Surface Discharge Development Research Articles

Filamentation of the surface plasma layer during the electrical explosion of conductors in strong magnetic fields

A model has been considered to describe the development of a surface discharge over a conductor electrically exploding in a strong magnetic field. A simulation performed using this model has shown that in the initial stage of the conductor explosion, a plasma layer of several tens of micrometers thick with an electron temperature of several electronvolts is formed on the metal surface. Based on the theory of small perturbations, the development of thermal filamentation instabilities that form in the surface plasma layer has been analyzed. The characteristic growth rates and wavelengths of these instabilities have been determined. The theoretical results were compared with the results of experiments performed on the ZEBRA generator (providing load currents of amplitude about 1 MA and rise time about 100 ns) and on the MIG generator (providing load currents of amplitude about 2 MA and rise time about 100 ns). For the conditions implemented with these generators, the filamentation model gives rise times of thermal filamentation instabilities of tens of nanoseconds at characteristic wavelengths of the order of 100 μm. These values are in good agreement with experimental data, which indicates the adequacy of both the surface discharge development model and the filamentation model.

Influence of gas flow on surface discharge development under AC voltage

Influence of gas flow on surface discharge development under AC voltage

Research on Leakage Current Waveform Spectrum Characteristics of Artificial Pollution Porcelain Insulator

The surface discharge development processes of polluted porcelain insulators for power transmission lines are tightly related with the development of leakage current (LC), the characteristics of LC, the insulating condition, and discharge intensity of the insulator surface have an important significance for revealing the contamination discharge state of insulators. In order to analyze the LC characteristics of porcelain insulators in the process of pollution flashover, artificial pollution flashover tests on porcelain insulators were conducted in the artificial fog cabinet, and the characteristics of LC waveforms in time-domain and frequency-domain were simultaneously measured and analyzed during the tests. The results indicated that the amplitude of LC, fundamental harmonic, the third harmonic, and fifth harmonic had a strong correlation, the maximum of LC(Im), the rate of total harmonics (THD), and the phase difference of fundamental harmonic (θ) were used for the representation of the characteristics of the LC waveform. The LC has the characteristics of high amplitude, low proportion harmonic, and small phase difference between the fundamental harmonic and voltage before the flashover occurrence. The test results provide effective reference for porcelain insulators in pollution flashover forecasting.

Surface discharge characteristics and influencing factors of polyimide insulations under high‐frequency sinusoidal voltages

The high-frequency power transformer (HFPT) has become a vital component of the power system. For HFPT insulations, surface discharge is a serious problem. Polyimide (PI) is a widely used insulating material for HFPT's. A needle-plate electrode structure-based experimental setup was developed to produce and measure the surface discharge. This paper first time used the fluid-kinetic model by adopting the finite element simulation method to address the micro-level study of surface discharge. This paper aimed at the discharge phenomenon of the PI under the direct action of high-frequency sinusoidal voltage and has not considered the overvoltage phenomenon caused by the waveform parameters that is the actual high-frequency action on the PI material. The development of surface discharge at various stages was analysed. Particle densities and electric field distributions versus discharge time were determined. Frequency, temperature, and air pressure have a significant effect on surface discharge behaviour. As a result, the applied voltage reaches its peak value at high-frequency, so the flashover time of surface discharge is reduced under the high frequency. The electron density of the streamer development increases as the temperature rises, and the time to flashover the channel becomes shorter. The higher the electron density and the longer the discharge channel develops while the gas pressure is lower. The simulation results have high accuracy and good agreement with experimental data.

Surface Discharge Status Diagnosis Based on Optical Image Chromaticity Coordinates

The physical and chemical processes of surface discharge are more complicated than other discharge forms due to the influence of gas composition and dielectric properties. Spectral diagnosis can be used to analyze the main species in the discharge process. However, the chromaticity characteristics are the comprehensive representation of all emission spectra. This article presents a new method to diagnose the state of surface discharge using chromaticity coordinates in color space. It is observed that the chromaticity coordinates slide along a straight line between the equal energy white point and a specific spectral color with the development of surface discharge. The relative position of the chromaticity coordinates on the line indicates the different states of the surface discharge. The slope of the line is affected by the material characteristics while external factors such as the camera's exposure time and the color of materials have no effect on it. It is found that the slope of the chromaticity coordinate curve has a fixed range, which means that there exists a specific color space of surface discharge. Finally, the effects of the ambient noise are corrected. This method uses the chromaticity coordinates in a long time scale to diagnose the microscopic process of surface discharge and it can be extended to other state diagnoses of partial discharge phenomena. From the point of view of potential application, this method also provides a new simple method to meet the requirement of long time scale state monitoring in industrial application.

Discharge propagation on a dielectric surface in a single-filament arrangement

Discharge development and streamer propagation on a dielectric surface were investigated in nitrogen-oxygen gas mixtures at atmospheric pressure. A coaxial pin-to-pin arrangement was used to generate single surface discharges driven by positive unipolar square wave high voltages of between 7 and 9 kV at 4.3 kHz. The development of surface discharges was recorded by ICCD and streak cameras. The discharges developed on the surface using pin electrodes attached directly to the dielectric plate. The accumulation over several discharge events showed a propagation front evolving from the electrode tip, while images of single discharges showed a non-uniform and branched structure of discharge channels. The electrode polarity influenced the discharge expansion and propagation velocity. Positive polarity of the metallic electrode (rising slope of HV pulses) led to a cathode-directed streamer with higher propagation velocities (5 × 105 m/s) than for negative polarity (relative to surface charges; falling slope). The increase in the O2 content in N2 from 0.1 to 20 vol% resulted in a decrease in discharge duration and an increase in streamer velocities.

Distribution of polymer surface charge under DC voltag and its influence on surface flashover characteristics

This study investigates the characteristics of polymer surface charge and their influence on DC surface flashover, which is significant to the study of surface discharge theories and the insulation design of high-voltage direct current equipment. Silicone rubber (SiR), epoxy resin (EP), and polymethyl methacrylate (PMMA), were selected as experimental materials in this paper. Using the active capacitance probe method, the charge accumulation, and dissipation characteristics of the polymers were studied under needle-plane electrode corona discharge. In cases of no surface charge accumulation, DC flashover tests were conducted in multiple electric fields. Furthermore, the influence of surface charge on flashover performance was quantitatively evaluated by means of optical and electrical tests as well as mathematical analysis. The results indicate that the surface potential of EP and PMMA increases more prominently as the applied voltage increases, but SiR is the least to do so. On the other hand, the surface potential of SiR and PMMA decays faster, with EP as the slowest of all. After the polymer accumulates surface charges, the flashover voltage, the luminance of the flashover arc channel, and the fractal dimension of the arc path all decrease. Besides, the flashover voltage of PMMA is extremely sensitive to surface charge, and the flashover voltage under extremely non-uniform electric field is less sensitive than that under the slightly non-uniform electric field. The analysis suggests that the surface charge exerts an influence on the flashover characteristics by affecting the surface field intensity and the surface discharge development process, and these factors ultimately determine the flashover characteristics.

Mechanism of the free charge carrier generation in the dielectric breakdown

Many studies have been conducted to investigate the effect of environmental, mechanical and electrical stresses on insulator. However, studies on physical process of discharge phenomenon, leading to the breakdown of the insulator surface are lacking and difficult to comprehend. Therefore, this paper analysed charge carrier generation mechanism that can cause free charge carrier generation, leading toward surface discharge development. Besides, this paper developed a model of surface discharge based on the charge generation mechanism on the outdoor insulator. Nernst’s Planck theory was used in order to model the behaviour of the charge carriers while Poisson’s equation was used to determine the distribution of electric field on insulator surface. In the modelling of surface discharge on the outdoor insulator, electric field dependent molecular ionization was used as the charge generation mechanism. A mathematical model of the surface discharge was solved using method of line technique (MOL). The result from the mathematical model showed that the behaviour of net space charge density was correlated with the electric field distribution.

The Correlation between Surface Tracking and Partial Discharge Characteristics on Pressboard Surface Immersed in MIDEL eN

This paper presents the investigation of the surface tracking on pressboard surface immersed in MIDEL eN oil. In this work, the development of surface discharge was analyzed by correlating the visual records of surface tracking on impregnated pressboard and the partial discharge (PD) activities. The PD activities during the surface tracking process were analyzed in terms of Phase Resolved Partial Discharge (PRPD) patterns. Throughout the experiment, surface discharge is found as the development of tree-like patterns in the form of white marks occurring on the oil-pressboard interface. This phenomenon is generally accepted as the drying out process that involves evaporation and decomposition of the oil molecules in the pressboard pores due to the surface discharge activities on the pressboard surface layer. The development of surface discharge on the pressboard surface can continue from minutes to months or even years until failure. Thus, condition monitoring system is important to characterize this type of faulty condition. The experimental results show that there is the decreasing trend of PD magnitude during the development of white mark hallway of a gap distance which is eventually suffered from an unexpected fault.

Energy eigenvector analysis of surface discharges for evaluating the performance of polymer insulator in presence of water droplets

Due to hydrophobic properties of polymer insulator, formation of discrete water droplets and the induced surface discharges on the insulator are typical phenomena during the insulator operation. To investigate the performance of polymer insulator in presence of discrete water droplets, maximum value, cumulative charge and energy eigenvector of discharge current were investigated. Effects of droplet numbers, droplet location, volume and contamination on surface discharges were analyzed. Obtained results reveal that with the development of surface discharges from initiation of corona discharge to arcing discharge and surface flashover, the dominate components in discharge current are the low frequency (0-78 Hz) components. For the condition of both single droplet and double droplets, flashover voltage and maximum value, cumulative charge and energy eigenvector of discharge current show the increasing tendency with increasing the droplet volume from 10 to 30 μL, but show the decreasing tendency under the condition of increasing the droplet volume from 30 to 40 μL and increasing the droplet contamination from 1.0 to 3.0 mS/cm. However, for triple droplets, the flashover voltage and investigated characteristics of discharge current show the decreasing tendency under the condition of increasing the droplet volume from 10 to 40 μL and increasing the droplet contamination from 1.0 to 3.0 mS/cm. Meanwhile, based on the comparison of droplet effects, it can be found that both droplet numbers and droplet location have significant influence on surface discharges.

Pattern identification of surface flashover induced by discrete water droplets on polymer insulator

The formation of water droplets and the triggered surface discharge have been considered as one of essential stages during the flashover process of outdoor polymer insulators. In order to determine the discharge pattern induced by discrete water droplets, laboratory experiments were carried out in an artificial climate chamber to simulate different morphology of water droplets on polymer insulator. And then, pattern and characteristics of the induced surface discharges were identified to establish the relationship between the flashover performance and the droplet parameters, such as the contamination, volume and location. Obtained results reveal that with the development of surface discharges from initiation of corona discharge to surface flashover, the gray-scale histogram of discharge image shows the increasing number of discharge pixels at the side of higher gray-scale value, the total luminance emitted by surface discharges shows the significant increase at the stage of surface flashover. Compared with the little affection of droplet contamination, the location and volume of water droplets show the significant influence on the distribution and strength of surface flashover. Meanwhile, both entropy and box dimension of surface flashover can be obtained to indicate the effects of discrete droplet parameters on the irregular and complicated properties of surface flashover. Therefore, pattern identification of surface flashover induced by discrete water droplets is available to further investigation on flashover performance of polymer insulator.

Pre-discharge and flashover characteristics of impulse surface discharge in vacuum

Surface discharge on solid insulators is one of the significant issues in the development of vacuum circuit breakers (VCB) and vacuum interrupters (VI). We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed and high sensitivity optical measurements. We identified the existence of charge activity with a small light emission on alumina dielectrics before the surface flashover development process, and defined this as a "pre-discharge" and measured its characteristics. We classified the charge activities into three processes: (1) pre-discharge, (2) surface flashover initiation, and (3) surface flashover expansion, in the development of surface discharge in vacuum. Utilizing this classification, we found that the surface predischarge in vacuum is generated by the electron emission from the cathode, and the subsequent flashover initiates with a steep increase in current and develops with the distortion of the electric field due to transiently-formed charging on the alumina dielectrics in vacuum.

Pre-discharge and flashover characteristics of impulse surface discharge in vacuum

Surface discharge on solid insulators is one of the significant issues in the development of vacuum circuit breakers (VCB) and vacuum interrupters (VI). We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed and high sensitivity optical measurements. We identified the existence of charge activity with a small light emission on alumina dielectrics before the surface flashover development process, and defined this as a "pre-discharge" and measured its characteristics. We classified the charge activities into three processes: (1) pre-discharge, (2) surface flashover initiation, and (3) surface flashover expansion, in the development of surface discharge in vacuum. Utilizing this classification, we found that the surface predischarge in vacuum is generated by the electron emission from the cathode, and the subsequent flashover initiates with a steep increase in current and develops with the distortion of the electric field due to transiently-formed charging on the alumina dielectrics in vacuum.

Understanding surface partial discharges in HV coils and the role of semi-conductive protection

An electrical insulating system in high voltage rotating machines is subjected to several stresses that cause changes of material properties during its lifetime. This phenomenon reduces progressively ability of the insulation to withstand the service stresses. Internal partial discharges can be located inside the high voltage insulation and usually they are dangerous for life-cycle duration. Surface discharges are generated at the solid/air interface in the gaps of machine stator slots or at the slot exits. Surface discharges can cause more intensive deterioration of the insulation, accelerate aging processes and definitively take aim toward premature failure of the machine. The application of conductive or semi-conductive protection layer provides certain protection for solid insulation from these discharges. Semi-conductive corona protection tapes are used on the coils at the slot exit region to electrical field control and prevent surface discharges. Laboratory measurements of the surface partial discharge quantities in specimens of high voltage coils with mica insulation were performed. The aim of the paper is to describe theoretically the development of surface discharges in rotating machines and to present surface partial discharge development in HV motors when semi-conductive layer protection is missing. The results can be found useful to understand better the surface discharge development and importance of high-quality semi-conductive protection technology.

Positive Nanosecond Surface Discharge and Wave-Front Steepness

The nanosecond surface discharge phenomena have been studied by paying attention to the rise time of impulse voltage using a dust figure technique. From the experiment result, it is found that irrespective of the applied voltage holding a constant, the size of discharge patterns reduces with increasing the rise time form 0 to 120ns, and that irrespective of the applied voltage being increased from 6 to 12kV, the size of discharge patterns becomes a constant with an increase in rise time. Furthermore, it is found that the cease voltage of surface discharge development is 2.5kV. Finally, we succeeded in theoretically estimating the electric field distribution and also space charge distribution in the positive corona streamer based on the voltage distribution which is obtained experimentally.

Optical studies of surface discharge under dc voltage in vacuum

Utilizing a high-sensitivity photomultiplier tube (PMT) and an intensified charge coupled device (ICCD) camera, the light emission from the surface of polytetrafluoroethylene (PTFE) and alumina ceramic samples was investigated in vacuum under a stepped dc voltage in order to study the initiation and development of surface discharge. Different from the classical electrode contact by butting electrodes on a sample directly, to achieve a fine contact between electrodes and the surface of a sample, the metalized electrodes were employed by sputtering two additional gold contacts on each sample in advance. The two types of materials showed quite different optical characteristics. It was suggested that there were several processes possibly related to the initiation mechanism of the surface discharge. Besides the field electron emission from the cathode triple junction (CTJ) for the classical electrode contact, there were charge injection and transport in the surface states of dielectrics, and charge trapping and detrapping phenomena. Electroluminescence (EL) emission was emitted due to the radiative recombination of electrons and holes injected. These processes were strongly dependent on the electrode contact manner and the surface states of insulating materials.

The role of leader re-illumination in the development of surface discharges in SF6 exposed to a very fast transient overvoltage

This paper describes the mechanism of discharge on the surface of an insulating plate in gas exposed to a very fast transient overvoltage (VFTO). The voltage consists of a sequence of steep short pulses on its front. A streak camera and a charge-measuring system were employed for the investigation. The leader channel, which has been formed during the first voltage pulse of the VFTO, flashes repeatedly upon each rise and fall of the successive voltage pulses. The luminosity at the voltage's rise consists of the light due to mere flashing of the previous leader channel and the light of the leader discharge in succession. The former phenomenon is called leader re-illumination in this paper. It plays a notable role in the leader's development. Such re-illumination of a leader channel occurs when the steepness of the applied voltage exceeds a critical value. The critical steepness increases with increasing pressure of gas, the thickness and the reciprocal of the permittivity of the insulating plate. A model for the leader discharge developing under VFTO stress is proposed.

Nanosecond surface discharge development using the computer simulation method

The authors have analytically studied the experimental results of the nanosecond surface discharge by using a simulation model. The results used were obtained using the dust figure technique with a nanosecond square pulse generator under the following conditions: an applied voltage of 5–20 kV, a pulse width of 3, 5, 8,..., 920 ns, and a gas pressure of 50, 100, 150,..., 760 Torr. Three kinds of simulation models were used: the first was based on the continuity equations together with the Poisson’s equation, without any photoionization effects; the second examined the gas photoionization effect on the first model; and the third considered further the photoelectric effect on the insulation surface. From a series of computations, it was found that by using the third simulation model, the experimental results such as the developing length of the corona streamer and its velocity were roughly confirmed, and the field intensity at the streamer tip was also obtained. Finally, the photoelectric emission coefficient θm at the insulation surface was evaluated.

5.0 Nanosecond surface discharge development

The nanosecond developing mechanisms of the positive and negative corona streamers on the insulation surface have been studied using the Dust figure technique (=Lichtenberg figure technique). This method has an advantage of measuring the velocity of corona discharge developments, because the distribution of the positive and negative charges produced in the discharge space can be colorfully separated and recorded on the insulation surface. The velocities of the corona streamer development were measured at the voltage of 8∼21 kV and the pressure of 50∼760 Torr. From the result, the velocities obtained at a voltage of 21 kV and a pressure of 150 Torr were 8.2×108 cm/sec in the positive and 5.8×108 cm/sec in the negative. The observed maximum velocity of negative corona streamer was 1.0×109 cm/sec at the conditions of 18 kV and 50. Torr. The result suggests that the velocity of the negative streamer will be larger than that of the positive at lower pressure and also nanosecond pulse voltage. Moreover, it was found that the field distribution in the corona streamer on the insulation surface could be estimated using Gauss's law.

Dynamics of the development of surface discharges in liquid nitrogen

In the present paper the results of experimental tests on the development of surface discharges in liquid nitrogen under a strongly non-uniform field in a point-plane electrode system are presented. On the basis of the discharge figures obtained on photographic film sheets with high voltage pulses of different duration, the average velocities of the development of positive and negative discharges have been calculated. Differences in the shape and structure of positive and negative discharge figures have been also described.