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Surface discharge in vacuum occurs under intense external field, also called flashover. The flashover threshold over which surface flashover occurs is of vital importance in vacuum-solid interface. It is widely assumed that flashover is initiated by single surface multipactor and develops as a plasma discharge along dielectric surface, namely secondary electron emission avalanche (SEEA). Yet SEEA model is not versatile for all practical vacuum-solid interfaces and there remain experiment results contradicting theory predictions without resolute answer. Here we further expand previous theories and consider conventional SEEA model under arbitrary field distributions. An example of conical insulator is then given to test the validity of SEEA model under arbitrary field. It is found that obtained flashover threshold is only consistent with experiment at low cone angle where field near cathode triple junction is strong, while remarkable discrepancies appear when anode field is significantly enhanced. The discrepancies can be understood if a different discharge model, i.e. anode-initiated flashover (AIF) is included. Both particle-in-cell simulation and optical diagnostics are employed to visualize this unique discharge, which are then formalized on theoretical ground to elucidate the early stage of AIF. Transition between two flashover phases are then clarified. Plasma-surface interaction and sheath dynamics during discharge are analyzed theoretically, showing distinct sheath evolutions in two flashover models. Additionally, a new factor reflecting field distortion is introduced to estimate discharge threshold under SEEA framework.

Atmospheric pressure cold plasma jet is widely used in the fields of material processing, bio-medicine and so on, because of its remarkable advantages. To further ensure the safety and expand the dimensions of the plasma jet, a three-electrode plasma jet device is developed by embedding the inner floating electrode in commonly two outer electrode configuration based on DBD theory. In this paper, the inner floating electrode is in three different positions respectively. Based on this device using helium gas, it has been studied that the effects of different inner electrode positions on optical and electrical properties of the plasma jet. In order to explain and verify the experimental phenomena, this paper simulates electric field and flow field distribution in the dielectric tube by COMSOL and FLUENT. The three-electrode structure obviously changes the field intensity distribution in the discharge tube, which makes discharging easily. The experimental results show that the three-electrode plasma jet configuration is more suitable for practical treatment on murine melanoma cancer which can provide some information and reference for future plasma jet treatment of cancer.

The electrical and optical characteristics of DBD are different from those of bare electrodes, while the Paschen curve reflects the relationship between breakdown voltage and gas pressure and gap distance. The specific characteristics in DBD need to be differentiated. The electrical characteristics of DBD are closely related to the discharge mode. The mode of DBD discharge can be divided into filamentous discharge and uniform discharge. The plasma contains a large number of excited particles, which radiate at low energy level. After being recorded by ICCD camera, the emission spectrum of plasma can be obtained. By analyzing the intensity and location of the spectrum, we can get a lot of information inside the plasma, such as electron excitation temperature, excited particle species, relative number density and the width of cathode falling zone, etc. The spectral diagnostic method used in this experiment is a non-contact plasma diagnosis method. First, based on the experimental platform of plasma, a DBD device for parallel plate electrode is set up. The medium frequency power supply is 22.0kHz, the working gas is argon, through the observation of the initial discharge voltage, discharge current, ICCD image, study the Paschen curves' characteristic of Ar-DBD, to investigate the mechanism of discharge, volt ampere characteristics and discharge mode evolution characteristics of argon gas, familiar with DBD. Then, through the ICCD camera emission spectra and nanosecond discharge dynamics image shooting, evolution of its time and space, including types, the discharge process of energetic particles in the reaction particle concentration, the cathode falls region width and electron excitation temperature relationship.

The insulation diagnosis method based on frequency domain dielectric spectrum is widely used in field test because of its nondestructive testing, rich insulation information and so on. FDS measurement results are easy to be affected by many factors. However, interpretation on the dielectric properties of oil paper insulation with moisture content, impurity ion concentration influence mechanism of the current research is not very ideal, so it is necessary to determine more appropriate and effective dielectric response simulation and experimental analysis of characteristics. In addition, moisture content (m.c.) is an extremely important factor which can determine the insulation state of Oil-immersed paper. Therefore, the evaluation of moisture content of oil-immersed paper has obvious significance. In this paper, based on the experimental results, the extended Debye model parameters of the field transformer are obtained. Then, the influence of the insulation resistance value in the Debye model on the FDS curve is simulated by MATLAB modeling. The simulation and experimental verification show that the regularity of the change of the FDS conductance process is obviously different from the moisture content influenced by the impurity ions. The impurity ions mainly affect the low frequency section of the imaginary part of the complex capacitor. A non-linear phenomenon in dielectric response with different moisture contents under different is observed. We found that the non-linear phenomenon has relationship with the hindrance effect of cellulose (m.c.

Oil-polypropylene (PP) composite insulation has been widely used in power capacitors, and also being extended to other equipment such as specific transformers, etc. As discharge can readily creep along PP surface, the interface between oil and PP is a weak point in the insulation structure. The oil-PP insulation degrades under the impact of thermal and electrical stress during normal operation. In this study, the influence of thermal degradation on surface discharge characteristics for oil-PP insulation is investigated. Degraded samples are obtained by thermally accelerated aging of virgin oil-PP samples. The microscopic morphology of PP samples are observed with SEM, and the physical and electrical properties of degraded oil are measured. In addition, the isothermal surface potential decay (ISPD) measurement is employed to investigate the charge keeping capability of PP samples. After that, surface discharge detection experiments under AC voltage are performed for the virgin and degraded samples. The ISPD results indicate that charge decay rate of degraded PP sample is faster than virgin sample, which imply that space charge memory effect of virgin PP samples is more remarkable. For this reason, the PD inception phase of virgin sample is smaller than those of degraded samples. It is found that thermal degradation of oil-PP insulation enhances the surface discharge activity. It is considered that rough surface profile and higher bubble conservation capability of degraded PP sample, and impurities in degraded oil are responsible for the enhancement effect of thermal degradation on surface discharge.

During the field service of composite insulators, high temperature vulcanized (HTV) silicone rubber (SIR) material used for insulator’s sheath and sheds is gradually aging, and thus how to effectively evaluate its aging state has become an inevitable issue. In this paper, artificially corona-aged and naturally site-aged SIR materials are employed as samples, and their properties such as hydrophobicity, leakage current, trap density/energy level, surface microstructure and chemical composition are investigated for comparative study. With the increasing of corona aging intensity, the contact angle of SIR samples drops gradually and recovers more slowly. The hydrophobicity of site-aged insulators also declines, from HC1 for 2-year service to HC5 for 15-year service. The leakage current of corona-aged and site-aged samples both increase with aging, which indicates that aging induces the increase of surface conductivity of SIR material. The peak trap density of corona-aged and site-aged samples increases with the aging duration or service duration remarkably. The changes of these properties are attributed to the changes of micro-structures and compositions in the surface layer of SIR. The scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analysis results reflect that a hardened inorganic silica-like (SiO2) layer with many polar chemical groups and distributed micro-pores is formed on the surface of SIR material after corona aging. Because of this silica-like layer, the hydrophobicity decreases while surface conductivity and trap density of SIR material increases. Besides the traditional properties like hydrophobicity and surface conductivity, the trap density is expected to be a novel parameter for effective evaluation of aging state of HTV silicone rubber material.

In this paper, the epoxy composite insulating material is selected as the experimental sample. After the samples being charged under different voltages with different duration in SF 6 of atmospheric pressure, the surface potential distribution is measured with a Kelvin electrostatic probe. The 2dimensional Fourier transform with Wiener filter technique is applied to calculate the surface charges density distribution. The results reveal that the surface charges density shows a good correspondence with the surface potential, and the whole measuring system presents a definitely high spatial resolution.

The microdischarges generated in spatially confined geometries, such as microcavities and micropores of various materials, represent a promising method to create stable nonequilibrium plasmas at atmospheric pressure. Hybrid plasmacatalytic system utilizing porous ceramics loaded with catalysts shows the synergistic effect on volatile organic compounds (VOCs) decomposition, carbon dioxide reduction and methane reforming. In this paper, we investigated on the physical characteristics of dielectric barrier discharge (DBD) in porous ceramics at atmospheric pressure methane generated by sinusoidal waveform (20 kHz, 0–20 kV) power supply. Discharge images corresponding to the variety of parameters were visualized by intensified charge-couples device (ICCD) camera system. Voltage - current curves and emission spectra captured by a mechelle spectrometer (Mechelle 5000, Andor) referring to electrical and optical characteristics of the discharges were measured. Experimental results show that two types of discharges, surface discharge and pore microdischarges inside the ceramics, can be observed and develop simultaneously when applied voltage and pore size are above a critical value. The onset voltage of pore microdischarges increases with the increasing thickness of ceramics,and it decreases significantly due to the increase of porosity.Abundant active carbon-based components, such as CH $(\lambda =314.05$, 389.02, 431.31nm) and $\mathrm {C}_{2} (\lambda =471.6$, 516.2, 563.3nm) in the excited level, are produced in discharge. Temperature of the active particles is calculated by LIFBASE software to better understand the mechanism of the discharge process in porous ceramics. The work should be beneficial to determine optimum parameters to enhance the plasma catalytic process.

Surface charge accumulation on insulating materials is a major factor that greatly distorts the electrical field and thus reduces the insulation level. In this paper, the relationship between the surface charge density distribution and the output voltage of electrostatic probe is discussed, and an improved algorithm is built to calculate the charge density distribution, which employs two-dimensional Discrete Fourier Transformation (2D-DFT) to reduce the amount of calculation and the Wiener filter to reduce the influence of noise. Furthermore, because of the similar relationship between the electric field strength distribution and the surface charge density distribution, the field strength distribution can also be calculated by adopting this algorithm. In addition, the electrostatic probe is taken into consideration in this algorithm, and the spatial resolution of this measurement system including the inverse calculation process is investigated. It is estimated that the spatial resolution of this measurement system with 1mm gap between the electrostatic probe and the surface of the insulation is 0.3341mm. The charge density distribution as well as the field strength distribution is calculated by this improved algorithm when a lightning impulse voltage is applied to a PMMA plate of 1mm thickness, respectively. The results reveal that this algorithm has great accuracy and stability and can exactly calculate the surface charge density distribution and the electric field strength distribution.

Streamer and breakdown behavior of transformer oil under impulse voltage are investigated with experimental observations and simulation. The optical observation by using Intensified Charge-coupled Device (ICCD) fast snapshot technique indicates that streamer channel branching is of a stochastic nature. The 2-D axially symmetric modeling based on hydrodynamic drift-diffusion model is employed to depict the streamer dynamics of transformer oil. The experimental and simulation results reveal that bushy negative streamer requires higher excitation than filamentary positive streamer. Positive streamer travels faster than negative streamer from experimental estimation.

A two dimensional axis-symmetric fluid model is developed to present the streamer discharge process in transformer oil under positive and negative sub-microsecond impulse voltage, which is based on the hydrodynamic drift-diffusion approximation to simulate the dynamics of discharge inception and propagation. The streamers' spatial evolution with time are obtained, i.e., the distribution of electric field and space charge density under different conditions such as the amplitude of applied voltage and rise time. Simulation results verify the breakdown voltage has a distinct polar effect: negative streamer requires more than 2 times voltage threshold than that of positive one. It is revealed that the negative streamer, which usually has a bushy profile, is much more diffusive than the positive one. The magnitude and rise time of the applied voltage have evident influences on the shape and speed of the streamer in oil. The formation of positive or negative streamers are both characterized by a self-generated field enhancement at the head, and space charges enhance the front field of ionization region to assist the growing discharge channel to propagate longer. In terms of the generation of charged particles, it is considered that field-dependent molecular ionization predominates the charge generation mechanism of streamer discharge processes in transformer oil.

Secondary electron emission (SEE) induced by interaction between positive ion and dielectric surface is essential physical process for maintaining sustained discharge and controlling the characteristics of atmospheric pressure dielectric barrier discharge (DBD). In this work, SEE is described based on Auger neutralization theory. Dielectric energy band structure and defect energy level, including hole and electron trap energy levels, are taken into account to build a hypothetical relationship γ i = f (σ) between secondary electron emission coefficient γ i and surface charge density σ. With the introduction of γ i = f (σ) in DBD model, the characteristics of DBD are discussed in both first and steady discharges. It is found that, in steady discharge, γ i has higher value during the beginning and development state of discharge pulse of DBD, and γ i decreases in extinction stage. The negatively charged dielectric surface enhances SEE before discharge, which benefits the formation of diffuse discharge, compared with DBD of constant γ i. Moreover, the increase of energy band gap of dielectric reduces γ i, leading to the contractive discharge in DBD. The relationship between γ i and σ can be further applied to study the influences of defect energy level parameters on SEE and DBD characteristics.

We propose a theoretical ground for emissive capacitively coupled radio-frequency plasma sheath under low pressure. The rf sheath is assumed to be collisionless, and oscillates with external source. A known sinusoidal voltage instead of current is taken as prerequisite to derive sheath dynamics. Kinetic studies are performed to determine mean wall potential as a function of secondary emission coefficient and applied voltage amplitude, with which the complete mean DC sheath is resolved. Analytical analyses under homogeneous model and numerical analyses under inhomogeneous model are conducted to deduce real time sheath properties including space potential, capacitance and stochastic heating. Obtained results are validated by a continuum kinetic simulation without ionization. The influences of collisionality and ionization induced by secondary electrons are elucidated with a particle-in-cell simulation, which further formalizes proposed theories and inspires future works.

Atmospheric pressure dielectric barrier discharge (DBD) has abundant application fields, and the influence of the barrier material on DBD characteristics has been the research focus in recent years. In this work, a two-dimensional fluid model is built to investigate the effects of the barrier dielectric surface layer with different permittivity distribution on DBD, including electric parameters, electron distribution, and surface charge evolution. It is demonstrated that higher dielectric permittivity can affect DBD by enhancing the electric field Espace of space charge and weakening electric field Esurf of the surface charge and have a more obvious influence on Esurf, which can make DBD transform from glowlike discharge to streamerlike discharge. The dielectric surface layer with linear distribution of permittivity is introduced in DBD, changing the discharge appearance and discharge channel forming sequence. This work may provide some theoretical experiences for the application of a functionally graded material as a barrier dielectric in DBD.

An investigation is conducted regarding the influence of surface roughness on the flashover strength of an insulator in vacuum. A series of experiments is first presented, showing the relationship between surface roughness and flashover voltage, combined with measurement of surface potential distribution. It is found that surface charging on a roughened dielectric is suppressed remarkably; also the flashover voltage threshold depicts a rise-and-fall trend with roughness increasing, exhibiting a voltage summit at a certain roughness value. In addition, optical observation is implemented to draw a parallel with an electron trajectory in vacuum. A proposal can therefore be made that multipactor expansion tends to be mitigated as electron bombardment on a dielectric occurs with a lower secondary electron yield. Then a theoretical model considering the microscopic physical process is established to explain the above phenomena, consisting of an internal charge migration layer, an interfacial electron absorption layer and an external electron obstruction layer. The validity of this theoretical model can be confirmed by obtaining the surface trap distribution, microscopic morphology and net secondary electron yield.

Much attention has been paid to different localisation algorithms to locate partial discharge (PD) sources in power transformers using the acoustic emission (AE) technique. Due to total reflection, AE sensors cannot always detect the signal of a direct wave. As a result, localisation algorithms based on an indirect wave can be more accurate. But little research has addressed arrival time (AT) methods of indirect PD signals. In this paper, a double-threshold (DT) method is introduced, and its improvement based on short-time energy and a zero-crossing rate is described. Because an indirect wave arrives earlier and has lower energy than a direct wave, current AT methods, such as the energy criterion, cross-correlation, and linear prediction residual methods, cannot determine its AT. However, the proposed DT method can determine the AT precisely. Moreover, a localisation experiment was conducted on a 35 kV transformer, and localisation results were calculated employing various AT methods and different localisation algorithms, such as the Chan, genetic, imperial competition, and improved-propagation-route-search algorithms. Characteristics of the proposed AT methods were compared and analysed to show their feasibility and accuracy.

The charge density distribution of the surface leader has never been measured before. Because the surface leader usually covers a long distance, and the surface potential caused by leader discharge is usually very high, this creates difficulties in measuring the potential distribution of the surface leader. In this paper, with a feedback type electrostatic probe based on a field-nullify technique, a charge density distribution scanning system is developed. A two-layer structure pipe is designed to lower the surface potential after discharge. In this way, the surface potential distribution caused by the residual charge of the leader discharge under the application voltage as high as to 40 kV can be measured. The surface charge density distribution including the leader and streamer is perfectly measured, which is in good agreement with the photograph.

Dielectric window is an important component of high-power microwave (HPM) devices. However, surface breakdown easily occurs at the vacuum/dielectric interface when HPM pass through the dielectric window. This greatly limits transmission of HPM and makes the window a bottle neck of HPM technology development. Secondary electron emission (SEE) from dielectric window plays an important role in its surface breakdown. This paper studies the total SEE (including true secondary electrons and backscattered electrons) coefficients of several inorganic and organic dielectric materials including polytetrafluoroethylene, polyethylene, alumina ceramic, and machinable ceramic. The measurements are implemented by using pulsed electron beam impacting the materials with energies from 200 eV to 5 keV. In addition, surface desorbed gas property is studied with the quadrupole mass spectrometer. The performances of different materials are evaluated. The obtained results are useful for the selection of HPM window materials.

The performance of high-voltage apparatus is seriously restricted by the surface discharge on dielectric materials. A surface charge density distribution scanning system is developed with a dc feedback-type electrostatic probe based on the field-nullifying technique. The residual charge density distribution after the occurrence of a positive leader discharge is measured. Using the 2-D Fourier transformation in the spatial domain and Tikhonov's regularization method, the surface potential distribution and electric field distribution are arrived at. The leader stem and radial lament streamer zone located at the head of the leader can clearly be distinguished from the distributions. There is an obvious knee point that divides the main channel of the discharge into a leader channel and a streamer channel. The knee point is at about 9 kV, which corresponds to about 320 pC/mm2.

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performance. The paper insulation degrades under the combined stresses of thermal, electrical and chemical stresses during routine operations, resulting in the reduction of its degree of polymerization (DP) and hence its mechanical and electrical strength. Space charges in a dielectric material have a close relation to its electrical performance. Therefore, in this work, the effect of paper-aged states (with different DP values) on the space charge behavior in oil-impregnated paper insulation is investigated, and the pulsed electro-acoustic (PEA) method is used for the space charge distribution measurement. In order to achieve this purpose, new oil-impregnated paper samples are thermally-accelerated aged for different time interval to obtain different DP values, and then the residual oil in each sample is removed, and the Kraft paper samples are dried in a vacuum dry oven. After that, all paper samples are impregnated with new oil, and PEA measurements are performed at room temperature. The experimental results show that, when the external dc voltage is applied on the oil-paper sample, homo-charge injection occurs for all samples. The samples with different aging states exhibit quite different space charge characteristics for both voltage-on and voltage-off conditions. When the aged states are worse, for voltage-on condition, more charges are induced at cathode and anode, as well as more space charges are accumulated in the bulk. For voltage-off condition, the space charge decay rate shows no obvious change for different aged samples.

Hundreds of features have been extracted from phase resolved partial discharge (PRPD) pattern and PD waveforms to represent and recognize typical defects. Several feature selection and dimension reduction methods for pattern recognition are presented in this paper. Feature selection algorithms including forward feature selection, backward feature selection and floating forward feature selection (FFFS) are adopted to optimally select the features. |Four dimension reduction algorithms such as principal component analysis, linear discriminant analysis, kernel principal component analysis and generalized discriminant analysis (GDA) are used to further reduce the dimension of features. In order to compare the effectiveness of different selection and reduction techniques, PD tests on artificial PD defect models are performed. The results indicate that the FFFS and GDA are the optimal selection and reduction method, respectively.

For dielectric-vacuum compound insulation systems, the discharges usually occur across the insulator surface with an onset voltage much lower than the breakdown voltage for vacuum gap of the same length, which is the main cause leading to insulation failure. It has always been a hot topic to improve the surface insulation strength in vacuum. In this paper, the relationship between the flashover time delay and electric field is studied under nanosecond impulse voltage. Experiment results suggest that the flashover time delay was proportional to N-th power of electric field for grooved insulators with different parameters. For grooved insulator, the flashover channel prefer to develop along the grooves rather than pass over the groove wall. The blocking effect of grooves to the electron multiplication in cathode-anode direction leads to the increased time delay. A narrow band of electric field with respect to time delay is formed over the flat samples. The breakdown electric field of a grooved insulator was increased by more than 100% than that of a flat one. The flashover electric field tends to be saturated when the groove depth is more than 2 mm for both PTFE and PMMA, which is correlated with the trajectory of electrons travelling across insulator surface. Grooves near electrode is more effective to increase surface insulation strength than grooves in middle area, which can be of great help to guide the structure optimization in engineering application.

With the pace of construction of electrical grid, more districts are likely to be influenced by galloping of overhead transmission lines in winter. The galloping of overhead transmission lines may cause mechanical damages to metal fittings, insulators and transmission line towers. So it's of great significance to monitor the state and movement of overhead transmission lines. In transmission line galloping monitoring, there was a great diversity in results and performance of all kinds of monitoring equipment to overhead transmission lines from different manufactures because of the absence of national standard or industrial standard. In order to cope with this, a verification system to overhead transmission lines galloping monitoring equipment based on draw-line sensors was introduced, which could offer a basis for entering electrical grid to all transmission lines galloping monitoring equipment. In this paper, the principle of draw-line sensors in verification system was firstly illustrated. The draw-line sensors outputted the length of the lines that connect to the testing transmission line to calculate the position of the transmission line. Secondly, the structure of the verification system was presented, which included hardware and software. Then the verification system was used to examine the outputs from acceleration sensors and video monitoring system to inspect the function of verification system and test the performance of those monitoring devices.

Dielectric barrier discharge (DBD) has drawn great attentions in recent years, and the discharge physic is one of the research focuses. In this paper, both Townsend ionization coefficient α and secondary electron emission coefficient γ (SEEC) in DBD are discussed. The value and partial derivative of α in pure helium are compared with that of argon theoretically, which is related to the breakdown voltage and discharge channel radius. The changes of discharge channel radius under different electrical field is obtained, and the high nonlinear relationship between α and electrical field (∂(α)/∂(E)) is believed to be the main reason for discharge contraction1. Therefore, we mix a little NH3 in argon to make the α different from that of pure argon and work in the region where the ∂(α/∂(E) is relatively low. In this way, the uniform discharge, similar with He-DBD, can be obtained in the Ar/NH3 DBD.

Non-equilibrium plasma-assisted combustion has been extensively studied over the last few decades and has shown promising effects on combustion controlling, such as reducing ignition delay time, increasing flame propagation speed, enhancing flame stabilization and so on. In this paper, we investigated on the characteristics of dielectric barrier discharge (DBD) in methane with parallel-plate and multi-needle-to-plate electrodes under low pressure with sinusoidal waveform (20 kHz, 0–30 kV) power supply. A mechelle spectrometer (Mechelle 5000, Andor) is used to capture the plasma spectra. The results show that the discharge produces abundant active components related to combustion, such as CH (λ=392.5, 430.1, 438.7, 440.9nm), and C2 (A=473.6nm) in the excited level. Temperature of the active particles is calculated by LIFBASE software and platinum-rhodium thermocouple. With the increasing of applied voltage, the gas temperature and the number and average energy of the high-energy electrons are increased, leading to more free particles which are in the excited state, and finally the corresponding emission spectrum intensity is enhanced. Under the same gas pressure and electrode gap except for generation by different the electrode structure, the temperature of gas in multi needle-plate electrode is lower than plate-plate electrode. And it is proven that multi needles in a honeycomb structure layout can be made in more stable discharge, which can be used in sub-atmospheric even under atmospheric pressure, and it is better for plasma-assisted combustion application.

UHF-based partial discharge (PD) early warning technique has been tried to monitor and localize PD activities in the whole Air-Insulated Substation (AIS). This paper presents a PD detection and localization system based on this method. This system is composed of four-antenna array (double-cone structure), signal conditioning unit (wide band filter, low-noise amplifier and band-stop filter) and high-speed acquisition unit The main sources of background noise are analyzed and corresponding rejection methods are developed to reduce their effect The probability-based localization algorithm which comprehensively consider the time differences of multiple signals are proposed to estimate the position of PD source. The developed system is applied to PD detection and location in substations, and the on-site results prove the effectiveness of the system.

Surface discharge on dielectric materials seriously restricts the performance of many electrical and electronic systems. Based on Pockels electro-optic effect, the surface discharge on polymeric insulating materials has been investigated under HVAC in Atmospheric Air. Different polymer materials such as polyimide (PI), polyethylene terephthalate (PET) and polyvinylidene fluoride (PVDF) films are employed. One single cycle of sinusoidal HVAC with peak value of 5 kV and duration time of 50ms is applied on a needle-plate electrode configuration and the applied voltage starts from the positive or negative half-cycle, respectively. During the development process of surface discharge on different materials, the dynamic observation of surface charge distribution is recorded in a cycle by a high speed camera. The experimental results show that there is significant voltage polarity effect observed, and the radius of negative charge distribution is smaller than that of positive charges. The positive charge density on PET and PI is lower than that on PVDF, which are closely related to the enhancement of electric field by permittivity and thickness of polymer films. Moreover, PI, PET and PVDF samples behave different surface charge distribution. Positive streamer channels on PI and PET are gradually growing into a snake-like pattern with voltage applying, and positive streamers on PVDF develop instantaneously and exhibit as a regular spoke-like pattern. The influence factors on different discharge patterns are discussed in the paper.

Surface charging commonly appears on dielectrics in vacuum in the presence of electron bombardment, seriously aggravating the superficial withstand strength of assorted devices. Nonetheless, a pre-conditioning technique is introduced in this paper capitalizing on surface charges to play an opposite role, enhancing flashover strength and suppressing the multipactor which is frequently found over vacuum insulator. A theoretical study is first performed, incorporated with particle-in-cell simulation to show the critical condition for a single-surface multipactor to initiate. Therewith, it is proven that a negative charge accretion in cathode adjacency can prevent the multipactor from commencing with efficiency. Subsequently, an analytical model is constructed to expatiate multipactor expansion with pre-set surface charges getting involved, illustrating an upper bound of its propagation velocity, influenced by pre-conditioning. Corresponding experiments are also conducted to corroborate previous conclusions, presenting both a deflected electron trajectory and an improved flashover threshold. In the end, a brief discussion is given on a possible method to generate desired surface charge distribution in practical applications.

We propose a novel theoretical formula to estimate the surface flashover threshold in vacuum using the intrinsic properties of dielectric material as well as dielectric-vacuum configuration. The method is based on an above-surface model called secondary electron emission avalanche (SEEA), where flashover development can be divided into multipactor discharge, outgassing and an ionization avalanche which eventually leads to a plasma discharge. The transition from SEE-dominated regime to discharge plasma is supposed to be of significance where gas breakdown takes place within a nonuniform desorbed gas cloud in the vicinity of dielectric surface. Flashover occurs when specific space charge multiplication is achieved, yielding the critical pressure and breakdown voltage. The depiction of previous stages involves the secondary electron emission of dielectric materials, desorption and diffusion of adsorbed gas, ambient temperature, electrode structure, etc. Both particle-in-cell simulation and experiment data can corroborate the obtained theoretical results.

A 2D simulation based on particle-in-cell and Monte Carlo collision algorithm is implemented to investigate the accumulation and dissipation of surface charges on an insulator during flashover with outgassing in vacuum. A layer of positive charges is formed on the insulator after the secondary electrons emission (SEE) reaches saturation. With the build-up of local pressure resulting from gas desorption, the incident energy of electrons is affected by electron-neutral collisions and field distortion, remarkably decreasing the charge density on the insulator. Gas desorption ionization initiates near the anode, culminating, and then abates, followed by a steady and gradual augmentation as the negatively charged surface spreads towards the cathode and halts the SEE nearby. The initiation of flashover development is discussed in detail, and a subdivision of flashover development is proposed, including an anode-initiated desorption ionization avalanche, establishment of a plasma sheath, and plasma expansion. The transform from saturation to explosion of space charges and dissipation of the surface charge are revealed, which can be explained by the competition between multipactor electrons and ionized electrons.A 2D simulation based on particle-in-cell and Monte Carlo collision algorithm is implemented to investigate the accumulation and dissipation of surface charges on an insulator during flashover with outgassing in vacuum. A layer of positive charges is formed on the insulator after the secondary electrons emission (SEE) reaches saturation. With the build-up of local pressure resulting from gas desorption, the incident energy of electrons is affected by electron-neutral collisions and field distortion, remarkably decreasing the charge density on the insulator. Gas desorption ionization initiates near the anode, culminating, and then abates, followed by a steady and gradual augmentation as the negatively charged surface spreads towards the cathode and halts the SEE nearby. The initiation of flashover development is discussed in detail, and a subdivision of flashover development is proposed, including an anode-initiated desorption ionization avalanche, establishment of a plasma sheath, and plasma expansion. Th...

The discharge mode and photoelectric characteristics of Ar/NH3 DBD (dielectric barrier discharge) are studied to discuss the stability of the discharge under the influence of Penning ionization and attachment reaction. There are three discharge modes, including stable uniform glow discharge, unstable glow discharge and unstable columnar discharge. Discharge instability, including nonuniform discharge and discharge channel split, occurs under low ( 0.35%) volume fractions of NH3, which are understood to be caused by an insufficiently strong Penning effect or a strong attachment reaction, respectively. The attachment reaction of NH3 can also lead to a weaker emission intensity for Ar/NH3 DBD, in particular regarding the emission of OH(A2Σ+). Ar/NH3 and He/N2 DBDs are also compared. In He/N2 DBD, the emission intensity of OH(A2Σ+) changes less with increasing N2 volume fraction, which may be attributed to the lack of attachment reaction. Compared with Ar/NH3 DBD, the single discharge channel in He/N2 DBD is narrower, but the discharge area is wider, which should be induced by the higher Townsend ionization coefficient nonlinearity of He/N2 and the absence of attachment reaction in He/N2, respectively. In the end, a one-dimensional fluid model of Ar/NH3 DBD is built to verify the explanation of experimental results.

Plasma bullet is the formation and propagation of a guided ionization wave (streamer), normally generated in atmospheric pressure plasma jet (APPJ). In most cases, only an ionization front produces in a dielectric tube. The present study shows that two or three ionization fronts can be generated in a single quartz tube by using a microwave coaxial resonator. The argon APPJ plumes with a maximum length of 170 mm can be driven by continuous microwaves or microwave pulses. When the input power is higher than 90 W, two or three ionization fronts propagate independently at first; thereafter, they confluence to form a central plasma jet plume. On the other hand, the plasma bullets move independently as the lower input power is applied. For pulsed microwave discharges, the discharge images captured by a fast camera show the ionization process in detail. Another interesting finding is that the strongest lightening plasma jet plumes always appear at the shrinking phase. Both the discharge images and electromagneti...

Partial discharge (PD) site in high voltage power equipment can be located by solving the time difference equations. Estimating the time difference between PD signals with high accuracy is of great importance for accurate locating. This paper presents a time difference determining algorithm using cumulative energy function and differential energy method. The differential energy characterize the increase of cumulative energy, and the first significant peak of the differential energy is recognized as the arrival time. After that, a Monte Carlo statistics procedure is adopted to analyze the influence of noise level on the accuracy of time difference. The proposed estimation algorithm is examined with UHF signals acquired from four antennas in laboratory. The results indicate that the algorithm is effective to determine the time difference between UHF signals.

The oil-paper insulation has been widely used in large power equipment, and thermal and electrical stresses are the two main origins causing the deterioration and failure of oilpaper insulation system. In this paper, accelerated aging experiments of oil impregnated paper are conducted and partial discharge (PD) measurement are performed under typical defect models, i.e. protrusion, surface discharge and cavity inside the oil-immersed paper. The PD experimental results of three typical defects indicate that discharges initiated from a protrusion in oil have the similarity with that in air but with a limited apparent charge. The surface discharge are much more disruptive for oil-paper insulation. The intervals between consecutive discharges has the good tendency of decrease in the whole discharge events.

The application of Functionally Graded Material (FGM) in the solid insulation of high voltage apparatus is discussed. Firstly, the concept of FGM and its effect on electric field (E-field) optimization is introduced. Secondly, optimization of permittivity FGM (e-FGM) insulator is studied. A numerical technique for the optimization of permittivity distribution in e- FGM spacer is proposed and simulated on three typical spacer models, i.e. cone type, disk type and basin type. It is confirmed that the FGM application could significantly improve the E-field distribution. Moreover, the effect of the shrinking coefficient in the algorithm is discussed. Finally, concept of a novel fabrication method for FGM spacers is proposed based on the rapid- developing 3D printing technology. The process and advantage of this ‘bottom to up’ method is discussed.

Low temperature plasma generated by dielectric barrier discharge (DBD) has been paid more and more attention in recent years for its wide range of potential applications. In this paper, atmospheric argon DBD is ignited in a small gas gap of 0.1mm between two parallel glass plates covered by circular copper electrodes with a radius of 0.7 mm. A two-dimension axisymmetric numeric model, coupled with fluid and Poisson equations, is built to clarify the discharge evolution. The basic process of discharge, mainly controlled by electron energy and electric field, is analyzed, which is divided into three stages including Townsend-like discharge, streamer development and discharge extinction. Through the simulation, the spatial-temporal distribution of electron, space charge, surface charge and electric field are investigated, which indicates that the space charge plays a key role in the development of discharge. When the DBD is driven by applied voltage, five discharge channels can be found at the end of the discharge current pulse. The electric field distorted by space charge is expected to be responsible for movement of ionization wave. The surface charge is believed to be the main factor affecting the distribution of five discharge channels.

The degradation of oil-paper caused by thermal aging seriously affects the lifetime of power equipment such as transformers. In order to investigate the relationship between partial discharge (PD) and degradation of oil-paper insulation, PD characteristics and trap parameters of the paper are measured. In this work, we fabricate oil-paper model with cavity discharge, and the adopted oil-immersed paper specimens with different degree of polymerization (DP) are achieved by thermally accelerated aging to simulate transformers with different operation time. For each sample, the phase resolved partial discharge (PRPD) spectrogram is recorded. The trap parameters such as density and energy of different aged paper specimen are obtained by thermally stimulated depolarization current (TSDC) method. Furthermore, the microstructure of each paper specimen is observed by scanning electron microscope (SEM). Experimental results show two levels of trap existing on the surface of oil-paper: shallow (0.10–0.59 eV) and relatively deep (0.60–0.90 eV) energy level. Further analysis illustrates that thermally aging makes the trap energy deeper. It is considered that the change of PD and trap characteristics is of great significance for insulation condition assessment, and the trap parameter is promising to be new characteristic for aged paper insulation.

Previous researches have certified that diffuse plasma can be generated in some mixture gas gap between needle and plate, which may have potential application in biomedicine, film surface modification etc. In this paper, a self-consistent, multiple species, two-dimension numerical model, based on hydromechanics and electric Poisson's equation, is established to study the properties of gas-mixture discharge in needle-to-plate structure with a 0.1mm gas gap, where the needle is connected to high voltage sources and the grounded plate electrode is covered by a 0.03mm PTFE film. Moreover, the gas-mixture is consisted of 99% helium and 1% air (79% nitrogen and 21% oxygen) while 11 kinds of particles and 27 kinds of chemical reactions are taken into account. After about one-week computation, models driven by both positive and negative 400 V pulse voltages are solved respectively, which releases that electron has similar and dispersive distribution in both positive and negative discharge when it is at the peak of discharge current pulse, differing slightly from that of streamer. Then, the spatial and temporal evolutions of neutral particles (O 3 , O, N, He∗ etc.) are analyzed, which demonstrates that the ozone and oxygen atom have their maximum density near the cathode regardless of the polarity of voltage, and the high concentration area of nitrogen atoms and metastable helium atoms are always behind the head of ionization wave. Through the model, it can be proven precisely that the electrical field can be distorted by the electric field created by 8 kinds of charged particles, where various ions have very different spatial and temporal distribution because of the different production mechanism and ion mobility. In the process of discharge, Penning reaction and charge exchange reactions are believed to be significant in the production of some ions (e.g. N 2 +, O 2 + etc.) and seeding electron.

More than 80% of the energy we consumed comes from the combustion of fossil fuels. Non-equilibrium plasma generated by dielectric barrier discharge (DBD) can play a key role i n hydrocarbon combustion reactions. In this paper, we investi gate the characteristics of dielectric barrier discharge with co axial electrodes structure driven by repetitive microsecond pulse (rise time 10 μs, PRF 6 kHz, pulse duration 45 μs, amplitu de voltage 15–20 kV) or sinusoidal (20 kHz, amplitude voltage 15–30 kV) power supply, respectively. A mechelle spectrom eter (Mechelle 5000, Andor) is used to capture the plasma spectra. The results show that the discharge plasmas produce abundant active components such as CH (λ=392.5, 430.1, 438.7, 440.9 nm), C 2 (λ=473.6, 516.2, 526.5, 563.3, 609.9, 711.5, 90 9.5 nm), OH (A2Σ→X2Π0-0, λ=309.0 nm), and O (3s3S0→3p 5P, λ=777.5 nm) atoms in the excited level. Temperature of the active particles is calculated by LIFBASE software. With the increasing of applied voltage and electric field, the number and average energy of the high-energy electrons increased, leading to more free particles which are in the excited state, an d finally the emission spectrum intensity is enhanced. Under t he same experimental conditions except for generation by different waveform of the power supply, methane-air premixed fuel gas can be ignited by sinusoidal power supply. This compared shows that the temperature of plasma driven by sinusoid al waveform is higher than that driven by microsecond pulse waveform. In this experiments it is proven that the active particles generated by the discharge plasma can accelerate the chemical reaction rate and the burning speed while reduce the ignition delay.

Dielectric barrier discharge (DBD) is widely applied in many fields, and the discharge characteristics of insert gas have been the research focus for years. In this paper, fluid models of atmospheric Ar and He DBDs driven by 22 kHz sinusoidal voltage are built to analyze their ignition processes. The contributions of different electron sources in ignition process are analyzed, including the direct ionization of ground state atom, stepwise ionization of metastable particles, and secondary electron emission from dielectric wall, and they play different roles in different discharge stages. The Townsend direct ionization coefficient of He is higher than Ar with the same electrical field intensity, which is the direct reason for the different ignition thresholds between He and Ar. Further, the electron energy loss per free electron produced in Ar and He DBDs is discussed. It is found that the total electron energy loss rate of Ar is higher than He when the same electrical field is applied. The excitation react...

Microwave breakdown phenomenon across dielectric window at the vacuum/dielectric interface limits the transmission of HPM and becomes a bottleneck of HPM technology development. Its avoidance or suppression is a major concern among researchers on HPM sources. Currently, the most effective approach is geometrical modification with the goal of altering the trajectories of electrons. In this paper, periodic rectangular grooves perpendicular to microwave electric field, are applied to improve the threshold of breakdown. To clarify the mechanism, the SEEA simulation model based on dynamic analysis and particle-in-cell (PIC) is built in this paper. The grooves change the electrons trajectories and eventually resonant multipactor condition may no longer be satisfied. The suppression effect is influenced by the grooves and each given width relates to an effective power range for suppression. Dielectric breakdown experiments under HPM of S-band at 2.86 GHz are conducted with the grooved dielectric window. The results of the experiment are consistent with theoretical analysis and simulation.

To improve impregnating property of Polypropylene film, surface modification of Polypropylene film with dielectric barrier corona discharge in atmospheric pressure is carried out in this paper. A dielectric barrier corona discharge (DBCD) generator is designed and employed for surface modification. The Electrical properties of DBCD, surface analyses of Polypropylene films including chemical components, microstructures and hydrophilic property of films before and after processing are carried out. The experiment results show that after plasma treatment, a great deal of oxygen-containing groups is introduced to Polypropylene films, the surface oxygen content of PP film increases to 10%, and contact angle decreases by 30%. The surface roughness of Polypropylene film is also enhanced and the impregnating property of Polypropylene films can be improved by DBCD plasma treatment.

Polymeric materials such as polypropylene films are widely used in electrical and electronic devices because of their good mechanical, chemical, and electrical insulation properties. The electrical behavior subjected to high electric-field and possible breakdown phenomena are of great practical interest. For high-voltage electrical equipment, insulation defects such as pointed end are unavoidable. Under high electric field, these defects may cause partial discharge, which deteriorates the insulation material and increases the probability of breakdown and failure. In this paper, the partial discharge characteristics of oil/polypropylene film under the needle-plate electrode configuration are investigated under the impulse voltage. A double exponential impulse voltage source generator is designed, and electrical partial discharge detection system based on micro-current transducer with anti-interference solution is then established. Experimental results show that at the rise stage of applied voltage and around peak value, the partial discharge pulses are mainly homopolarity with the excitation, while at the wave tail time, reverse discharge could be often observed. Surface charge on the barrier dielectric plays an important role in the formation of hetero-polarity discharge. Moreover, the influences of voltage magnitude and pulse width are also analyzed. It is very useful to investigate surface charge during surface discharge on polymeric materials, which is helpful for understanding the development of surface discharge and making proper use of polymeric materials.

The charge accumulation on the surface of dielectric materials is a long-term academic research focus. As part of an effort to understand the origins of this behavior, surface charge distribution on silicon rubber under DC high voltage before and after flashover in atmosphere was measured by an electrostatic voltmeter with a vibrating Kelvin probe. Three types of electrode systems were employed in the study, i.e. circle-circle electrodes, ellipse-circle electrodes and tip-circle electrodes. The results showed that surface charging processes are close related with uniformity of electric field and voltage polarities. Once flashover happens, there exists a channel linking HV and grounded electrodes, which presents a low potential.

During the long-term operation of electrical equipment of HVDC system, accumulated space charge in oilpaper insulation can greatly distort the local electric field, which can cause the aging and breakdown of the equipment. Therefore, it is important to understand the influence of the space charge on breakdown characteristics of oil-paper insulation dielectrics under DC voltage. In this paper, four kinds of oil-paper samples with different aging status are obtained by accelerating thermal aging. At room temperature, each sample is applied with DC voltage till breakdown. During the processes of the breakdown, the space charge distribution is measured using pulsed electro-acoustic (PEA) method, and space charge distribution is analyzed at the breakdown moment. The experimental results show that positive space charge moved towards to opposite electrode under DC voltage and the more serious deterioration of the oil-paper insulation dielectrics, the deeper positive space charges moved. At breakdown moment, space charge and electric field polarity are reversed, and space charge density and electric field decrease with the delay time after breakdown.

Summary form only given. The electrical behavior of dielectric liquids such as transformer oil subjected to high electric-field and the possible breakdown phenomena is of great practical interest, since this liquid has been widely used in high-voltage power apparatus and pulse power systems, accordingly has led to extensive research work. This paper presents streamer discharge process such as initiation and propagation until breakdown in transformer oil under positive and negative nanosecond pulse voltage. Simulation computation via finite element method is undertaken through developing a two dimensional axis-symmetric fluid model. In this fluid model, three charge carriers, such as electron, positive and negative ions are utilized to account for the charge generation, recombination, and transport mechanisms in the discharging process. The streamer discharge profile and distribution of electric field and space charge density are obtained under different conditions such as polarities, the amplitude of applied voltage, rise time. Simulation results show that the discharging phenomena of positive and negative streamers are fundamentally different, such as electric-field profiles, initiation voltage (IV), breakdown Voltage (BDV) and propagation speed of positive and negative streamer are basically distinguished from each other. Generally, positive streamers tend to be filamentary in appearance, while negative streamers form in a bushy shape, which possibly imply that physical mechanisms behind both polarity streamers are different. It can be found that space charges play an key but different role in positive and negative streamers initiation and propagation. During streamer approaches the ground electrode, “ionization wave” which propagates toward ground can be observed. The rise time of applied voltage have evident influences on the average speed of streamer propagation. It is considered that field-dependent molecular ionization predominates charge generation mechanism of streamer discharge process in transformer oil. Field-dependent molecular ionization is firstly introduced by Zener Ionization in solid dielectric. Space charge effect contributes to further developed ionization until the whole gap eventually breakdowns.