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To lower the lightning impulse withstand voltage of gas insulated switchgear (GIS) while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding researches, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five to six kinds of non-standard lightning impulse waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. Then, the dielectric breakdown voltage - time characteristics were measured under several different conditions on the quasi-uniform SF6 gas gaps and partly the cone- shaped insulating spacers that represent insulation elements of GIS for six kinds of non- standard lightning impulse waveforms. In this paper, the resultant breakdown voltages were evaluated in terms of the overvoltage duration, which led to their formulation in a unified way. On the basis of these insulation characteristics and their unified formulation, the paper investigated a method for converting non-standard lightning impulse waveforms into standard lightning impulse waveforms with equivalent stress for the insulation. When the constructed algorithm was applied to five examples of representative two type waveforms in the lightning surge time region, they were converted into standard lightning impulse waveforms with crest values reduced by 20% to 34%, suggesting potentiality for reduction of lightning impulse insulation specifications of GIS. [ABSTRACT FROM AUTHOR]

In this paper, a model based on dynamic electric field analysis has been developed to predict the flashover voltage of the ice-covered HV insulators, under dc voltage. The potential and electric field calculation models before and after air gap breakdown are built respectively based on finite element method (FEM). The arc initiation process is determined based on the model before air gap breakdown. The critical applied voltage and leakage current to maintain an arc with certain length are obtained based on the electric field calculation model after air gap breakdown and the U-I characteristic of the arc. Moreover, the improved Hampton criterion has been employed to determine the critical flashover of the ice-covered insulator. The results obtained from the dynamic electric field analysis model have been compared with other mathematical and experimental results of other researchers and got a great agreement. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the effect of a high-frequency high-voltage impulse conditioning technique on the inrush current interruption phenomena. In this paper, the inrush current interruption phenomena referred to the actual interruption and the currentless period following interruption. First, Six 7.2 kV vacuum interrupters (VIs) were conditioned by 400 batches of voltage impulses. The duration of each batch were set to 0.1 s. The peak value of each impulse could reach up to 100 kV. The repetition frequency of the impulse was set to 1000 Hz. The contact gap was 0.8 mm. The VIs were tested for the inrush current interruption performance with inrush current set to 2 kA and 5 kA, with a frequency of about 3800 Hz. The experimental results showed that there were two different types of inrush current interruption depending on high-frequency high-voltage impulse conditioning. The inrush current interruption type I referred to the inrush current interruption during the prestrike process before the contacts touched. This type inrush current interruption could be caused by the erosion of prestrike arc. Moreover, the high frequency voltage impulse conditioning influenced the occurrence frequency, the number of interruptions and the duration of the inrush current interruption type I. The inrush current interruption type II referred to the inrush current interruption during the contact bouncing process after the contacts touched. The inrush current interruption type II occurred only in the case of the VIs after conditioning. Furthermore, the occurrence frequency of the inrush current type II decreased with increasing inrush current. [ABSTRACT FROM AUTHOR]

The space charge accumulation can affect the dielectric performance of oilimpregnated pressboard in convertor transformers. In this paper, the space charge behaviour in 1mm thick impregnated pressboards is investigated by the means of pulsed electroacoustic method. The dried pressboards are separately impregnated in fresh oil and service aged oil as comparison. The space charge distributions are investigated under ?15 kV/mm and ?25 kV/mm at room temperature. The homocharge injection can be observed for all the samples, which is much more significant in the aged oil impregnated samples. The amount of injected charges and the depth of the injected charges accumulation in the pressboard bulk are closely related to the oil properties, amplitude and polarity of the applied voltages. The service aged oil shows the dominant effects on the local electric field distortion, about 48% enhanced in the aged oil impregnated samples while about 15% enhanced in the fresh oil impregnated samples. The estimations of the apparent charge mobility and trap depth also indicate the great impacts caused by the service aged oil. [ABSTRACT FROM AUTHOR]

When digitally acquiring partial discharge (PD) signals, the bandwidth, the parameters of the measuring circuit and the arrangement of the test set-up play a role in preventing the signals from being affected by oscillations, distortions and noise. In this paper, a compact design of test set-up for PD measurements and a method for characterization of the frequency response of the measuring circuit is proposed. The frequency characterization was based on the induction of a fast current pulse in the closed HV loop of the measuring circuit. Frequency domain analysis and the use of a RLC model led to a practical estimation of the circuit inductance L. Experimental results showed that the resonance produced by the circuit inductance and the combined capacitance of the test object and the coupling capacitor C can produce resonance peaks within the bandwidth of the measuring PD sensor, which results in large oscillations. In addition, it was shown that adding a critical resistance Rc damps the oscillations, thus reducing the error in parameter computation. [ABSTRACT FROM AUTHOR]

Contamination of free conducting particles is a major threat to gas insulated systems. In HVDC GIL, free conducting particles can be even more harmful due to the fact that particles can easily cross the gas gap under DC voltage and distort the electric field. In HVDC systems, impulse overvoltage can be generated and superimposed on the DC operating voltage. As a result, GIL will experience superimposed DC and impulse voltage. In this paper, the behavior of free conducting particles in HVDC GIL is observed and breakdown characteristics of particle contaminated GIL under superimposed DC and impulse voltages are studied. Two motion patterns of free conducting particles in HVDC GIL are mainly found: bouncing and firefly motions. Particles tend to firefly motion near the enclosure under positive DC voltage and near the high voltage bus bar under negative DC voltage. With increased DC voltage amplitude, the probability of firefly motion increases while the probability of bouncing motion decreases. It is found that the insulation strength of GIL is greatly reduced by free conducting particles under superimposed DC and impulse voltages. With increasing particle length, the breakdown voltage of GIL under superimposed voltage decreases sharply. Breakdown voltage decreases slightly with the increase of particle radius. Breakdown voltage of GIL is lower under superimposed voltage of negative DC and positive impulse than under negative DC and negative impulse with the same size of particle. The results indicate that superimposition of DC operating voltage and impulse overvoltage can be a critical condition for HVDC GIL if free conducting particles exist. Therefore, a superimposed DC and impulse voltage on-site test can be considered for HVDC GIL to check for insulation performance. [ABSTRACT FROM AUTHOR]

In many cases of on-site PD measurement, it is difficult to acquire the perfect phase angle of the PD signal synchronized with the applied voltage. This paper describes a new method of PD diagnosis which can classify the types of defects for unknown phase-shifted PDs using decision tree method. 327 cases of on-site PD data were collected in the field. 5 valid parameters were extracted and the decision tree-based diagnostic algorithm was used to classify types of 6 PDs and 4 noises clearly. Finally we confirmed the new method of PD diagnosis can be applied to on-site GIS diagnosis. [ABSTRACT FROM AUTHOR]

The inner area close to cap and pin of insulator is sometimes heavier polluted than the outer area, causing the contamination pattern on insulator surface look like ring-shape. This kind of non-uniform pollution may affect insulator flashover voltage. In this paper, artificial pollution tests of 4 types of porcelain and glass insulators with ring-shaped non-uniform pollution were carried out. Experimental results indicate that the SDD (salt deposit density), the non-uniform pollution degree K (the SDD ratio of inner part to outer part), the heavily polluted area with radius r, and the insulator profile all have significant effects on insulator flashover performance. The effects of K and r on flashover performance can be reflected by the change of pollution layer conductivity during flashover. Under ring-shaped non-uniform pollution, the relationship between Uso and SDD can still be expressed by negative power function, while coefficient a varies obviously with pollution non-uniformity, but exponent n does not. Finally the correction coefficients of flashover voltage considering non-uniform pollution degree K were also calculated for outdoor insulation design. [ABSTRACT FROM AUTHOR]

Inductive fault current limiters (FCLs) have several advantages, such as significant current limitation, immediate triggering and relatively low losses. Despite these advantages, saturated core FCLs have not been commercialized due to its large size and associated high costs. A major remaining challenge is to reduce the footprint of the device. In this paper, a solution to reduce the overall footprint is proposed and discussed. In arrangements of windings on a core in reactors such as FCLs, the core is conventionally grounded. The electrical insulation distance between high voltage winding and core can be reduced if the core is left at floating potential. This paper shows the results of the investigation carried out on the insulation of such a coil-core assembly. Two experiments were conducted. In the first, the behavior of the apparatus under high voltage conditions was assessed by performing power frequency and lightning impulse tests. In the second experiment, a low voltage test was conducted during which voltages of different frequencies and pulses with varying rise times were applied. A finite element simulation was also carried out for comparison and further investigation. [ABSTRACT FROM AUTHOR]

The surface discharge injected current were measured by using High Frequency Current Transformer (HFCT). It is observed that the rise time of the current pulse is about 800ps. In addition, the rise time of the current pulses has not altered in copper sulphide (CuS) diffused Oil Impregnated Pressboard (OIP) material, but the width of the current signal has increased with the increase in the amount of copper sulphide diffusion into the pressboard. It is confirmed that the injected current pulse radiates Ultra High Frequency (UHF) signal and the dominant frequency of the UHF signal formed due to the surface discharge injected current is about 0.9 GHz. The characteristic frequency contents of the UHF signal formed due to surface discharge activity is the same with the virgin and copper sulphide diffused pressboard material. The surface discharge inception voltage is high with the virgin specimen compared with copper sulphide diffused oil impregnated paper insulating material and is the same irrespective of the amount of diffusion of copper sulphide to it. The Phase Resolved Partial Discharge (PRPD) analysis indicates that at the point of surface discharge inception, the discharge occurs at the peak of the applied AC voltage and when the applied voltage magnitude is increased, the discharge occurs at the rising portion of the applied AC voltage. The characteristics of the surface discharges formed are the same with the copper sulphide diffused oil impregnated pressboard material. It is noticed that with increase in ageing time, a reduction in partial discharge magnitude is observed. Attenuated total reflectance Fourier transform infra-red spectroscopy (ATR-FTIR) analysis was carried out to understand the characteristic changes that occur in the degraded zone formed due to surface discharges. The measurement of surface charge indicates that ageing of oil impregnated paper material increases the surface charge accumulation. Energy Dispersive Spectroscopy (EDS) confirms the presence of copper and sulphur in the aged Oil Impregnated Pressboard material. The Atomic Absorption Spectroscopy (AAS) results indicate that the amount of copper content in pressboard material have increased with ageing time. [ABSTRACT FROM AUTHOR]

This paper focuses on the effects of hindered phenolic antioxidants AO300 and AO736 with similar molecular structure on the electrical properties of isotactic polypropylene (iPP). Experiments of conductivity, space charge distribution, and DC breakdown strength after the hetero-polarity DC prestress are performed. The experimental results show that these two antioxidants can increase the conductivity-temperature coefficient, reduce the accumulation of space charge and improve the DC breakdown strength after the hetero-polarity DC prestress of iPP. By calculating the molecular orbitals of these two antioxidants, it is found that the antioxidants introduce local states in the forbidden band of iPP matrix to capture high-energy charges. Besides, the bond dissociation energy (BDE) of the hydrogen-oxygen bond in AO736 is lower than that in AO300, representing a stronger ability of proton transfer, which scavenges the radicals, inhibits the growth of microvoids and prevents the process of breakdown. It is concluded that the improvement of space charge and breakdown properties is attributed to the coupling effects of deep trap sites and radicals scavenging introduced by antioxidant. This research shows that AO736 has potential applications in PP insulation. [ABSTRACT FROM AUTHOR]

In this paper, a model of the Kelvin probe is established in COMSOL Multiphysics based on electrostatic field theory, and it is verified by measuring the surface potentials. The influence of the probe is also investigated. An improved charge inversion method based on surface potentials measured online is proposed. The surface charge density is directly obtained by the Gauss's theorem to avoid the calculation of the inversion matrix. The improved method is verified by comparing the surface charge density calculated by the analytical method. When considering the effect of the probe, the maximum difference in surface charge density is 0.3 µC/m2 comparing to that when ignoring its effect, and the percentage of the difference is as high as 41%. The results indicate that the effects of the voltage of the probe under online measurement conditions must be considered. [ABSTRACT FROM AUTHOR]

Gas insulated switchgear (GIS) is used in power system more and more widely. With the increased voltage level of power system, field impulse voltage withstand test becomes increasingly important. It would be beneficial for a comprehensive insulation evaluation to carry out the detection of partial discharge (PD) under impulse voltage withstand test in field. Metal protrusions or particles on HV conductor are typical defects in GIS and are critical at lightning impulse voltages. This paper therefore aims at PD characteristics study on needle-plane defect under oscillating lightning impulse (OLI). PD detection system was established based on ultra-high frequency (UHF) and pulse current detection. Then, the PD of a needle-plane defect under negative OLI was measured. The relation between statistical time lag, PD amplitude, PD times and the applied voltage was analyzed respectively based on UHF detection. The results show that UHF detection has high sensitivity and it is suitable in field impulse voltage withstand test. PDs occur around each wave peak of the applied voltage. With the increase of peak voltage, PD magnitude and PD times in each oscillation cycle both increase linearly. Further analysis indicates that the occurrence and development of PD under negative OLI can be interpreted by electron emission from metal surface, field detachment from negative ions and corona stabilization effect. This approach is valuable for PD measurement and analysis in field impulse voltage withstand test for GIS. [ABSTRACT FROM AUTHOR]

According to the vacuum breakdown model based on Cranberg “clump” hypothesis, micro-particles on contact surface can be detached from the contact surface and accelerated across the vacuum gap towards to the opposite contact under an applied voltage in vacuum interrupters (VIs). Finally they can impact on the target contact surface with high velocities, which probably triggers a breakdown in the vacuum gap. Thus, the mechanism of the impact phenomena may be the key to understanding the mechanism of the vacuum breakdown initiated by the micro-particles. Moreover, the detailed nature of the dynamic impact process is a subject of the hydro-dynamical theories on the projectile impact. The objective of this paper is to study the micro-particles impact phenomena in VIs by introducing a mesh-free numerical calculation method Smoothed Particle Hydrodynamics (SPH) method which is an efficient method of the projectile impact studies. In this paper, the materials of the micro-particle and the target contact were assumed to be copper and stainless-steel, respectively. The radius of the micro-particle was assumed to be 0.1 µm. The characteristics of the deformations occurring during the high-velocity impact processes are investigated under the different applied voltages (10 to 60 kV). As a result, it is found that the deformations of the micro-particles and the target contacts are severer with the increase of applied voltages, due to the increase of the impact velocities. With the applied voltages 50 kV and even to 60 kV (the impact velocity 526 and 631 m/s), there can be damages generated on the contact surface accompanying by a large number of high-velocity secondary particles. Furthermore, the influencing factors of the micro-particles impact phenomena in VIs, such as the size of the micro-particles, the microscopic electric field distribution, the contact materials and so on, have also be considered. The result of this paper may provide some useful information to understand the vacuum breakdown initiated by the micro-particles. [ABSTRACT FROM AUTHOR]

The aim of this paper is the study of the effect of the isolation and the surface condition of an insulating barrier on the performance of a non-uniform field electrode system. The study is carried out under positive DC voltage. The dielectric properties of the material were measured using the Schering bridge. The effect of the material isolation mode in the point-plane air gap system on the optimisation of its performance was analyzed. Moreover we present findings of experiments which allow quantifying the effects of the clean or polluted atmosphere and the contamination severity level on the electric strength of the air gap system. Finally, this investigation has been supported by laboratory observations of the discharge phenomena in the air gap from inception to full flashover in all cases using a video camera system. The results from this study show that there is a distance limit of the isolation barrier beyond which its performance is optimal. In addition, a limit level of pollution, from which its minimal electric strength is equivalent to that of a conducting barrier, has been detected. [ABSTRACT FROM AUTHOR]

This paper presents the relationship between ac flashover voltage and the varying positioning of iced insulator strings under different icing condition. The results were obtained through laboratory test in multifunction artificial climate chamber located at Chongqing University, China. Also difference of the ac icing flashover performance of single insulator string, double insulator strings, inverted v-type insulator strings and insulators connected with alternately large and small diameter sheds (CALSDS) were analyzed in this paper. According to test results, it is found that the distorted electric field distribution on iced insulator string is the main reason that results in the decrease of flashover voltage. With the same icing degree, the icing flashover voltage of either double insulator strings or as an inverted v-type is lower than that of single insulator string. If insulators are arranged with the type of CALSDS, the speed of icicles to bridge the insulators will decrease. To ensure the height of conductor from the earth remaining unchanged, the ac icing flashover voltage of insulator string can be increased by arranging the insulator strings as an inverted v-type or arranging the insulators with the types of CALSDS with the pattern of "3+1" or "4+1". [ABSTRACT FROM PUBLISHER]

The main contribution of this paper is to investigate the correlation between Tan δ diagnostic measurements at 0.1 Hz (Very Low Frequency-VLF) and VLF breakdown performance for Medium Voltage (MV) cable samples through a laboratory test program. The cable samples used are a set of 15 kV, Cross-linked Polyethylene (XLPE), unjacketed cables removed from the field, the same service area, and having experienced similar operating conditions for almost four decades. The test program includes Tan 6 measurements at different voltage levels and a subsequent VLF extended step-withstand test. The VLF step test allows the evaluation of risk of failure during VLF Tan δ testing and assessment of the ultimate performance of the cables. The Tan 6 diagnostic measurements are represented by the Tan δ value and Tip-up, which are considered the classical metrics. However, the paper also suggests the use of a new additional diagnostic feature that takes into account the scatter in the Tan 6 measurements for a particular test voltage level. [ABSTRACT FROM AUTHOR]

In this paper the behavior of partial discharges (PDs) on epoxy insulation under AC and impulse superimposed voltages are experimentally investigated in SF 6 gas. The lightning impulse (LI) and the switching impulse (SI) voltages with different polarity are superimposed on different phases of the AC voltage. PDs are measured by three sensors, a 50 Ω measurement impedance, an ultra-high frequency sensor and a photomultiplier tube. The max PD amplitude, PD pulse number and PD duration under AC voltage after an impulse voltage are summarized and analyzed. The effect of the impulse superposition phase, the impulse type and the impulse polarity on the characteristics of AC PDs are discussed. Results show that after a positive impulse voltage was applied on the positive semi-wave of AC voltage (0 to 180°), PDs in the negative half cycle of the AC voltage can be remarkably excited. A negative impulse voltage was applied on the negative semi-wave (180 to 360°), AC PDs can be initiated in the positive half cycle of AC voltage. PDs excited by a LI voltage is severe than excited by a SI. PDs excited by a positive impulse is intense than excited by a negative impulse. [ABSTRACT FROM AUTHOR]

The objective of this paper is to propose a method for quantitatively estimating the breakdown voltage of double short vacuum gaps in series under the lightning impulse voltage. The study found a quantitative relationship between the breakdown voltage and the maximum electric field strength at the cathode. This relationship is validated by previous experimental works when the total gap distance is less than 20 mm. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on epoxy insulator installed in gas insulated switchgear (GIS) or gas insulated transmission line (GIL) has been recognized as an important factor for electric field distortion that results in reduction of surface flashover voltage. This paper reports on surface charge accumulation behavior on a actual epoxy insulator for 100 kV GIL prototype with multi-arc surface profile. A coaxial electrode system is used to investigate the charging behavior under DC voltages with metal particles positioned at various positions of the surface so as to estimate its influence on the charge accumulation behavior using a Kelvin type probe. The results show that positive charges are accumulated mainly on the nonplanar region around the high voltage electrode in the shape of speckles, and the average charge distribution along the radial direction presented a double-peak feature. Moreover, the charge accumulation is enhanced by metal particles contacting to the grounded electrode. Using a numerical simulation method, it is suggested that the charge accumulation is mainly determined by gaseous ionization which is enhanced by the metal particle induced electric field distortion. [ABSTRACT FROM AUTHOR]

The paper comprises a new systematic approach to determine the critical ambient field needed for positive streamer propagation in nonuniform field gaps. It is shown that such critical streamer penetration field is a function of the basic value previously established for uniform field gaps and the degree of ambient field nonuniformity. The latter quantity is defined by the ratio of the maximum to mean ambient electric fields along the streamer propagation path, which can be determined either analytically or by charge simulation. Such functional dependence of the critical streamer penetration gradient on field nonuniformity has been determined for different rod topologies of rod-plane gap as well as for single and bundle conductor plane gaps. The model is used to determine critical streamer penetration fields and streamer lengths under different impulse voltage levels for different gap configurations and for a tall ground structure exposed to ambient impulse field. The findings of this method are compared to available experimental results and the agreement is found satisfactory. [ABSTRACT FROM AUTHOR]

The composite insulation system in a wind turbine generator (WTG) is subjected to either the AC or the repetitive pulse voltage, which may lead to degradation in the insulation. In this paper, the effects of voltage amplitude, waveform, repetition rate and composite insulating material constitution on the electrical tree morphology and development are studied. The results indicate that higher AC voltage produces sparser branches, shorter initiation time, and higher growth rates in the impregnating varnish and along interfaces. Electrical trees are initiated and grow easier under repetitive pulse voltage than AC voltage. Increasing repetition rate of pulse voltage promotes the initiation and propagation of electrical trees. It has been found that the peak of the pulse overshoot on the rising edge plays an important role in the initiation and propagation of electrical tree, while the average of pulse voltage dominates the breakdown time of the composite insulation system. [ABSTRACT FROM AUTHOR]

With the development of high-voltage direct-current (HVDC) transmission, DC gas insulated lines (GIL) with SF 6 /N 2 gas mixtures as insulating medium have been successfully adopted. Space is typically the weak link of equipment insulation where surface discharge is one of the common discharge forms in power equipment. Therefore, research on the surface discharge characteristics of SF 6 /N 2 under DC voltage can contribute to insulation design of SF 6 /N 2 insulated DC GIS, GIL and other power equipment. In this paper, a DC high-voltage experimental system is established to study partial discharge (PD) along insulation surfaces. The characteristics at the initial discharge and the discharge development stages are analyzed in detail. Additionally, the synergistic effect of mixed gas is investigated based on the discharge characteristics and the synergistic effect coefficient is introduced. The experimental results show that the synergistic effect of mixed gas partial discharge inception voltage (PDIV) under high pressure is more obvious compared to low pressure and the discharge quantities of mixed gas is smaller than that of pure SF 6 at the discharge development stage. This study on the multiple influencing factors of synergistic effect is expected to provide important insight for exploring the SF 6 /N 2 discharge mechanism and helps decreasing the usage SF 6. [ABSTRACT FROM AUTHOR]

To reduce the electric field concentration induced flashover in AC gas insulated line (GIL), a novel concept of surface functionally graded material (SFGM) is proposed in this paper, and the continuously graded high-Ä layer was fabricated by depositing BaTiO 3 on the insulator surface. The iterative method was introduced to optimize the thickness distribution of the BaTiO 3 layer. After four iterations, the layer thickness distribution almost reaches its optimal solution, and the maximum electric field strength is reduced by approximately 70% compared with that of the conventional insulator. To fabricate the optimized SFGM insulator, a rotatable baffle with the designed gap was placed above the insulator during the sputtering process. For comparison, a uniformly sputtered insulator with the 4μm BaTiO 3 layer and a discretely graded insulator with four gradients were prepared. Both electric field simulations and experiments were performed to evaluate the electrical performance of different insulators. The results show that, compared with the conventional insulator, the sputtered insulators, especially the optimized SFGM insulator, can significantly improve the uniformity of the electric field distribution along the insulator surface under a 50 Hz voltage. Accordingly, the discharge inception voltage and flashover voltage of the insulator both increased significantly. Under negative standard lightning impulse voltage (1.2/50 μs), the sputtered insulators show insulation performance comparable to that of the conventional insulator. [ABSTRACT FROM AUTHOR]

There is not sufficient data on the breakdown voltage-time characteristics for gas insulated switchgear (GIS) at power-frequency voltages, especially in the long-term region, for complete insulation specifications to be developed. This paper gives voltage- time breakdown characteristics for a 72-kV GIS at power-frequency voltages, both for clean conditions and with metallic particles added, from times of the order of a second to times of the order of 1000 hours. The Weibull parameters were calculated to be n = 95 and a = 0.32 to 0.47 in the clean case; and in the metallic-particle case n = 31 and a = 0.92 to 0.98 for a short-term region and n = 66 and a = 0.40 to 0.47 for a long-term region, with the boundary between these two regions located at 10 minutes. Based on the statistics for the metallic-particle case, the reliability of the power-frequency withstand voltage tests were then estimated using as examples the GIS in a 500-kV system (an effectively grounded system) and a 66-kV system (a non-effectively grounded system). The paper also studied the effect of the type of grounding and the influence of the presence of metallic particles on the test voltage level. [ABSTRACT FROM AUTHOR]

High voltage Marx generator implementation using IGBT (Insulated Gate Bipolar Transistor) stacks is proposed in this paper. To protect the Marx generator at the moment of breakdown, AOCP (Active Over-Current Protection) part is included. The Marx generator is composed of 12 stages and each stage is made of IGBT stacks, two diode stacks, and capacitors. IGBT stack is used as a single switch. Diode stacks and inductors are used to charge the high voltage capacitor at each stage without power loss. These are also used to isolate input and high voltage negative output in high voltage generation mode. The proposed Marx generator implementation uses IGBT stack with a simple driver and has modular design. This system structure gives compactness and easiness to implement the total system. Some experimental and simulated results are included to verify the system performances in this paper. [ABSTRACT FROM AUTHOR]

This paper presents measurement results for the random variable "vacuum breakdown voltage", obtained by the application of pulse, dc, and ac voltage. Parameters varied during the measurements were: 1) vacuum pressure and 2) inter-electrode gap width. The measurements were performed under strictly controlled laboratory conditions. Conclusions about the influence that the variable parameters in the experiment exert on the parameters of theoretical statistical distributions have been drawn, based on the statistical analysis of the obtained experimental results. The obtained results have been explained by the mechanisms constituting the process of electrical breakdown in vacuum. [ABSTRACT FROM AUTHOR]

Recently a number of new non-destructive diagnostic techniques have been investigated for the reliable condition assessment of the ageing of metal oxide surge arrester (MOSA). Among them polarization/depolarization current and return voltage measurement techniques showed very promising results. This paper presents an insulation polarization model for investigating polarization-based diagnostics for metal oxide surge arrester. The simulations of the proposed model are validated and verified by comparing the data from the polarization/depolarization current measurements. The simulation of return voltage for MOSA is also performed to validate the insulation polarization model. Finally, the usefulness of this model for MOSA diagnostics has been highlighted in this paper. [ABSTRACT FROM AUTHOR]

In order to meet the requirement of mechanical properties, multiple parallel suspension insulators will be used in UHV lines. In this paper, pollution flashover performance of multiple parallel insulators has been investigated based on artificial contamination tests under both AC and DC voltage. Four types of multiple parallel suspension mode are used in the tests, which has 2 strings, 3 strings, 4 strings and 6 strings, respectively. Experimental results indicated that multiple parallel insulators? pollution flashover voltage was lower than that of single string. Study showed that the result is not only affected by the multiple string leads to reduced pollution flashover probability, but also affected by the ionization of air. When the parallel number was less than 4, pollution flashover voltage of multiple parallel porcelain insulators was about 4% lower than that of a single string. If the parallel number was greater than or equal to 4, the value of flashover voltage was reduced by more than 10%. For composite insulators, the voltage drop of the multiple parallel insulators is not as obvious as that of porcelain insulators. In actual project application of the multiple parallel strings, the number of porcelain insulators and the insulation length of composite insulator was proposed to increase. The conclusion of this work can provide technical reference for the external insulation design and construction of UHV transmission lines. [ABSTRACT FROM PUBLISHER]

The flashover performance of insulators is significantly affected by the degrees and ways of wetting the pollution layer. However, there have only been a few reports on the wetting condition and its effects on the flashover characteristics of pollution on insulators to date. In this paper, a phase-angle difference-based method is proposed to test the wetting conditions of the pollution layer on insulators. A corresponding measuring device is also developed. The wetting process of the pollution layer on aerodynamic-type glass insulators is studied in an artificial climate chamber using the developed device. The related flashover performance is studied via the even-rising voltage method. Test results show that the phase-angle difference (PAD) curve accurately reflects the entire wetting process of the pollution layer from the initial stage, through the saturated stage to the oversaturated and washing away stages. Over the wetting process, the PAD curve initially drops sharply, and then remains stable for a few minutes before eventually rising slowly to the initial value. The PAD curve has its lowest value when the pollution layer is wetted to saturation, which occurs after approximately 5 min in our experiments. The flashover voltage characteristics of insulators are also significantly affected by the wetting conditions of the pollution layer. The flashover voltage in the saturated stage is shown to be lower than that in the other stages. Therefore, to improve the accuracy of the artificial pollution tests, quantitative methods (including PAD) should be used to obtain the exact time of wetting to saturation. [ABSTRACT FROM PUBLISHER]

The introduction of the test voltage function or k-factor in IEC 60060-1 Ed. 3 has been an important step to ensure reproducibility and traceability in high voltage tests techniques for lightning impulses with superposed oscillations and overshoots. The standard recommends computed fitting methods to extract a base curve from the oscillating recorded voltage curve and, applying the test voltage function, it is possible to calculate the impulse parameters. Testing external insulations with positive polarity lightning impulses at 50% breakdown voltage or lower level, the disruptive discharge usually arrives after ten microseconds. For these cases the test voltage function and procedures for calculating impulse parameters proposed by the standard may lead to divergent results. In this paper a new approach is proposed for fitting the base curve for metric air gaps based on the relevance of the leader propagation phase, highlighting the influence of the instantaneous voltage between the impulse crest and the time-tobreakdown. A comparison of three calculation methods for test curves with long timeto- breakdown is discussed and a new test voltage function obtained by measurements on a 1 m rod-plane air gap is proposed. Compared to the standard test voltage factor, this approach evidences a negligible influence of oscillations and the relevancy of the impulse tail voltage for testing external insulations with moderately distorted lightning impulses. Finally, a generalization of the voltage test function is proposed for several types of insulation considering mainly the average breakdown times. [ABSTRACT FROM PUBLISHER]

The growing industrialization needs techniques to diagnose the incipient faults in induction motor at inception stage itself for avoiding the downtime of the production. In this regard detecting the faults in a 3-phase induction motor at an early stage is a vital component in process industries. The condition of the supply unbalance, under voltage and sudden load changes are other involuntary issues which may tend to exhibit current signature similar to the stator winding insulation faults. This paper proposes a robust technique to detect, classify various stator winding insulation faults and severity of stator inter-turn faults when an induction motor works under various operating conditions. In the present work, disturbance features are extracted from three phase residues which are obtained from wavelet multiresolution analysis. Three modular neural networks are implemented, in which one is used to classify various disturbances such as single phasing, supply unbalance, under voltage, stator inter-turn faults, sudden load change and phase faults, second one is used for classifying the stator winding phase faults and third one is used for identifying the faulty phase and severity level of stator inter-turn faults. Simulation and Experimental data demonstrate the validity of the proposed method and improvement in classification accuracy as compared to traditional method. [ABSTRACT FROM AUTHOR]

Non-contact measurements performed with the auto-compensated electrostatic induction probe of an electrostatic voltmeter are often employed to characterize the charging state of insulating materials, such as films or non-woven media. The aim of the paper is double: (i) describe a simple method for assessing the resolution of such a probe; (ii) discuss the significance that can be attributed to the value displayed by the electrostatic voltmeter when the charge is non-uniformly distributed at the surface of the bodies examined by the probe and no constant surface potential can be defined. The measurements were performed for some simple experimental models that simulate surface potential non-uniformity of constant-charge insulating bodies. By examining the different surface potential distribution curves that were obtained for each of the situations under study, it is possible to estimate the resolution of the probe. A commercial computer program based on the superficial charge simulation method was employed for the numerical analysis of the electric field in a simplified 2-D representation of the geometric system formed by the probe, the experimental model, and the grounded plate. The potential that anneals the electric field at the surface of the probe (to simulate its principle of operation) was calculated by an iterative method. A good agreement was found between the results of the numerical simulations and the experimental data. The errors that can be made in the interpretation of the results of surface potential measurements are illustrated by the case of a non-uniformly-charged non-woven polypropylene fabric. [ABSTRACT FROM AUTHOR]

The transient electric field distribution under lightning impulse voltage (LIV) is vital to the reliability of 750 kV composite cross-arm. However, the transient electric field of electrical equipment under impulse voltage has usually been substituted by the equivalent sinusoidal voltage, without considering the variation of permittivity with frequency. Using the frequency decomposition computation method (FDCM) based on finite element method (FEM), this paper presents the study of transient electric field under LIV for a 750 kV composite cross-arm. The LIV waveform has been decomposed via Fourier transformation. The complex dielectric functions of the insulation materials used in composite cross-arm have been investigated, and the computation parameters for the dc and each harmonic component of LIV have been obtained. Then, the transient electric field distribution can be computed through the superposition of responses from dc and each harmonic component. Moreover, the computation results have been compared with that using traditional method. The comparison indicates that the potential distribution of composite cross-arm is similar. The maximum percentages of electric field strength difference on fittings and silicone rubber sheath are 2.9 %, 14 %, respectively. [ABSTRACT FROM AUTHOR]

Converter transformers use oil-pressboard as their main dielectric medium and are operated mainly in AC-DC combined electric field. For oil-pressboard insulation, the electric field under DC voltage shows distinctive characteristics from that in AC electric field. Compared to conventional AC power transformers, the electric filed distribution in the converter transformers is much more complicated and remains a big challenge for optimization of insulation design. Existing simulation research in this field falls short of experimental verification and shows limitations in the insulation design of the converter transformers. This paper attempts to fill the gap by measuring the dynamic process and distribution of electric filed in oil-pressboard insulation under the combined field so as to provide reference for insulation design. With the established Kerr-effect electric field measurement platform, the present paper recorded the dynamic process and distribution of AC and DC electric fields at different AC/DC ratios for the double-layer overlapping oil-pressboard insulation model. The research results indicate that 1) the AC electric field distribution in oil is homogeneous and symmetric. The field strengths form linear relation with the amplitudes of applied voltages; 2) the DC electric field distribution in oil is heterogeneous and asymmetric. Under the application of positive DC voltage, the field strength around the upper electrode is much smaller than that of the lower one. The DC electric field strength shows no linear growth with the increase of the applied voltages; 3) the accumulation of interface charges forms the major cause resulting in the asymmetric distribution of the DC electric field at different positions of the insulation. The research results imply that although RC model is recognized as effective simulation method for insulation design of conventional transformers, it shows its limitations and should be enriched and adjusted according to the electric field distribution under AC-DC combined voltage and taking into full consideration of the accumulation of interface charges. [ABSTRACT FROM PUBLISHER]

This paper investigates nonlinear characteristics of leakage current (LC) for analyzing and monitoring AC flashover performance of ice-covered suspension insulators. The experiments were carried out in a cold climate room of CIGELE Laboratories according to the methods recommended by IEEE Std. 1783/2009. The maximum withstand voltage (VMW = 81 kVrms) and minimum flashover voltage (VMF = 84 kVrms) were obtained to reflect electrical performance of suspension insulators under a most dangerous ice type. The ice accretion process, the length, width and number of icicles associated with the appearance of partial discharges were observed and discussed to present the final formation of a relatively uniform ice layer and vertical icicles along the insulator string. This ice-covered insulator string was characterized by air gaps on the grounded-side shed space and complete ice bridging of the other shed spaces, which have significant influence on the development of arcing discharges leading to flashover. Nonlinear characteristics of LC were analyzed by using a recurrent plot (RP) technique, which graphically gives nonlinear variations of LC on RP maps as well as nonlinear indicators for quantitative prediction of the flashover. The obtained results reveal that dynamic behavior (initiation, propagation, extinguishment and re-ignition) of arcing discharges on ice-covered suspension insulators can be visually reflected by the texture transition of RP maps of LC, which correlate with different stages during the flashover process. The quantitative indicators including a recurrence rate (RR) value lower than 0.24 and a deterministic (DET) value lower than 0.965 are proposed as the predictive values to indicate the highest probability of flashover occurrence. The results obtained will be helpful to predict most hazards of ice-covered outdoor insulators; this is one of monitoring methods based on LC characteristic analysis. [ABSTRACT FROM PUBLISHER]

The main objective of this paper is to present a new analysis, founded on partial arc activities, for evaluating the flashover performance of EHV post station insulators equipped with booster shed (BS) under heavy icing conditions. Based on observations from ordinary and high-speed cameras, novel discussions on partial arc characteristics, ice-free leakage distances, and flashover arc path were presented. Moreover, innovative axisymmetric simulations of partial arcs were implemented by using COMSOL Multiphysics™ in order to calculate their effects on potential distributions during the flashover process of the 4-, 5-, and 6-BS configurations. Simulation results of the BS configurations showed a redistribution of voltage drops along the air gaps after the appearance of the partial arc along the first air gap. This redistribution of voltage drop leads to the occurrence of partial arcs along the other air gaps which may somewhat explain occurrence of the flashover. The presented experimental and simulation studies can provide better designs for practical application of booster sheds. [ABSTRACT FROM AUTHOR]

The existing industrial tribo-electrostatic separators have proved their efficiency for sorting mixtures of millimeter-sized granular plastics. The objective of this paper is to validate a new tribo-aero-electrostatic separator designed for processing granular mixtures of insulating materials (average size: 10–100 μm). The device consists of two vertical plate electrodes of dimensions 20 cm × 30 cm, connected to two DC high voltage supplies of opposite polarities. Both electrodes perform “back-and-forth” motions inside a fluidized bed containing the micronized materials to be separated. Fluidization is achieved by injection of air using a variable speed air blower. The experimental study is carried out with samples of white PVC and gray PVC having an average size of 50 μm, and enables the analysis of the effects of several factors on the separation efficiency: the linear speed of the electrodes, the electric field, the mass of processed materials and the fluidization air flow rate. The new separation process is characterized by high product recovery rates. The purity of the separated products, estimated by visual examination, exceeds 90% in all experiments. [ABSTRACT FROM AUTHOR]

This paper describes experimental investigations focused on the glow discharge created by a plasma actuator and used to shock wave modification over a flat plate in a Mach 2 air flow. The model is equipped with a plasma actuator composed of two electrodes. A weakly ionized plasma is created above the plate by generating a glow discharge with a negative dc potential applied to the upstream electrode. ICCD images of the discharge without and with the Mach 2 flow show the influence of the flow field on the discharge morphology. In addition, ICCD images of the modified flow revealed that when the discharge is ignited, the shock wave angle increased with the applied voltage. Thermal measurements of the flat plate surface carried out with an IR camera showed that the spatial temperature distribution is not uniform along the plate and its maximum, near the leading edge, increases with the applied voltage. Previous results showed that surface heating is responsible for roughly 50% of the shock wave angle increase, meaning that purely plasma effects must also be considered to fully explain the flow modifications observed. The focus of this paper is the study of the properties of the glow discharge to better understand the interaction between the supersonic flow and the purely plasma effects which are responsible of flow field modifications, in particular ionization degree and thermal disequilibrium upstream the model. [ABSTRACT FROM AUTHOR]

The incorporation of inorganic nano particles (TiO2, Al2O3 and SiO2 etc.) with polyimide (PI) greatly improves its dielectric property. The dielectric properties of PI films are largely influenced by the absorbed water and humidity. What is more, the water absorption will be affected greatly by nano particles. In this paper, to study the effects of absorbed water on dielectric properties of moistened PI/Al2O3 composite film, PI film specimens were prepared by incorporating 12 wt% nano Al2O3 particles with PI. The original specimens were also prepared for contrast. Then the specimens were immersed in high purity water for 12, 24, 48, 72, 96, 120, 144 and 168 h. The water absorption and breakdown strength under different immersion time were measured. Meanwhile, given that the existence of surface charge had a critical influence on the breakdown characteristics, the corona charging tests were conducted at room temperature under pulse voltage. The pulse frequency was 300, 600 and 900 Hz, respectively. The pulse amplitude was +4 kV. The charge dissipation was investigated based on the demarcation energy theory. Obtained results show that the moisture condition affects more on the nano Al2O3 composite films than the original ones. Compared to original PI films, the water absorption is larger and the breakdown strength falls faster for the nano ones. The decay process of surface charge and the change of detrapping charges in nanocomposite films are affected greatly by the absorbed water. [ABSTRACT FROM PUBLISHER]

There are cross-linked metal sheath and voltage drop in the long distance power cable which make difference between the short and the long distance power cables on on-line insulation monitoring technology. In order to solve the problem, two terminal synchronous tanδ measurement as a new method of on-line insulation monitoring had been put forward in the paper. The working principle and theoretical basis of calculation were given. The feasibility of the method was demonstrated when there was synchronization error in the system. The Matlab software was used to simulate the circuit of three-phase cable and to calculate tanδ of cable insulation. The results show that the load current will not influence on tanδ measurement if choose the average of vector sum of voltage at the two ends of the cable. And the harmonic and frequency fluctuation had little influence on the methods. The methods were proved to be validity and feasibility in the paper. It can be obtained that the methods were suitable for all kinds of cable insulation monitoring whether the metal sheath of three-phase cable was cross-linked or not. [ABSTRACT FROM PUBLISHER]

In order to study the effect of glaze erosion on the performance of porcelain post insulator, an inclined plane (IP) test similar to IEC 60587 standard was conducted on a cylindrical insulator sample under laboratory conditions for ac voltage, and a new test method, called graphite-layer-based glaze erosion test was proposed in this paper and performed on the sheds of a 40.5 kV post insulator. The experimental details of the graphite-layerbased glaze erosion test was introduced, the technical points of which are coating a graphite layer on the surface of the shed using a brush mold which contains multiple hollowed-out rhombic grids, and arranging high voltage electrode at shed root and grounded electrode at shed edge respectively at both ends of the graphite layer, so as to form continuous and stable surface arcing under ac voltage and therefore achieve desired glaze electrical erosion effect. The graphite layer is a uniform viscous mixture made up with flake graphite and liquid glue according to a quality ratio of 1:1.2. The arcing process, arc current characteristics, surface temperature, and glaze erosion appearance were analyzed. Leakage current under power frequency voltage and flashover voltage of the eroded cylindrical insulator by IP test were measured, while porosity test and scanning electron microscopy (SEM) tests were conducted on samples taken from the sheds after graphite-layer-based glaze erosion test. The results indicate that glaze erosion will decrease both electrical and mechanical performance of the porcelain post insulator. The new test method proposed in this paper has been proved to be effective for glaze erosion test and consumes much less time compared with IP test, which provides a new idea for accelerated tracking and erosion test of glaze and porcelain materials so as to study the aging characteristics of porcelain insulators. [ABSTRACT FROM AUTHOR]

To study the dc insulation design of gas insulated switchgear (GIS), the insulation characteristics under lightning impulse (LI) voltage with a superimposed dc voltage (superimposed voltage) must be clarified. The paper experimentally examined the GIS breakdown characteristics under this superimposed voltage. The test models simulating an insulator creepage surface were used for which consideration of the influence of dc voltage among various other GIS insulating elements is particularly important. To be specific, a cylindrical model made of epoxy resin or fiber-reinforced plastic (FRP) as the material and a conical epoxy spacer model were tested. For the cylindrical model, a cap-shaped electrode was placed on the insulator and a small gap was established between the end of the electrode and the insulator. When the dc breakdown voltages were measured using these samples, they were higher for the applied voltage of positive polarity than that of negative polarity for all samples. The post-test observation of the electrification condition revealed greater electrification on the insulator surface for the applied voltage of positive polarity. The electrification charges are considered to have relaxed the electric field and increased the breakdown voltage. Subsequently, the breakdown test was conducted using a superimposed voltage, whereby a foregoing dc voltage was applied to samples for a certain period, whereupon a LI voltage was applied with the dc voltage continually applied. The breakdown voltage when the LI voltage and dc voltage had equivalent polarity was approximately same to the LI alone breakdown voltage. Conversely, when they were opposite in polarity, the breakdown voltage under the superimposed voltage obviously tended to decrease from the LI alone breakdown voltage. It is considered attributable to the fact that the insulator surface was electrified by the foregoing dc voltage and applying LI voltage opposite in polarity to this electrification intensified the electric field where the breakdown started to occur. Accordingly, it emerged that the GIS breakdown characteristics changed significantly depending on the polarity combinations of LI and dc voltages. The influence of these polarities must be taken into consideration when studying the GIS dc insulation design. [ABSTRACT FROM AUTHOR]

Power transformers are designed to withstand system abnormalities such as over voltages and external short-circuits. Transformer monitoring and diagnostics are the effective techniques in preventing the eventual failures and contribute to ensure the plant's reliability. Transformer life mainly depends on the integrity of its solid insulation (cellulose). The solid insulation in transformers degrades with time at rates which depend on the temperature and the amount of moisture, oxygen and acids in the insulation system. Moisture and oxygen cause the paper insulation to decay much faster than normal. Moisture in a transformer accelerates the aging process and causes severe deterioration in the mechanical and electrical properties of insulation system. The aging of paper insulation is irreversible. The degree of polymerization value is a main relation between insulation deterioration and formation of aging products. To predict a failure or an unusual behavior, an accurate interpretation of monitoring, diagnostics and maintenance data is required. The monitoring of insulation system and verification of core and winding mechanical integrity allow optimization of the lifecycle management of an asset. This paper presents an investigation and condition assessment of 152 MVA, 512 kV, 1-phase, 2-winding generator step-up transformer having a history of combustible gases. [ABSTRACT FROM AUTHOR]

Although the adverse effects of using power electronic conversion on the insulation systems used in different apparatuses have been investigated, they are limited to low slew rates and repetitions. These results cannot be used for next-generation wide bandgap (WBG)-based conversion systems targeted to be fast (with a dv/dt up to 100 kV/ws) and operate at a high switching frequency up to 500 kHz. Frequency and slew rate are two of the most important factors of a voltage pulse, influencing the level of degradation of the insulation systems that are exposed to such voltage pulses. The paper reviews challenges concerning insulation degradation when benefitting from WBG-based conversion systems with the mentioned dv/dt and switching frequency values and identifies technical gaps and future research needs. The paper provides a framework for future research in dielectrics and electrical insulation design for systems under fast, repetitive voltage pluses originated by WBG-based conversion systems. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) in gas insulated switchgear (GIS) has been studied extensively under AC voltage or transient voltage applied alone. However, the transient voltage in an actual system is superimposed on the AC operating voltage, which has rarely been studied. In this paper, the effect of AC and switching impulse superimposed voltage on PD characteristics was studied in SF6 gas. A test platform of generating superimposed voltage with controllable phase was used to study free particle and floating potential defect models. When the AC voltage was 90 and 110% of PD inception voltage (PDIV), the standard switching impulse with amplitude of about twice the PDIV was superimposed on different phases of the AC voltage. When the AC voltage is below the PDIV, results show that PDs of free particle defect model can be initiated when switching impulse is superimposed on 45 and 90° of AC. The PDs in the subsequent cycles of AC reignited into a self-sustaining discharge. For the floating potential defect, PDs have a marked difference duration, which is initiated by switching impulse in different phase. However, there is no change in PDs in each model when the AC voltage is above the PDIV. [ABSTRACT FROM AUTHOR]

With the miniaturization of electrical devices, the insulation width and the separation between electrodes have been accordingly reduced. Consequently, electrical breakdown across micrometer-scale gaps is of great practical interest for insulation design. In this paper, electrical breakdown across micrometer scale surface gap in atmospheric air was investigated under inhomogeneous field. The breakdown was measured under negative impulse voltage and the generation and movement of neutral and charged particles were simulated based on PIC-MCC (particle-in-cell with Monte Carlo collision). The result shows that the breakdown phenomena across micrometer scale gaps is mainly determined by field emission current from the cathode surface, similar to the discharge process in vacuum. The positive ions generated by collision of electrons to neutral particles enhance the electric field near the cathode, resulting in the increase of the field emission current from the cathode. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) taking place in a solid dielectric-bounded cavity involves physical processes such as free electron supply, discharge development and surface charge decaying, which bring about memory effects and become the main reasons for stochastic behavior of PDs. This paper reviews numerical modeling of cavity PD in the past 30 years. In the first place, physical processes relevant to PD activity are summarized, and modeling methods for discharge development are classified. Then some differences of PD modeling at AC and DC voltages are distinguished. Subsequently, reproducing methods from simulations to experiments are introduced, as well as their comparison under different conditions and with the emphasis on voltage frequency and PD aging. At last, some problems about current simulation models are discussed, and our suggestions for future work are proposed. [ABSTRACT FROM AUTHOR]

Cross-linked polyethylene (XLPE) insulation DC voltage endurance coefficient (VEC) is an important basis in the selection of DC cable insulation design and factory test parameters. Studies on the degradation of XLPE insulation performance under DC electric field can also provide technical support for the reliability evaluation of cable operation. The step stress method as an efficient electrical life test method is widely used to obtain VEC. The parameters of step stress method present considerable influences on VEC. An analytical method based on the trajectory of the damage curve and equivalence between step stress test and constant stress test within zone division is proposed in this paper. According to the proposed method, several sets of step stress tests with different test parameters have been carried out. The accuracy of the test results has been confirmed via analyzing test data. The experimental results show that the VEC (n=12.0) obtained by the step stress test whose parameters are selected by proposed method has good equivalence with the result (n=12.2) under constant-stress method. [ABSTRACT FROM AUTHOR]

In this paper, the effects of DC voltage, pulse voltage and pulse frequency on the electrical treeing characteristics in polypropylene (PP) under DC and pulse combined voltages were investigated. The rise time of the pulse voltage is 50 μs. The DC amplitude range is from −25 kV to +25 kV, and the pulse amplitude range is from −35 kV to +35 kV. The pulse frequency range is from 400 to 800 Hz. The results show that the electrical tree structures change with different DC amplitudes, pulse amplitudes and pulse frequencies. With the increase in the positive DC voltage, the treeing process is accelerated. However, with the increase in the negative DC amplitude, the breakdown time decreases first and then increases. This phenomenon is attributed to the different charge transport processes under different DC amplitudes. With the pulse amplitude and frequency increasing, the growth rate and accumulated damage increase significantly. The pulse amplitude and frequency affect significantly the breakdown time. The pulse voltage with the same polarity as that of the DC voltage is more likely to cause breakdown than the pulse voltage with opposite polarity of that of the DC voltage because of the larger effective values of the composite voltages. [ABSTRACT FROM AUTHOR]