar pulse, Escherichia coli, sterilization, three-phase discharge.

Electrospinning is an old but yet immature process which is now used to form nanoscale polymer fibers. This paper describes the use of SEM and measurements of jet currents to investigate the effects of key parameters on fiber formation during electrospinning of a polyethylene oxide solution (PEO). The ambient humidity and voltage polarity are key factors influencing fiber formation, but this process has not been carefully analyzed. The influence of the ambient humidity can be overcome by choosing a solvent with a larger difference between the saturated vapor pressure and the actual vapor pressure in air or by lowering the ambient humidity when water is used as the solvent. The process works with both negative and positive voltages, but the polarity influences the stability and the fiber diameters. Variations of the current were found to correspond with variations of the fiber diameters. Short target distances were found to prevent fiber formation while long distances resulted in a larger distribution of fiber diameters. The experiments also showed that ultra fine fibers were found between the ends of broken fibers due to a strong electrical force on fibers with little solvent. Index Terms--Electrospinning, humidity, voltage polarity, fibers.

Emission spectrum of the bipolar pulsed discharge in water-air mixture has been measured in the near-ultraviolet (UV) region using an Inspectrum 300 charge-coupled device (CCD) spectrograph. Effects of several parameters including peak-to-peak voltage, pulse repetitive frequency, etc., on the relative UV radiation intensity have been investigated. Experimental results show that bipolar pulsed discharge in water-air mixture emits strong UV radiation. The emission spectrum of the discharge has a wide range. The applied peak-to-peak voltage, puise repetitive frequency, and air flow rate have positive effect on the relative intensity of the second positive band of nitrogen at 337.1 nm. However, solution conductivity has negative effect. Index Terms--Bipolar pulse, plasma, ultraviolet (UV) radiation, water-air mixture.

In order to measure the grounding impedance of a grounding system more precisely and easily, two key problems of the measurement system: the location of the voltage electrode and the elimination of the noise and interference must be conquered. The new theoretical analysis and measurement system of the grounding impedance based on short-current lead wire and a swept frequency ac source is proposed. The compensating position of the voltage electrode to obtain the real grounding impedance is evaluated by analyzing the soil structure, the grounding system under test, and the position of the current electrode. This new system measures the ground impedance at several frequencies other than power frequency, then interpolates them to acquire the ground impedance at the power frequency. The mutual inductance between the potential and current wires is analyzed and eliminated. The industrial computer-based grounding impedance measurement system is developed and the analysis software is programmed. A field test was performed and compared with the traditional method in the substation and the novel system worked well. Index Terms--Grounding impedance, quasi-synchronous sampling algorithm, substation, swept frequency ac source.

Water drops on surface of polymer insulation materials may cause corona because they can enhance the electric field nearby. Water drop corona plays an important negative role in the long-term performance of composite insulators. In this paper, based on a model with two parallel electrodes water drop corona is studied. The relationship between initial corona voltage and different sets of water drops is obtained from experiments. Then several factors, which may affect the electric field analysis of water drop corona, are studied by numerical calculation on computer. Calculation is mainly on a 2-D model. However, a 3-D model is also used to study the effect of water drops distributed in space. Index Terms--Composite insulation, corona, electric fields, numerical analysis.

The shielding failure is the occurrence of a lightning stroke that bypasses the overhead ground wires and terminates on the phase conductors. The correct design to improve the shielding effect of transmission line to lightning is one of the key problems of transmission line design. A numerical analysis model for shielding failure of transmission line based on electromagnetic field theory is proposed in this paper. The electrical field of the system including lightning leader, transmission line and ground is solved, the range of lightning current striking the phase conductor is obtained, and the shielding failure probability of transmission line is calculated. The lightning leader model and the critical breakdown electrical field intensity of long gap are discussed. The analyzed results by numerical analysis model are compared with those by the electrogeometric model. Index Terms--Critical breakdown electrical field, electrical field, electrogeometric model, lightning leader, numerical analysis model, shielding failure, transmission line.

Grounding system is very important to maintain the safe and reliable operation of power network and ensure the safety of power apparatus and operators. A new technique to decrease grounding resistance of substation is presented in this paper, which makes use of deep ground well to decrease the grounding resistance. The ground well is formed by metal tube with water percolation apertures in the soil with groundwater, which has the ability to gather the groundwater and wet the surrounding soil. The principle of deep ground well to decrease grounding resistance is to decrease the resistivity of the soil region surrounding the grounding well by leading the groundwater moving to the deep ground well under pressure, and consequently decrease the grounding resistance of the deep ground well. This novel method was applied in a substation grounding engineering, and good effect was achieved. Index Terms--Deep ground well, grounding electrode, grounding resistance, grounding system, soil resistivity, substation.

Analyzing the nonuniformity of electrical parameters of ZnO varistors is useful to select and coordinate the ZnO varistors in parallel operation and improve the protection capability of surge-protection device (SPD). The nonuniformity of 1-mA dc voltage and impulse residual voltage of commercial low-voltage ZnO varistors were estimated by statistic analysis in this paper. The Minimum Gobble Distribution was used to describe the distribution of 1-mA dc voltage and impulse residual voltage, ZnO varistors were verified conforming to the Minimum Gobble Distribution by the Kolmogorov-Smirnov test. The relationship between the impulse residual voltage and 1-mA dc voltage has a chaos phenomenon, and they have a close relative degree by t-test, the impulse residual voltage can be estimated by voltage ratio and 1-mA dc voltage. The nonuniform phenomena of electrical parameters of ZnO varistors were simulated by Voronoi network. The current overload of ZnO varistor with reduced residual voltage is serious, in order to keep safe operation of SPDs, when we designa SPD with ZnO varistors in parallel, a safe margin coefficient of discharge withstand current should be considered according to the actual relative standard deviation of surge residual voltages of used ZnO varistors. Index Terms--Discharge withstand current, electrical parameter, Kolmogorov-Smirnov test, minimum gobble distribution, nonuniformity, statistic analysis, surge-protection device, surge residual voltage, ZnO varistor, 1-mA dc voltage.

Frozen soil leads to the change of the soil model. This would affect the safety of the grounding system. The design of the grounding system considering the seasonal frozen soil should be based on the full investigation of the actual maximum depth of the frozen soil and the actual layered soil models. The final design scheme of the grounding system should be determined synthetically from two phases-first, the grounding system is designed in the normal soil model and its safety is checked in the frozen soil model, second, it is designed in the frozen soil model and its safety is checked in the normal soil model. The influences of the frozen soils on the optimal design of grounding systems in homogeneous and double-layer soil models were systematically analyzed, and the respective design methodologies of the grounding systems were proposed. The influence of vertical electrodes on the design of the grounding system considering the seasonal frozen soil was discussed. The analyzed results state that the safety of grounding system can be sufficiently improved by vertical grounding electrodes added to the horizontal grounding grid. Index Terms--Frozen soil, grounding resistance, grounding system, optimal design, safety, step voltage, touch voltage, vertical grounding electrode.

How to precisely measure the grounding resistance of substations is the fundamental factor to guarantee the safe operation of power systems. The placement rules of test electrodes for the grounding resistance measurement of grounding system in vertical-layered soil were analyzed by the numerical method in this paper. The actual demanded location of the test potential electrode respective to the true grounding resistance of grounding system in different measuring directions changed (in a wide range) when the grounding system was arranged in different soil models. The test error of 0.618 method to measure grounding resistance sometimes is very high. The test route that the test circuit is in parallel with the vertical boundary is recommended for measuring the grounding resistance of grounding system in the vertical-layered soil area. The scientific and suitable measuring method of grounding resistance is based on analyzing the actual soil model and the grounding system structure to obtain the suitable test route and choose the correct compensated point location of the potential electrode. Index Terms--Fall of potential method, grounding resistance, grounding system, substation, vertical-layered soil.

The electromagnetic environments (EMEs) of the planned Zhongguancun Software Park near transmission lines, including electrical field, magnetic field, and ground potential rise under three cases of lightning stroke, normal operation, and short-circuit faults, are assessed by numerical analysis. The power frequency EMEs of the software park are below the maximum ecological allowed exposure values for the general public;, nevertheless, the power frequency magnetic field may interfere with the sensitive computer display unit. The influence of short-circuit fault in two different cases of remote short circuit and neighboring short circuit on the software park is discussed. The main problem we must pay attention to is the ground potential rise in the software park due to neighboring short-circuit fault; it would threaten the safe operation of electronic devices in the software park. On the other hand, the lightning stroke is a serious threat to the software park. How to improve the EMEs of the software park is discussed. Index Terms--Electromagnetic environment (EME), electromagnetic field, ground potential rise, lightning, pulse magnetic field, short circuit, software park, transmission line.

The nonuniformity of electrical characteristics of grain boundaries in ZnO varistors was systematically analyzed. The high nonuniformity exists in barrier voltages and nonlinearity coefficients in different grain boundaries. The barrier voltages have normal distributions, only a few grain boundaries were electrically active, and the grain boundaries can be simply classified into good, bad, and ohmic ones according to the electrical characteristics of grain boundaries. The average barrier voltage is equal to 3.3 V by the direct method, but it is only 2.3 V by the indirect method. There is a high difference between the barrier voltages by direct and indirect measurement methods. These few good grain boundaries are responsible for the good varistor effect, and control the leakage current of ZnO varistor at low values of applied voltage. The [Al.sub.2][O.sub.3] dopants affect the electrical characteristics of grain boundaries by changing the electron status in grain boundary and intragrain. Index Terms--Barrier voltage, electrical characteristics, grain boundary, nonlinearity coefficient, nonuniformity, normal distribution, ZnO surge varistor.

Nonuniformity of potential distribution of metal-oxide (ZnO) surge arresters reduces service lifetime of the arresters. The metal-oxide surge arresters of polymer housings developed can be suspended in different places. A combined method of electrical field and electric circuit is proposed to analyze the potential distribution of the suspended arresters. The equivalent electric circuit is obtained from a charge simulation method (CSM) and matrix transformation, and the potential distribution is solved from the electric circuit analysis by the first law of Kirchhoff. The complicated electromagnetic field problem is then converted to a simple electrical circuit problem. The calculation results show a good agreement with experimental results. Potential distributions of surge arresters suspended on a conductor by a wall, on a framework, and on a transmission-line tower are presented. Index Terms--Charge simulation method, numerical analysis, potential distribution, surge arrester.

Reducing grounding resistance of a transmission tower grounding device is an important method to improve performance of transmission lines against lightning. Principles of impulse characteristics of grounding devices were discussed. Impulse experiments using grounding device models were systematically performed to analyze the influence of different factors on the impulse characteristics of grounding devices. During the experiments, soil resistivity was changed in the range of 100 to 5103 [OMEGA].m. Effects of different parameters on impulse grounding resistance and impulse coefficients of different transmission tower grounding devices were discussed. Formulae to calculate impulse coefficients and power frequency grounding resistance of different grounding devices were obtained. Index Terms--Transmission tower, grounding device, impulse grounding resistance, impulse coefficient, simulation experiment.

The low or high resistivity soil layer formed in raining or freezing season affects the safety of grounding system, and leads the changes of grounding resistance of the grounding system, step and touch voltages on the ground surface. This paper systematically discusses the seasonal influence on the safety of grounding system by numerical analysis method. If the thickness of the low-resistivity soil layer formed in raining season is smaller than the burial depth of the grounding system, the touch voltage would increase. On the other hand, the raining season leads the limit of touch voltage is smaller than the actual touch voltage, even if a surface granite layer is used. The influences of raining on the safety of grounding system must be comprehensively considered. When the high resistivity soil layer is formed in freezing season, the grounding resistance, step and touch voltages would increase with the thickness or resistivity of the freezing soil layer. When the thickness of high resistivity soil layer exceeds the burial depth of the grounding system, the grounding resistance, step and touch voltages would increase sharply, and the touch voltage would exceed the limit of touch voltage decided by a surface granite layer. In order to ensure the safety of the grounding system in freezing season, the grounding system must be designed carefully, and special measures should be used, long vertical grounding electrodes added to the grounding system can effectively attenuate the seasonal influence, and improve the safety of the grounding system. Index Terms--Freezing season, grounding resistance, grounding system, raining season, safety, step voltage, surface material, touch voltage, vertical grounding electrode.

A model for predicting the flashover voltage of ice-covered insulators energized with ac voltage is presented in this paper. The model takes into account the variation of ice surface conductivity as a function of freezing water conductivity. The effects of the length of the arc on its own characteristics, as well as the ac arc reignition condition, were determined in a laboratory investigation using triangular ice samples. The model is applied to a short string of 5 IEEE standard insulator units. There is an excellent concordance between the 50% flashover voltage calculated from the mathematical model and the experimental results.