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Surface charging phenomena on DC GIL spacers is a complicated process affected by lots of factors, among which surface conductivity is an important dielectrics parameter. In this study, experimental and theoretical research are carried out to investigate the effects of non-uniform surface conductivity on surface charge accumulation behaviours of downsized HVDC GIL spacers. A SiC/epoxy layer is partially coated on alumina/epoxy spacers to form a non-uniform surface conductivity. The coating patterns include sector-shaped coating and ring-shaped coating. The applied stress is DC voltage. The accumulated surface charges show a rising first and then decreasing trend with the increase of $S$ iC contents in coated area. When surface conductivity is high enough, there is almost no charge accumulation. Bipolar charges, which are generally observed on raw alumina/epoxy spacers, are found to be preferable to accumulate at interface region between high and low surface conductivity. The orientation of bipolar charges is determined by the distribution of inhomogeneous surface conductivity. The corresponding mechanism is revealed on the basis of experiment and simulation results which would be helpful for designing highly reliable and stable HVDC GIL.

Surface charge accumulation phenomena greatly restrict the development of DC power equipment. Suppressing surface charges are of great significance to promote surface insulation strength of polymeric dielectrics. In this study, we report surface charge and flashover regulation on alumina/epoxy spacers by graded conductivity coatings under DC voltages. The spacer surface is divided into different circular-ring areas from high voltage (HV) to grounded (GND) electrode. High conductive coatings consisting of titania particles of different contents and epoxy matrix are fabricated to spray on different circular-ring areas of spacer surface, which forms graded distribution of conductivity. Surface charge distribution on epoxy-based spacers is obtained by a Kelvin electrostatic probe after DC charging. Flashover voltage is measured in 0.1MPa 15%SF 6 /85%N 2 mixtures. The results show that graded conductivity coating can achieve excellent control of surface charge accumulation, and therefore improve flashover performance.

Cables play an extremely important role in the transmission of electrical energy and communication signals in modern large-scale electrical systems. Early cable insulation defects often cause cable insulation breakdown, leading to permanent insulation damage. However, it is often very difficult to accurately detect these potential local defects. This paper proposes a novel non-destructive detection method for cable defect based on broadband impedance spectroscopy. Firstly, a simulation model of cable impedance spectrum is established. The correlation between impedance parameters and cable defects is well analyzed, and the influence of different defect locations on the cable impedance spectrum is studied. The effect of frequency spectrum on the accuracy of fault location is also analyzed. Finally, several cables of different lengths are simulated and the results show that this method has great spatial resolution within 0.3% for cables from several meters to hundreds of meters. It indicates that this non-destructive default location method has great potential in field application of cable diagnosis.

With the continuous increase of electric power load, the capacity and peak overload can no longer meet the demand of power supply well. The current carrying capacity of the cable should be increased as much as possible to solve the above problems. The current carrying capacity of power cables is difficult to determine because it is affected by many factors such as the laying method, operating conditions, and surrounding environment. Therefore, accurately calculating the current carrying capacity of power cables under complex conditions has great significance for ensuring the cables' safety and economy. Power cables are the main medium for urban power transmission. Accurately calculating the current carrying capacity and planning the laying method of power cables are of great significance for reliable power supply. Because cables are always laid by complicated methods and harsh environments, which mean that various factors can affect the ampacity and temperature field distribution, it is difficult to entirely rely on the current IEC standard for calculations. Based both on the IEC standard and heat transfer principles, this paper establishes models for buried cables in different soils and laying methods. The finite element algorithm is used to analyze the current carrying capacity and temperature field distribution of power cables.

Thermal stress would affect the surface charge accumulation of insulation, causing unpredictable threat to insulation safety. It is necessary to investigate the effect of temperature on surface charging characteristics. In this paper, epoxy/SiC composite sheets with different SiC contents were fabricated. The corresponding surface potential under different temperatures were measured. The results show that the thermal stress affects the surface charging characteristic by changing the insulation conductivity. For epoxy/SiC composites, since the bulk conductivity is sensitive to the temperature, increasing sample temperature can decrease surface charge obviously. A simulation model was established to verify the effect of temperature on surface charging.

At present, protection devices such as low-voltage circuit breakers and fuses are commonly used in low-voltage distribution networks, which can effectively prevent short circuits, overloads, and ground leakage. However, this method is out of work in detecting series arc faults caused by poor contact, insulation failure, etc. Therefore, how to achieve accurate detection of series arc faults has become a hot issue in current research. Wavelet transform is usually used for series arc fault detection. But it exists the problem of spectral aliasing, the false detection rate is still high. This paper uses detection method based on the current waveform to carry out research. By building an arc fault platform to simulate series arc faults, normal and arc fault data under different loads have been obtained. The structure of deep learning algorithm can be established through these experimental data. The accuracy of the algorithm is improved by using mini-batch gradient descent, exponential decay learning rate and Adam's optimization algorithm. By establishing test data for diagnostic verification, it was found that the algorithm has an excellent recognition rate.

Polymers, especially polyethylene (PE), are widely employed as insulating materials in electrical power transmission systems. However, the insulation still faces the problem of space charge, which distorts the electric field distribution and accelerates electrical aging. Experimental results show that after the fluorination process, less charge injection occurs compared with pure PE. To clarify the mechanism, classical molecular dynamics was employed to build a PE/fluorinated layer interfacial model and first principles calculation was utilized to get the band offset at the interface. The results calculated by both the bulk plus band lineup method and the layer-decomposed density of states method show that the energy band of the fluorinated PE layer is overall lower than that of the PE side, and the band offsets are around 2 eV. Charge transport results based on Marcus theory and kinetic Monte Carlo simulations also show that charge can easily accumulate at the interfacial area under an electric field and the band offset can suppress charge injection. The conduction band offset acts as an energy barrier for the excess electrons at the fluorinated layer side to cross the interface, while the valence band offset has the same effect on hole transport because of the energy barrier caused by the inverted region. Our findings provide a fundamental and theoretical basis for material modification and space charge inhibition.

Bushing is the most important part of transformer to connect external component. Bushing fault is one of the prime reasons causing transformer faults. Moisture is an important factor leading to aging of bushing insulation, so it is necessary to evaluate the moisture content of bushing. In the current research, it can confirm whether the oil-paper insulation bushing is uniform damp. But there is no research related to the moisture location of oil-paper insulation bushing. We use Comsol software to carry out a simulation study of the finite element analysis method for nonuniform damp bushing. And the distribution characteristics of hydronium ions and hydroxide ions under electrode polarization can be obtained from the simulation analysis. In this paper, the polarization and depolarization current (PDC) experimental platform was set up and models of nonuniform damp bushings were made. The position information of the high-moisture capacitive screen can be obtained through two different wiring methods. The validity of the method can be verified by the distribution law of hydrolysis ions in this simulation.

Pulse discharge in water can not only produce ozone, hydrogen peroxide and other active substances with high oxidation potential, but also ultraviolet light and shockwaves, which has been widely used in biomedical, environmental, mining and other fields in recent years. The formation of active substances in water is closely related to the formation characteristics of discharge channels in the process of pulse discharge, as a result of which it is of great importance to study the characteristics of discharge channels in water for further development of high voltage pulse discharge technology. In this study, pulsed voltage produced by Marx pulse generator with frequency set to be 1Hz is applied for analyze characteristics of discharge channels induced by pulsed discharge in water. With the increase of voltage amplitude, transition of discharge mode from streamer discharge to spark breakdown is realized. Based on the fundamental results of discharge channels, preliminary experiments of its application in solar panel recycling are performed, which show that high voltage fragmentation is an effective method to deal with solar panels and the optimal parameter is 193J/g imported in this research. Circular holes and dendritic discharge channels are formed on the surface of the back sheets, and the ablation on the surface of the back plates shows a rugged pore structure on the micro level. Main metals in the broken products are concentrated in specific size fractions, which is convenient for resource recovery.

In order to realize the diagnosis of water distribution, this paper analyzes the interface polarization and macroscopic space charge polarization mechanism when the water distribution is non-uniform. The experimental results of this paper and bushing show that when the moisture distribution is non-uniform, there is a significant loss peak in the tan&delta, f curve. The loss peak shifts to higher frequencies as the non-uniformity coefficient increases. There are common intersection points between multiple tan&delta, f curves. Further, this paper realizes the diagnosis of the location of moisture distribution through Frequency Domain Spectroscopy (FDS) testing of different voltages and different wiring methods based on the macroscopic space charge polarization. In the single-cycle FDS test, when the positive electrode is first added to the area with higher moisture content, the amplitude of the tan&delta, f curve is smaller. The tan&delta, f curves under different wiring methods constitute a &ldquo, ring-shaped&rdquo, loss peak. As the voltage increases, the peak value of the loss peak shifts to the lower frequency band. As the temperature increases, the peak value of the loss peak shifts to higher frequencies. Based on the above rules and mechanism analysis, this research provides a new solution for the evaluation of moisture content of oil-immersed polymers equipment.

The localisation of partial discharge (PD) sources using the acoustic emission (AE) technique has attracted increasing research attention. The complicated propagation routes and wave-type conversion in power transformer can induce considerable localisation error. In this paper, the catadioptric phenomenon of AE wave propagation is explained in detail. With the incident angle varying, the wave type of the direct wave in tank wall could convert and the velocity will change accordingly. A transformer model is established in which each node refers to a suspected PD position and a shortest route search algorithm is proposed to calculate the shortest path between two nodes in this model. However, the fastest route is most significant to PD localisation rather than the shortest route when TDOA method is used. As a result, an improved propagation route search (IPRS) algorithm, which can recreate the propagation process and calculate the fastest AE routes, is proposed to localise the PD origin. To verify the feasibility of the IPRS algorithm, localisation experiments were performed in 35 and 110 kV transformers, respectively. Compared with other present localisation methods, such as the Chan algorithm, the genetic algorithm and the imperial competitive algorithm, the proposed algorithm can effectively reduce localisation error.

In this work, samples of oil-immersed paper with different moisture levels and aging condition were studied. Frequency domain spectroscopy (FDS) of the samples was examined at 40 °C under different excitation voltages. A non-linear phenomenon in frequency domain dielectric response with different moisture contents and aging condition was observed. The non-linear phenomenon appears to be related to the hindrance effect of cellulose on the movement of ions and the Poole-Frenkel effect on the concentration of ions. With aging, the nonlinear loss peaks at different frequencies caused by the effect of nonlinear conductance and interface polarization appear in the FDS results. The non-linear phenomenon was modeled and verified experimentally. The characteristic parameters and quantitative method to estimate moisture content and the degree of aging degree of oil-immersed paper can be obtained by the proposed method.

Capacitive high-voltage bushing is one of the typical oil-immersed paper insulation equipment, which is an indispensable external connection component for power transformers. In order to evaluate the effectiveness of frequency domain spectroscopy (FDS) for diagnosing moisture content of oil-immersed paper insulation bushing, this article studied the frequency-domain dielectric characteristics of oil-immersed paper insulation samples and field bushing. At the same time, the basic principle of interfacial polarisation is used to analyse the samples and field bushing with non-uniform moisture content. The results show that the moisture content affects the whole frequency band of the FDS curve. When the distribution of moisture content is non-uniform, the paper with high moisture content has a great influence on the whole FDS curve of oil-immersed paper insulation. In addition, the FDS curves will show a significant loss peak. The frequency of the peak point gradually moves towards the high frequency as the non-uniformity of the moisture content distribution increases. The experimental results show that the variation law of oil-immersed paper insulation can be used to quantitatively evaluate the moisture content and moisture type of oil-paper insulation power equipment.

This paper is aimed to identify the mechanism that atmospheric pressure plasma jet (APPJ) damages malignant melanoma (B16-BL6 cells) cultured in vitro through the induction of oxidative stress. After being exposed to APPJ with different time durations, the reactive species of H 2 O 2 and NO 2 - inside cell culture medium, together with a series of biochemical parameters including the cell viability, cell morphology, DNA damage, intracellular reactive oxygen species (ROS) and antioxidant capacity, are measured. The results reveal that there are obvious increases of H 2 O 2 and NO 2 - in the plasma-irradiated medium. And the cell viability, intracellular total antioxidant capability (T-AOC), and glutathione (GSH) contents decrease with the exposure to APPJ in a time-dependent manner compared with the untreated cells, i.e., 0-s group (p

Surface charge accumulation on spacer is an important factor to cause surface flashover. An experimental set-up is established to measure surface potential over a truncated cone-type spacer under DC voltage in SF 6 gas at atmospheric pressure using a Kelvin vibrating electrostatic probe. The surface charge density is calibrated based on multipoint measurement technique, which takes the influence of electrostatic probe into account. The experimental results show that surface charge distribution patterns can be divided into three classes including centrosymmetric distribution, spot-like distribution, and cloud-like distribution. The symmetrically distributed charges are injected by electrode, which migrate along spacer surface through the surface conductivity. While the charges showing spot-like and cloud-like distribution are generated by the ionisation of SF 6 due to local electrical field concentrated. Besides, the hetero-charges are more easily to be saturated with different charging durations and the polarity of the net charge is the same as that of applied voltage.

In this paper, the effects of aluminum nitride and alumina ceramics on the discharge mode of He/O2 mixed gas are studied and compared. It is found that when the O2 volume fraction is in the range of 0.2%–1%, dielectric barrier discharge (DBD) with Al2O3 is always in the unstable filamentary mode, and homogeneous and filamentary modes are observed in DBD with AlN. Under the homogeneous discharge mode, there is only one current pulse each half-voltage cycle, and the intensified charge-coupled device discharge image shows that it is glow discharge. The relative permittivity and surface charge decay characteristics of AlN and Al2O3 are measured. The experimental results show that the relative permittivity of AlN is lower than that of Al2O3, but its surface charge decay is obviously slower than that of Al2O3. The derived electron trap density from the surface charge decay of AlN ceramic is higher than that of Al2O3, which shows AlN has a strong charge capacity and can emit more electrons in the second half cycle of the discharge. The higher secondary electron emission coefficient can make the discharge ignition voltage lower and the formation of stable homogeneous discharge easier. This paper provides a new control method for expanding homogeneous discharge in He/O2 mixed gas.

In this study, a novel ultra-high frequency (UHF) sensor using an inner grading electrode inside an insulting spacer is employed to detect gas-insulated switchgear (GIS) partial discharge (PD). The simulation of the waveguide modes of the 252 kV GIS and the frequency response characteristics of this UHF sensor are performed by using the finite difference time domain (FDTD) method. Three kinds of typical defects are employed to investigate the PD detection characteristics of this UHF sensor. It is shown that the resonance frequencies of this sensor are extremely abundant below 3 GHz, and the major resonance frequencies around 0.5–1.2 and 2.2–3 GHz. The phase-resolved PD (PRPD) images accumulated by using this UHF sensor present the typical PRPD images of defects. The main frequency spectrum of the metal protrusion on a high-voltage conductor and floating potential defects are distributed below 1.5 GHz, which is consistent with waveguide mode simulation. A resonance frequency of a grading electrode UHF sensor appears at about 115 MHz, which is close to the simulation value of a resonance frequency at 140 MHz. The PD detection sensitivity of this sensor is much higher than that of an external sensor; however, it is a little lower than that of an internal sensor.

The flashover performance of insulating materials plays an important role in the development of high-voltage insulation systems. In this paper, silicone rubber (SIR) is modified by CF4 radio frequency capacitively coupled plasma (CCP) for the improvement of surface insulation performance. The discharge mode and active particles of CCP are diagnosed by the digital single-lens reflex and the spectrometer. Scanning electron microscopy and x-ray photoelectron spectroscopy are used for the surface physicochemical properties of samples, while the surface charge dissipation, charge accumulation measurement, and flashover test are applied for the surface electrical characteristics. Experimental results show that the fluorocarbon groups can be grafted and the surface roughness increases after plasma treatment. Besides, the surface charge dissipation is decelerated and the positive charge accumulation is inhibited obviously for the treated samples. Furthermore, the surface flashover voltage can be increased by 26.67% after 10 min of treatment. It is considered that strong electron affinity of C–F and increased surface roughness can contribute to deepening surface traps, which not only inhibits the development of secondary electron emission avalanche but also alleviates the surface charge accumulation and finally improves the surface flashover voltage of SIR.

Surface charge accumulated on insulators plays an important role in flashover process in vacuum, which has not yet been fully understood. In this paper, charging behaviors on insulator surface were experimentally examined under negative DC and impulse voltage. For the case of DC voltage, two charging stages were observed, i.e., the area near cathode was at first negatively charged under lower voltage (stage I) and then followed by a positively charged cathode-anode surface (stage II). Surface charging at stage I is considered to be caused by Schottky injection of homo-charges from electrode into sample surface layer. While at stage II secondary electron emission (SEE) process is responsible to the positively charged anode-cathode surface. However, charging at stage I is quite sensitive to pre-stress process, as well as the electrode-insulator interface conditions. The ongoing injection of electrons from cathode can lead to gradually decreasing in surface charge accumulation as the voltage duration increases. For the case of impulse voltage, Schottky injection of homo-charges has little influence on surface charging as the time is too short for charge injection and migration in surface layer. The SEE process will govern surface charging due to its extremely fast establishment process. A good linear relationship is obtained between saturated surface charge density and applied voltage, which is generally consistent with the numerical calculation result based on secondary electron emission avalanche theory. At last, it is confirmed that the ionization of desorbed gas from insulator surface and subsequent neutralization of electrons with positive surface charge is the main cause to eliminate surface charge in flashover.

Methods to infer the location of partial discharge (PD) in high-power transformers using acoustic emission (AE) data have been extensively studied. The inner complex structure of the transformers is one of the most critical points in localization with AE method. Windings and cores affect acoustic wave propagation by changing the arrival time because of inhomogeneous propagation. A transformer physical model has been established herein by taking these complex factors into consideration. Each node in the model is a potential PD position, and an acoustic wave route comprise a series of nodes. The velocity and propagation factors are set for each node according to its acoustic wave propagation characteristics. A propagation-time estimation algorithm is proposed to calculate the propagation-time. Based on the transformer physical model, a particle-swarm-optimization route-searching (PSORS) algorithm is employed for searching the position of the PD source. By comparing time differences of measured AE signals and the ones estimated by the PSORS algorithm, the velocities and positions of particles are continually adjusted, which can ensure their convergence to the PD source position. Localization experiments were performed in 35 and 110 kV transformers, respectively, to verify the applicability of the proposed algorithm. A protrusion defect is used to trigger PD pulses, and four AE sensors with two different arrangements are employed. The results confirm that the accuracy of proposed localization method is insensitive to the presence of metal structures blocking acoustic wave routes.

Ultra-high-frequency (UHF) antenna array system is applied to detect and locate partial discharge (PD) sources in air-insulated substation (AIS). Time-difference localization algorithm is generally adopted in such system in which the antenna array is of large scale. In this paper, the phased array theory from radar field is introduced to estimate the direction-of-arrival (DOA) of PD source to reduce the array size. The DOA estimation results are calculated by two-sided correlation transformation (TCT) focusing algorithm and multiple signal classification (MUSIC) algorithm. Based on that, the optimum arrangement of antenna array is investigated considering both high directional accuracy and small array size. The DOA estimation performances of three array geometries namely uniform linear array (ULA), uniform circular array (UCA) and uniform Y-shaped array are compared through Cramer-Rao lower bound (CRLB), numerical simulation and experimental testing. Moreover, the number of elements and element spacing are optimized. The results indicate that 4-elements UCA with 0.2m element spacing is the optimum array arrangement for PD location in AIS.

The temporal behaviors of charging on insulator surface at flashover initial stage is studied under dc voltage in vacuum. It suggests that the surface charging process develops synchronously with multipactor phenomena. A saturated region is firstly formed nearby the cathode triple junction (CTJ), and the insulator surface is charged gradually from cathode toward anode as the multipactor develops forward. The surface charge density increases linearly with applied electric field, but a stronger applied electric field will slow down the charging velocity as the results of violent collisions of secondaries with dielectric surface due to the stronger normal field built by surface charges. The charging velocity mainly depends on the secondary electron yield (SEY) for dielectric, i.e., the higher SEY, the greater the charging velocity is. The emitter height from CTJ has great influence on surface charging at areas adjacent to cathode, but little effect on charging behaviors at other areas. Once the multipactor is triggered by adequate initial electrons released from cathode, the surface will be charged spontaneously even if the field emission process at cathode is terminated, implying the generation of secondary electrons turns into a self-sustaining process. This study strongly supports the speculation in our previous publications regarding the mechanisms of stable luminescence preceding flashover, which is attributed to be initialized by Fowler-Nordheim emission of electrons at CTJ and finally generated by the radiative de-excitation of excited atoms/molecules in the self-stabilizing secondary electron emission (SSEE) process.

As an emerging technology for flexibly generating large-area atmospheric plasma, jet arrays are facing an urgent problem to control discharge modes caused by jet-to-jet interactions, apart from issues of stability and uniformity. Here, two modes in one-dimensional ns-pulsed He plasma jet arrays are reported, namely coupling and collimated modes. Discharge characteristics and transition rules of the two modes are explored by optical, electrical, spatial-temporal resolved and Schlieren methods. Results reveal that the coupling mode appears only if multiple conditions are satisfied, including low gas flow rate, high voltage frequency and small jet separation. Fluid dynamics and ICCD images prove that the mode transition depends on hydrodynamic interaction, which exceeds the effect of electrostatic repellence between ionization waves. Shrinkage of He flows is visualized to be enhanced at coupling mode, contributing to inward deflection of outmost plasma jets. Eddies on both sides of jet array and low-pressure regions near nozzles are generated by electric winds, promoting the transversal momentum transfer and gas fusion among adjacent He flows, thus tending to jet couplings. Fresh insights are provided into jet coupling and generation of controllable large-area plasma sources. The results could help to understand the importance of hydrodynamic interaction among proximal plasma jets, and plasma action on adjacent gas flows.

Pulsed mode as a common phenomenon appears in many kinds of DC corona discharge, whose characteristics can be affected by some specific factors. In this paper, an important research field of pulsed mode, pulsed–pulseless mode transition, is investigated in needle–plate electrodes in nitrogen at atmospheric pressure, and we discuss the effect of external circuit, gas temperature, and associative ionization on mode transition by experiment and simulation. The external circuit coupling with plasma can make the pulseless mode be achieved when there is a balance of charge between loss by discharge and gain by source before discharge quenches. The time-averaged gas temperature remains at 700 K which is regardless of source voltage and discharge mode, so gas heating is not a critical factor for mode transition. We investigate the effect of the associative ionization involving metastable particles by comparing the results with and without associative ionization reactions in the simulation; we find that the associative ionization is vital to determine the cathode voltage, discharge current, and the concentrative shape of discharge in the pulseless mode. Finally, we compare the pulsed–pulseless mode transition in nitrogen and air to clarify the effect of specific factors that depend on electronegativity of gas.

Partial discharge (PD) is the symptom of insulation breakdown in power equipment. The high rising steepness of PD current radiates electromagnetic waves which can be coupled with antenna. In this study, an antenna array is applied to PD detection and localisation in air-insulated substations, in which the locations of PD sources are found with time difference of arrival localisation algorithm. The influences of antenna deployment including the size, aspect ratio and shape of antenna array on localisation accuracy are systematically investigated. Three approaches including Cramer–Rao lower bound, numerical simulation of root of mean square error and experiments are adopted to evaluate the localisation accuracy. Three kinds of antenna arrays with diamond, rectangle and Y shapes are designed for locating the coordinates of PD sources. Their localisation performances are compared by these evaluation approaches, and the optimum antenna configurations for coordinate estimation are derived. The results indicate that the optimum array is dependent on aspect ratio of array size. If the aspect ratio is higher than 0.5, the rectangle array gives minimum localisation error and wide bearing ranges with low error. For array with aspect ratio smaller than 0.25, the diamond array is optimum among the three shapes.

For enhancing the surface electric withstanding strength of insulating materials, epoxy resin (EP) samples are treated by atmospheric pressure plasma jet (APPJ) with different time interval from 0 to 300s. Helium (He) and tetrafluoromethane (CF 4 ) mixtures are used as working gases with the concentration of CF 4 ranging 0%-5%, and when CF 4 is ∼3%, the APPJ exhibits an optimal steady state. The flashover withstanding characteristics of modified EP in vacuum are greatly improved under appropriate APPJ treatment conditions. The surface properties of EP samples are evaluated by surface roughness, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and water contact angle. It is considered that both physical and chemical effects lead to the enhancement of flashover strength. The physical effect is reflected in the increase of surface roughness, while the chemical effect is reflected in the graft of fluorine groups.

A new kind of N-containing heterocyclic borate ester (MTMDB) as lubricant additive was synthesized and characterized. The tribological behaviors of MTMDB and ZDDP in 100N base oil were evaluated through four-ball machine. The surface morphology of the worn surface lubricated by MTMDB was investigated by scanning electron microscope (SEM) and the tribological mechanisms were analyzed by energy dispersive spectrometer (EDS). The results show that the synthesized additive exhibits high hydrolytic stability, good anti-wear property and excellent extreme pressure performance. When 2.5% MTMDB was added into the 100N base oil, the smallest wear scar diameter (WSD) 0.48 mm is obtained. Furthermore, S and B elements were detected on the surface of the abraded steel. This can be confirmed that MTMDB has adsorbed and reacted with the steel surface during the rubbing process and generates a protective film.

The far-field acoustic radiation of a cylindrical shell with finite length submerged at finite depth from the water surface is studied. Two steps are utilized to solve the problem. The first step is to determine the vibration response of the submerged cylindrical shell by using an analytical method and the second one is to determine the far field sound radiation with the boundary element method. To address the vibration responses of the shell analytically, the cylindrical shell and surrounding fluid are described by the Flugge shell equations and Laplace equation in the cylindrical coordinate system respectively. The free surface effect is taken into consideration by using the image method and the Graf's addition theorem. The reliability and efficiency of the present method are validated by comparison with the finite element method. Then, based on the vibration responses of the shell obtained from the first step, the far-field sound pressure is computed by using the boundary element method. It is found that the vibration of the cylindrical shell submerged at finite depth from the free surface tends to be the same as that in infinite fluid when the submerged depth exceeds a certain value. The frequency and the submerged depth have crucial effects on the fluctuation of the far-field sound pressure, while for the curve of sound pressure level versus immersion depth, the ratio of the distance between the adjacent peak points of sound pressure to the wavelength is independent of the frequency. Moreover, the petal number of the directivity of the far-field sound pressure increases with the increase of the frequency and the submerged depth. The work provides more understanding on the vibration and acoustic radiation behavior of a finite cylindrical shell submerged at finite depth.

Two kinds of Mo/B oleic diethanolamide derivatives (coded as YXM and YXB) were synthesised, and their tribological properties were evaluated using a 4-ball machine. Results indicate that the 2 novel additives show excellent antiwear and extreme pressure properties. When 2.5 wt% YXM was added into the base oil, the wear scar diameter was reduced by 42.2%, and the PB value was increased by 170.4%; when 2.5 wt% YXB was added into the base oil, the wear scar diameter was reduced by 23.1%, and the PB value was increased by 167.1%. The worn surfaces of the lubricated GCr15 steel were analysed by using scanning electron microscopy and X-ray photoelectron spectroscopy. It is indicated that the variation of tribofilm species produced by a chemical reaction between the additives and the steel surface plays an important role in inhibiting wear and friction. A model was used to analyse the action mechanism. According to the analysis, the tribofilm is composed of an adsorption layer and a reaction layer, and it could effectively protect the steel surfaces from direct contact.

Ultra-high-frequency (UHF) sensing technique has been introduced to detect and localize partial discharge (PD) sources in air-insulated substation (AIS). This paper presents a probability-based algorithm to localize multiple PD sources which may occur simultaneously in different power equipment. Assuming that the time difference of arrival (TDOA) between all pairs of antennas in a array are normally distributed, the probability density function (PDF) of PD source coordinates can be obtained by substituting the linearized form of time difference equations into PDFs of TDOAs. When large number of PD signals are recorded, the joint PDF (JPDF) can be calculated from the product of PDF of each TDOA. Then the PD coordinates to be solved are regarded as with highest probability, and can be solved by taking the derivative of JPDF. In the case of multiple PD sources, mixed UHF signals are separated by clustering the TDOA vectors with K Means clustering method. PD experiments are performed to test the presented algorithm, and the localization accuracy of proposed algorithm is compared with other typical methods such as Newton-Raphson, Particle Swarm Optimization and plane intersection method. The results indicate that the probability-based localization algorithm reasonably integrates the TDOAs of continuous signal sequence, which can effectively reduce the influence of TDOA estimation errors and improve the localization accuracy.

Partial discharge (PD) measurement is an effective tool for insulation condition assessment of high-voltage power equipment. The occurrence of multiple PD sources causes great difficulty on pattern recognition and failure risk assessment. This paper presents a self-adaptive PD separation algorithm based on optimized feature extraction of cumulative energy (CE) function. The CE functions in time domain (TCE) and frequency domain (FCE) are calculated from PD waveforms and their FFT spectrums, respectively. By using an oblique line to cross the CE curves, width features are extracted from the intersection points between them. Through the mathematical morphology gradient (MMG) operation, sharpness features are extracted to characterize the rise steepness of CE. It is found that the separation capability of width and sharpness are dependent on the pre-selected oblique line and the structure element length (SEL) in MMG, respectively. In order to obtain satisfactory PD separation results for various experimental conditions, a density-function based parameter is proposed to evaluate the separation capability, and the oblique line and SEL are optimized with the goal of maximizing the evaluation parameter. A clustering algorithm is adopted to discover different clusters in feature space and separate PD signals. The separation algorithm is examined with mixed PD current pulses and ultra-high-frequency (UHF) signals acquired from experiments in laboratory and on-site equipment. The results indicate that the self-adaptive separation method is immune to the change of experimental conditions, and is effective for separating mixed PD signals.

Time-resolved evolution of secondary electron emission and trapped charge accumulation in polyimide film is investigated during two interval electrons bombardment, derived from the measurement of displacement current and secondary current via a hemispherical detector with the shielded grid. Under various irradiation current, secondary electron yield (SEY σ ) at a certain injected energy decreases exponentially from initial amplitude σ 0 to self-consistent steady value σ ∞ close to 0.93. The time constant τ of charging process is characterized as a function of incident current I p , and the results indicate that the formula I p × τ is fitted by a hyperbolical law. The influence of I p on the amount of trapped charge is studied and no significant change in its saturation value is observed. The evolution of SEY σ and trapped charge is dependent on incident dose Q p but not the incident rate I p . Furthermore, the trap density and capture cross section are discussed.

The oil-paper insulation in a transformer gradually ages due to the combined effects of thermal and electrical stress that occur during its operation. In this paper, for investigating the changes that occur within the oil-paper during the aging process with different aging stresses, partial discharge (PD) characteristics and trap parameters are measured. An internal discharge cavity model is designed for the electrical stress, and the thermal stress is generated by a baking oven simultaneously. The experiments are designed such that the dominant factor on the aging process is either thermal, electrical, or a unity of both stress factors. The phase resolved partial discharge (PRPD) spectrograms are recorded at different aging time, and the trap parameters are extracted by thermally stimulated depolarization current (TSDC) method. Additionally, the surface morphologies of these samples are observed by using a scanning electron microscope (SEM). The results show that the trap energy presents a decreasing tendency when the electrical stress plays the dominant role, and the discharge energy becomes larger over the aging time until pre-breakdown. When the thermal stress dominates the aging process, the trap energy shows an increasing tendency over the aging time, and there exists a process of ‘material conditioning’ that occurs at the initial stage of the aging process. The change of trap parameters in aged oil-paper will further affect PD characteristics. Correlation analysis of PD and trap characteristics can promote more accurate condition estimation of aged oil-paper.

During the manufacture and installation of power transformers, or after long-term operation, there may be various partial discharge (PD) sources inside the power transformer. In order to accurately assess the insulation state, it requires accurate measurement and diagnostics of type and location of PD source. Therefore, it is necessary to find an accurate method to extract effective features from phase-resolved partial discharge (PRPD) patterns, which can effectively characterize the PD type. In this paper, four keypoint extraction algorithms (the scale invariant feature transform (SIFT), the speeded up robust features (SURF), oriented fast and rotated brief (ORB) and BRISK methods) are introduced. For different types of PDs, different methods were used to extract a fixed number of key points. Then, the key points of the PRPD pattern of same defect type were matched. Thus, keypoint extraction methods were compared according to the degree of matching. According to the matching results, the ORB algorithm and the SURF algorithm have better performance, and the SIFT has the worst result.

As a traditional insulating gas, SF 6 has a serious greenhouse effect. At present, SF 6 /N 2 gas mixture, as an alternative to environmental protection, has good application prospects and attracts wide attention of scholars. In order to investigate the corona discharge characteristics of SF 6 /N 2 gas mixture under DC voltage, an experimental platform for needle-plane model corona discharge is built in this paper. The effects of N 2 content on discharge characteristics of SF 6 /N 2 gas mixture are studied under the experimental conditions of 25℃ and 0.3MPa pressure. The development process of corona discharge in SF 6 /N 2 gas mixture is analyzed, and the variation of initial voltage and the main pulse characteristic parameters of gas mixture with different N 2 content under negative DC voltage are measured. The results show that the corona discharge process in SF 6 /N 2 gas mixture can be divided into four stages. With the increase of N 2 content, the initial voltage will decrease and the pulse repetition rate will increase. The rise time will not change with N 2 content, the fall time and the pulse duration have no obvious regularity, and the pulse amplitude shows different trends under different N 2 content.

In experiment, it was found that the electrons injected into fluorinated polyethylene (PE) were much less than pure PE. In order to clarify the microscopic mechanism of this phenomenon, the PE/fluorinated layer structure is constructed with the help of molecular dynamics. The constructed model’s density is in correspondence with the actual material. After that, the constructed structure is introduced to first principle to calculate the electronic structures. The calculation shows that a little amount of charge transfer happens from the PE side into the fluorinated layer, leading to the formation of a weak dipole moment at the PE/fluorinated lay interface. This effect suppresses electrons’ cross-interface movement.

Although many techniques now are available for transformer fault diagnosis, one of the main issues need to be further investigated, i.e. how to address the incomplete and uncertain monitoring information in a fault diagnostic task. In this paper, we propose a transformer fault diagnosis method via approximation relations in approximation space to accomplish decision-making under incomplete information. Firstly, we build a decision-making table of transformers based on Rough Set (RS) theory in which each decision-making rule includes some conditional attributes and a correspondingly decision attributes. Hence, approximation relations are used to calculate the dependency of attributes in the approximation space, which provide the criterions to determine the optimal reduction sets of the table. When the conditional attributes in a diagnostic task are determined by monitoring information, we can use the reduction sets to match the task for obtaining the diagnostic results. It comes to conclusion that this proposed method shows a promising results of transformer fault diagnosis with high accuracy of 75.41% under incomplete information. In addition, the method could be improved by new symptoms-fault knowledge discovered.

This study experimentally presents how the flashover evolves from cathode-initiated to anode-initiated. An intensified CCD camera was employed to capture the surface flashover development process across 95% alumina ceramics in vacuum under seven kinds of electrode-sample structures with different electric field distributions. Experiments results suggested that flashover could initiate at either cathode or anode, which strongly depended on the competition between the localized field stress and electrical strength of the material. A basic relationship between surface flashover development process and its insulation performance was established. This study may contribute to guide the design of facilities used for pulsed power and electro-vacuum fields to achieve higher insulation capability.

SF 6 is a strong electronegative gas and used as insulating material in gas-insulated switchgear (GIS). For the efficient and reliable operation of GIS, it is necessary to investigate non-uniform field breakdown in SF 6 . Generation of free electrons is an essential prerequisite for initiation and propagation of positive discharges. A 3D particle-in-cell MonteCarlo collision (PIC-MCC) model is used to investigate the initiation of positive needle-to-plane streamer discharge in SF 6 with field ionization at pressures ranging from 1 to 5 bar. Free electrons are generated from impurity molecules with low density and low ionization energy in SF6 because of field ionization. The local electric field dependent electron generation rate is calculated using Zener’s model. Two stages of tree like streamer discharge are observed. In first phase, a spherical shape inception cloud is formed around the needle tip, which eventually destabilizes into multiple branches in the next stage. With the increase in pressure, the decrease in particle density, number of branches and total propagation length of streamer discharge is observed. The simulation results show that field ionization could be the possible source of ionization in SF6 in the presence of high electric field.

Surface charge accumulation on insulators is an important factor to incur surface flashover. The present study investigates the effects of surface nonlinear conductive coatings on surface charge behavior of alumina-filled epoxy resin spacers. Five SiC/epoxy coatings with different SiC content were prepared and coated on spacer surface. The surface charge distribution in atmospheric pressure air under DC voltage was measured. The results show that with the increase of SiC content, surface charges increase firstly then decrease on SiC/epoxy coated surface compared with uncoated spacer. With the increase of SiC contents, surface charge accumulation can be divided into three regions: the aggravation region at low SiC content, the balance region at moderate SiC content and the suppression region at high SiC content. The game process between the increased trap density and the increased non-linear conductivity caused by the surface coatings is considered to be the main reason for the formation of these three regions.

Influence of metal particle contamination on surface charging behaviors as well as flashover characteristics for PTFE insulators in vacuum was experimentally investigated under impulse voltage under a planar electrodesample structure. The surface charge was measured by a vibrating Kelvin probe used in conjunction with an electrostatic voltmeter. The effects of metal particle positioned on insulator, e.g., near cathode, at the middle of cathode-anode surface and near anode, on surface charging and flashover voltage were discussed. It is confirmed that the secondary electron emission process governs surface charge accumulation behaviors under impulse excitation, which initiates from cathode triple junction (CTJ) and develops towards anode. Surface charging is much more severe with metal particle contamination, and flashover voltage decreases as the size of metal particle increases due to the enhanced electric field at CTJ. This study is helpful to make a better understanding of the mechanism of surface flashover for insulators with metal particle contamination.

The plasma sheath is the non-neutral space charge region that isolates bulk plasma from a boundary. Radio-frequency (RF) sheaths are formed when applying RF voltage to electrodes. Generally, applied bias is mainly consumed by a RF sheath, which shields an external field. Here we report evidence that an intense boundary emission destroys a normal RF sheath and establishes a type of RF plasma where external bias is consumed by bulk plasma instead of a sheath. Ions are naturally confined while plasma electrons are unobstructed, generating a strong RF current in the entire plasma, combined with a unique particle and energy balance. The proposed model offers the possibility for ion erosion mitigation of a plasma-facing component. It also inspires techniques for reaction rate control in plasma processing and wave mode conversion.

Active species generated during the underwater discharge process play a pivotal role in the application fields. The electrical parameters including voltage amplitude and voltage pulse width make various contributions to the generation of active species. In this paper, we investigate the influences of discharge patterns (streamer and spark discharge) and voltage pulse widths on hydrogen peroxide (H 2 O 2 ) generation. The experimental results show that the production rate and energy yield of H 2 O 2 increases with the rise of applied voltage in streamer discharge region. While after spark discharges occur, the energy yield shows a saturated trend and the production rate even decreases in the spark discharge region. The increase of voltage pulse width also makes a positive contribution to improving the production rate of H 2 O 2 . While the energy yield of H 2 O 2 production is independent on the variation of voltage pulse width. Besides, it is inferred that the generation of H 2 O 2 mainly occurs during the wave tail time instead of rising time.