Discharge Phenomena Research Articles

Multiphysics simulation of plasma channel formation during micro-electrical discharge machining

A 2D axisymmetric plasma model for micro-electrical discharge machining (μEDM) is developed, and the discharge phenomenon is discussed in this paper. Variations in different plasma properties, such as density, temperature, and collisions of the electrons bombarding the anode and cathode electrodes, were simulated to comprehensively explain the discharge process. The said properties of the plasma channel will be extremely helpful in determining the heat flux available at the tool and workpiece of μEDM. The governing equations of electrostatics, drift-diffusion, and heavy species transport were coupled together and solved simultaneously for computing the properties of the plasma channel in water vapor. The simulation describes the movement of electrons and ions in the inter-electrode gap during the discharge initiation under the applied electric field. The anode spot responsible for the material removal was formed much earlier compared to the cathode spot formed at the tool. Both the temperature and the density of the electrons were observed to be higher near the workpiece, compared to the tool electrode. The temperature of the electrons and the current density of the plasma obtained during the simulation will be useful to determine the heat flux responsible for the material removal. The non-equilibrium nature of the plasma sheath is responsible for the steep changes in the collisional power loss and higher capacitive power deposition near the workpiece electrode.

High Temperature Insulation Materials for DC Cable Insulation—Part II: Partial Discharge Behavior at Elevated Altitudes

Utilizing hybrid electric propulsion systems in the next generation of aircrafts with elevated DC voltage level has raised the concern about the degradation of insulation subjected to partial discharge at high altitude and harsh environment. In this work, the surface discharge behavior of commonly used high-temperature insulation materials in aviation systems (FEP, ETFE, and PEEK) under positive and negative DC and ramp voltage was studied, including the pulse waveform, frequency spectrum, PD count and magnitude. Pressure changes as a significant factor influencing surface discharge phenomenon was thoroughly studied. Dust figure technique was employed after voltage ramping tests for both positive and negative polarities at low pressure to investigate the surface charge accumulation and surface discharge traces, and unveil the involved physical mechanisms at different stages of streamer propagation. The content of this paper provides a reference for surface discharge studies and evaluation of high temperature materials for medium voltage direct current power distribution in the future aviation systems.

Preparation of silver nanoparticles in a high voltage AC arc in water

The article presents for the first time the synthesis of silver nanoparticles in an electric arc of high-voltage alternating current with a frequency of 50 Hz. In particular, the method and apparatus necessary for the preparation of nanoparticles in water solution is discussed. Current–voltage characteristics depending on the mutual distance between the electrodes are presented which show a very high stability of the generated discharge phenomena. The obtained nanoparticles were examined using various analytical techniques such as UV–Vis spectroscopy, dynamic light scattering (DLS), zeta potential, energy dispersive X-Ray analysis (EDS), X-ray diffraction (XRD), and X-ray fluorescence (EDXRF). The morphology, surface and size of the obtained nanoparticles was carried out using transmission electron microscopy (TEM) and scanning TEM (STEM) equipped with the annual dark-field imaging scanning atomic-scale chemical mapping (STEM). The designed simple power supply unit consisting of an autotransformer and a microwave oven transformer (MOT) makes the preparation of silver nanoparticles both simple and economical.

Voltage behavior in lithium-ion batteries after electrochemical discharge and its implications on the safety of recycling processes

The demand of lithium-ion batteries (LIBs) is exponentially increasing, largely due to the ongoing transition towards electric transportation. To support the raw material supply for LIB manufacturing, there are significant ongoing efforts to recycle battery materials. Nevertheless, end-of-life LIBs entering recycling processes may still contain remnant energy, representing a potential hazard during handling and processing. Despite the urgency to improve LIB recycling, there is a lack of serious discussion in the literature regarding discharging strategies for LIBs. The electrochemical discharge using aqueous salt solutions route for example, has been widely mentioned without proper evidence of its usefulness. Among the discharge phenomena so far overlooked is the voltage recovery effect of batteries (a.k.a. voltage rebound/relaxation), where battery power appears to spontaneously surge, even after readings of full discharge in a circuit. In this work, a systematic study on the behaviour of LIBs during discharge in aqueous salt solutions is presented to better understand this unit process, addressing the challenges to fully drain energy from spent batteries prior to recycling. We demonstrate that the voltage recovery effect creates false readings for the battery charge level that represent risks during processing. If electrochemical discharge is employed, we present a methodology to decrease open circuit voltage in aqueous salt solution to 2.0 V, suitable for mechanical processing.

Analysis of Cavity PD Characteristics’ Sensitivity to Changes in the Supply Voltage Frequency

The supply voltage frequency effect on partial discharge (PD) phenomena has continued to draw research interest. Although most high voltage equipment operates at power frequency (50/60 Hz), testing is often done at different frequencies for various reasons. Despite some agreements and inconsistencies for the research findings of PD activity’s frequency dependence, there has been consensus on the recognition of the discharge mechanism parameters that influence how the supply voltage frequency affects PD activity. These parameters include statistical time lag, discharge area surface conductivity, and the residual charge decay. In this paper, a 3-capacitor model (ABC) is used to simulate how the changes in the discharge mechanism parameters influence PD characteristics as a function of the supply voltage frequency. The findings are that the phase-resolved partial discharge pattern (PRPDP) and PD repetition rate (PDRR) characteristics are more sensitive to variations in the probability of the seed electron availability at higher frequencies of the supply voltage. The opposite trend is observed for the cavity surface resistance. At lower resistance of cavity surface, the PRPDP and PDRR characteristics are more sensitive to changes in the supply voltage frequency than at higher resistances. The paper also confirms that incorporating equivalent resistances in the ABC model makes it more authentic than the model comprising of capacitors only.

Experimental study on single-mode microwave-induced tungsten wire discharge for NO conversion in NO/N2 atmosphere.

Single-mode microwave-induced tungsten wire discharge was conducted to investigate discharge phenomena in Ar, N2, NO, and their mixtures, as well as the effects of parameters, including diameter and number of tungsten wire, initial NO concentration, total gas flow rate, and microwave power, on NO conversion. The discharge phenomena verified that intense discharge could be observed in pure Ar or N2, but the discharge was considerably weakened in gas mixtures. The results of NO conversion showed that the increases of the tungsten wire diameter (0.1-0.12 mm), the tungsten wire number (1-3 wires), and microwave power (400-700 W), or the decreases of the total gas flow rate (2-0.5 L/min), and the initial NO concentration (800-200 ppm) could effectively lead to the increase of NO conversion. A maximum NO conversion of 91.5% can be achieved under the optimal conditions in the examined range. Besides, spectral analysis showed that W, O, and N ions were found in the discharge zone. After reactions, depositions were found on the inner surface of reaction tube, and the results of EDS (energy dispersive X-ray spectrometer) tests show that the depositions were composed of W, O, and N. Therefore, a portion of NO was inferred to be consumed by tungsten ions through the formation of tungsten oxides and tungsten nitrides.

Comprehensive thermodynamic analysis of the CAES system coupled with the underground thermal energy storage taking into account global, central and local level of energy conversion

A key aspect of CAES is the optimal configuration of the thermodynamic cycle. In this paper, the situation of cooperation between the current conventional power plants and wind farms is analyzed, and then, based on thermodynamic models, the process of storing thermal and electrical energy in the CAES system coupled with a heat storage and recovery is developed. Therefore, three levels can be distinguished, namely: the global level of energy production; the central level of energy conversion and storage; and the local level of heat storage. Two different types of a CAES, with and without an Underground Heat Exchanger (UHE), were analyzed. The round-trip efficiency of CAES with a UHE approaches 52.26%. The article also analyses transient phenomena of charging and discharge the heat storage with UHE. The heat transfer rate and temperature received from the underground thermal energy storage was estimated. For this purpose, preliminary thermal-FSI approach was coupled with zero-dimensional model of CAES. The newly proposed CAES with a UHE, never studied before, can be used for applications requiring a high round-trip efficiency.

Micro gap air discharge based on fractal theory

Micro-gap discharge is a form of gas discharge in which the discharge gap is on the order of sub-millimeters orless. To study the initial path of micro-gap discharge and the mechanism and law of particle density change during discharge, in this paper a micro-gap discharge experiment and discharge image acquisition device under atmospheric pressure is built and the COMSOL simulation software is used to simulate the electron density and space charge in the process of micro-gap air discharge. Furthermore, the MATLAB software is used to calculate the fractal dimension and probability development index of micro-gap discharge. The air discharge phenomena produced by applying positive DC voltage to needle tip at atmospheric pressure and room temperature with gap distance ranging from 50 μm to 150 μm are studied. It is found experimentally that there are twists and turns in the discharge channel, and the number of bifurcations in the discharge process with a short gap is less than that with a long gap. Observation of the micro-gap air discharge process with a gap of 100 μm under atmospheric pressure shows that the discharge process is divided into the following three processes: needle tip corona, corona breakdown streamer, and spark discharge channel. Based on the analyses of these experimental results, it can be concluded that the discharge mechanism follows Thomson's theory, supplemented by the streamer theory. The cathode secondary electron emission (including positive ions colliding with the cathode and photoelectron emission) and the space charge distortion electric field form a secondary electron avalanche to maintain the discharge together. The seed electrons formed by a small amount of space photoionization also form an electron avalanche under the action of the space charge distortion electric field. There are tortuous sections in the discharge channel, but the number of branches is small and the degree of tortuosity is low. Therefore, there are weak streamer forms. The discharge channel is tortuous and branched, but the number of bifurcations is relatively small, and the tortuousness is low. In addition, it is also found that a sheath is formed at the cathode, the distortion of electric field is 3–8 times that of original electric field, and the electron density reaches 2 × 10<sup>21</sup> m<sup>–3</sup> during discharge, obtained from the COMSOL simulation. Meanwhile, the fractal theory simulation is used to simulate the micro-gap discharge. In the process of research, the fractal dimension is found to be proportional to the voltage and the gap distance. When the probability development index <i>η</i> = 1.18–1.3, the fractal dimension of the simulated discharge process is closer to the experimental result. The findings in this paper lay the foundation for further exploring the discharge theory of sub-micro- and nano-scaled gaps.

The Research on the Microscopic Mechanism and Dynamical Characteristics of DC Positive Corona Discharge in Pure O₂

We systemetically research the microscopic physical mechanism of positive corona discharge and comprehensively analyse the regular and irregular pulsed oscillations of discharge outputs. Especially, we discover the formation mechanism of irregular pusled discharge phenomenon for the positive corona and give a profound explanation. Accorinding to the physical process of discharge, we propsoe a new dynamical model which can describe the Trichel pulsed discahrge outputs and the characteristics of waveforms. Based on nonlinear dynamics method, we predict and analyse differernt stages of discharge. In particular, the experimentally observed irregularly pusled discharge on positive corona is reproduced by the presented theory. Our study reveals that the positive ions as current carriers predominate in the total current and the photons occurring can be treated as the feedback signal in the nonlinear process of discharge system. We find that the photo-ionization is an indispensable role as the source forming electron avalanches for sustaining the positive corona discharge. Moreover, according to quantum mechanics we demonstrate the high frequency photons with energy higher than the ionization energy may photo-ionize oxygen molecules in pure oxygen. We point out that the recombination radiation is competent to photo-ionize neutral molecules producing the seed electrons. Besides, we analyze the characteristics of diversely pulsed outputs and further explain the microscopic menchnism generating the symmetric and asymmetric sharp jagged pulses. Furthermore, we discover the intrinsic physical relations between positive ion and surface electric field, and the significant influnces of the attributes of different particles on the discharge behaviors.

Partial Discharges in Electrical Machines for the More Electric Aircraft—Part I: A Comprehensive Modeling Tool for the Characterization of Electric Drives Based on Fast Switching Semiconductors

The arrival on the market of new power devices based on wide bandgap semiconductors has raised a relevant interest due to their superior properties compared to conventional technologies. On the other hand, these devices are inherently characterized by high rates of voltage changes over time, which may result in reliability challenges in electric drives adopting them. In fact, dangerous voltage overshoots at the motor terminals and uneven voltage distributions within the machine windings may occur. These phenomena can trigger a high insulation stress and partial discharges and, as a consequence, they may concur to the premature failure of the dielectric materials. This paper proposes a flexible and comprehensive modelling approach for the accurate analysis and estimation of both voltage overshoots and voltage distributions in a typical converter-cable-motor system intended for more electric aircraft applications. The modelling results are validated against experimental measurements carried out on a physical prototype comprising a wide bandgap-based converter, a connecting cable and an electrical machine stator. The findings are then used in the companion papers (part II and part III) to investigate the dependence of partial discharge phenomena on these voltage waveforms, highlight reliability challenges in modern ±270 V DC bus voltage drives for the more electric aircraft and discuss solutions.

Detection and Localization of Partial Discharge in Connectors of Air Power Lines by Means of Ultrasonic Measurements and Artificial Intelligence Models.

According to the statistics, 40% of unplanned disruptions in electricity distribution grids are caused by failure of equipment in high voltage (HV) transformer substations. These damages in most cases are caused by partial discharge (PD) phenomenon which progressively leads to false operation of equipment. The detection and localization of PD at early stage can significantly reduce repair and maintenance expenses of HV assets. In this paper, a non-invasive PD detection and localization solution has been proposed, which uses three ultrasonic sensors arranged in an L shape to detect, identify and localize PD source. The solution uses a fusion of ultrasonic signal processing, machine learning (ML) and deep learning (DL) methods to classify and process PD signals. The research revealed that the support vector machines classifier performed best among two other classifiers in terms of sensitivity and specificity while classifying discharge and surrounding noise signals. The proposed ultrasonic signal processing methods based on binaural principles allowed us to achieve an experimental lateral source positioning error of 0.1 m by using 0.2 m spacing between L shaped sensors. Finally, an approach based on DL was suggested, which allowed us to detect a single PD source in optical images and, in such a way, to provide visual representation of PD location.

State‐of‐charge estimation for LiNi0.6Co0.2Mn0.2O2/graphite batteries using the compound method with improved extended Kalman filter and long short‐term memory network

SummaryBecause it is necessary for electric vehicle (EV) driver to know the remaining power of the EV battery during driving, thus it is very valuable to research the precise state‐of‐charge estimation, which can update the residual capacity of battery and prevent the occurrence of overcharge and over discharge phenomenon for battery. As compared with most model‐based methods, neural network‐based methods can directly use battery surface temperature as the input parameters of the model. In here, the state‐of‐charge of LiNi0.6Co0.2Mn0.2O2/graphite batteries is estimated using the compound method with improved extended Kalman filter and long short‐term memory network. The improved extended Kalman filter algorithm is composed of recursive least square method with forgetting factor algorithm, generalized regression neural network and extended Kalman filter algorithm. For the compound state‐of‐charge estimation method, the state‐of‐charge values are first estimated using the improved extended Kalman filter, and then optimized using long short‐term memory network. The datasets of dynamic stress test at 25°C are applied to train the long short‐term memory network, while the datasets under different conditions are used as validation datasets. The results reveal that the mean absolute error, maximum absolute error, and root mean square error for their combination method are less than 1%, 3%, 1.1%, respectively. Among three kinds of state‐of‐charge estimation methods, the compound method has the highest estimation accuracy under different working conditions.

Three-dimensional hybrid simulation of single cathode spot vacuum arc plasma jet under axial magnetic field

Vacuum arc is a special metal vapor discharge phenomenon, because its discharge medium totally comes from the evaporation and ionization of electrode materials. In the case of low current, the vacuum arc is completely composed of plasma jets emitted from discrete cathode spots on the cathode surface and the current carried by each spot depends on the cathode material. When the arc current exceeds a certain value, a certain number of cathode spot plasma jets will appear. Vacuum arcs play a very important role in some industrial applications such as vacuum circuit breakers, vacuum coatings and electric thrusters. As an important plasma control method, the external axial magnetic field (AMF) has an important influence on the macroscopic morphology and microscopic parameter distribution of the vacuum arc. Various studies of vacuum arc under AMF have been carried out and some progress has been made. However, the existing literature about the simulation research of vacuum arc is mostly concentrated in the case of large current, and less attention is paid to the case of small current. The reason is that the traditional methods, magneto-hydrodynamics or particle-in-cell, are limited by either accuracy or efficiency, and cannot be effectively applied to the low current vacuum arc plasma jet simulations. In this paper, we develop a fully three-dimensional hybrid plasma simulation algorithm to study the single cathode spot vacuum arc plasma jet under AMF. In this model, ions are modelled as particles while electrons are treated as massless fluid, and the self-generated magnetic field is also considered. To simplify the condition, the cathode spot in our model only exists as a plasma jet source, thus the detailed mechanism of producing plasmas is neglected. And the movement of the cathode spot is not considered either. The results show that the single cathode spot plasma jet diffuses into the interelectrode in a cone shape after leaving the cathode spot, and the ion density drops rapidly from cathode to anode. Under the simulation conditions in this paper (<i>I</i> ≤ 150 A), the self-generated magnetic field will not have a significant influence on the plasma jet itself in the case of low current. The external AMF has a compressive effect on the diffusion of the vacuum arc plasma jet. Under the AMF, the radial movement of the ions is suppressed, and the decrease of the ion radial velocity leads to a smaller diffusion radius of the jet. This compression effect of the AMF on the plasma jet is related to both the intensity of the external AMF and the magnitude of the arc current. In the case of a constant arc current magnitude, the compression effect gradually increases as the value of the AMF intensity gradually increases; in the case of a constant value of the external AMF, the compression effect gradually decreases as the current gradually becomes larger.

Wilhelm Conrad Röntgen: The scientist and his discovery.

Röntgen's discovery of a new type of radiation is the epochal event in a series of highlights of physics emerging within only a few decades of the late 19th century. As these discoveries are directly or indirectly rooting in the study of the phenomenon of electric discharge in gases a brief look at the physics scenario in the immediate pre-X-ray era is presented in the first section. Rather than just the fortune with the bold it is Röntgen's character as a diligent, self-critical and ingenious scientist which made his discovery possible. This will be illustrated in the sections on Röntgen's personal life and some specific details of his experiments leading to the discovery of X-rays. Finally, a short overview is given on the potential and the large variety of applications of X-rays.

Study on Surface Characteristics of E-glass Fiber Reinforced Epoxy Resin Composites in Different Stages of Tracking

To study the variation of surface characteristics of glass fiber reinforced epoxy composite insulation materials during the development of tracking, this paper established an experimental platform for tracking under the inclined plate method and prepared samples of glass fiber reinforced epoxy resin. In this paper, according to the experimental discharge phenomenon, discharge repetition rate phase diagram and corrosion degree of materials, the process of tracking was divided into four stages: initiation, stability, development and outbreak stages. Scanning electron microscope was used to observe the change of micromorphology of samples in different stages of tracking. The content of elements in different stages of tracking was determined by energy dispersive spectrometer. The surface characteristic functional groups in different stages of tracking were measured by Fourier transform infrared spectroscopy. The results show that with the change of surface morphology and the formation of surface products during tracking, the content of C element in the spherical region of the material decreased first and then increased, and the content of O and Si increased first and then decreased. The epoxy group of the material was gradually decomposed. Carbonyl group was generated on the surface of the material, and then decomposed during the outbreak stages. In addition, the deterioration mechanism of thermal aging and tracking was quite different. Thermal aging provided convenient routes for the electron injection into the material during tracking, thus reducing the tracking and erosion resistance of the material.

3D Model of partial discharge in defects with different sizes and positions in power cable for distribution and transmission networks

The knowledge of partial discharge (PD) phenomena inside electrical insulation of power cables is an important issue for assessing the insulation condition and its degradation state, obtaining information about the likelihood of failure. PDs cause signals to propagate along the cable, as noise phenomena, and contribute to the insulation degradation, culminating in a disruptive fault with the interruption of power supply. Therefore, PDs are considered the best ‘early warning’ indicators of insulation degradation and their modelling, with the development of on-line PDs location methods, are important topics to increase the networks’ electricity security. In this study, a three-dimensional (3D) model of PD events using the CST STUDIO® Dassault software is proposed. PDs inside air inclusions in epoxy-resin are analysed with different shapes, positions and sizes of the defect. The electric-field distribution is evaluated with the conduction current inside the void and the apparent charges induced on the electrode. The effectiveness of the model is validated by comparing the simulation results with other published experimental results. Finally, a description of a 3D–1D hybrid model useful to describe the propagation of PD signals in power networks is given.

Research on discharge phenomenon caused by cross‐adsorption of linear insulating fibre and metal dust under DC voltage

Research on discharge phenomenon caused by cross‐adsorption of linear insulating fibre and metal dust under DC voltage

Insulator leakage current prediction using surface spark discharge data and particle swarm optimization based neural network

The paper has completely built an online monitoring system to detect deteriorative insulator by predicting the leakage current which assist maintenance personnel in determining whether insulators need to be cleaned or replaced and improve maintenance efficiency in Taiwan. The real-time monitoring systems are installed on the 69 kV and 161 kV transmission towers. The local meteorological data such as temperature, humidity and dew point, and images of spark discharge phenomenon are collected to identify the correlation with the leakage current of deteriorative insulators. The forecasting model is enhanced by using the image features of surface spark discharge phenomenon (SSDP). The percentage of spark area and the change in the brightness of region of interest (ROI) can be utilized as exogenous inputs to the predicting model. Correspondently, a particle swarm optimization based back propagation neural network (PSO-BPNN) is applied to improve the accuracy in prediction. The proposed developing model combined enhancement inputs are compared with persistent models, such as back propagation neural network (BPNN), radial basis function neural network with K-means cluster (RBF-K-means) and support vector regression (SVR), to evaluate the performance based on common statistic metrics. The simulation experiments have proved the integrated surface spark discharge features combined PSO-BPNN to improve the accuracy and stability of predicting leakage current comparing with other models. Therefore, the surface spark discharge data have strong correlation with the leakage current. In addition, the PSO-BPNN also achieves higher accuracy, better effectiveness and faster convergence comparing with other persistent models.

Research on the electromagnetic propagating characteristics of hypervelocity impact on the target with aperture and different potential conditions

In view of the key problems of the discharge phenomenon caused by the impact of small space debris on the satellite's charged surface and the electromagnetic field induced entering the interior of the satellite after coupling through the aperture. In this paper, the characteristics of the coupling plasma, the plasma induced discharge and the propagation of the electromagnetic field induced by discharge through the aperture are studied by using the two-stage light gas gun loading system, the discharge charge monitoring system, the plasma diagnosis system and the electromagnetic testing system; Combined with the characteristic parameters of plasma inside and outside the Simulated satellite, the curves of discharge current and voltage, and the spectrum characteristics of electromagnetic radiation inside and outside the simulated satellite, the characteristics of electromagnetic field propagating into the Simulated satellite through the aperture under different potential on the surface of the simulated satellite are analyzed. The experimental results show that the characteristic parameters of the ionized plasma produced by the ionization of the target and the projectile splash increase with the increase of the surface potential of the target, and the ionized plasma determines the discharge characteristics and electromagnetic field characteristics; After the electromagnetic field generated by the impact side propagates through the aperture into the inner of the simulated satellite, the electromagnetic field in the simulated satellite appears signal attenuation in most frequency bands, but signal enhancement occurs in a few frequency bands; The main reason of electromagnetic signal attenuation is that the electromagnetic field directions at the edge of the aperture and the back of the target are opposite to that at the impact side; The enhancement of electromagnetic signals in some frequency bands is caused by the plasma entering the simulated satellite.

Entire process of surface discharge of GIS disc‐spacers under constant AC voltage

Surface contamination on the disc-spacers is one of the main threats to the safe operation of gas insulated switchgear (GIS) insulation. The authors’ research has confirmed the phenomena of intermittent discharge of such defects under AC constant voltage and concluded that long-intermittent discharge may be the cause of sudden flashover failure of GIS. To explore the characteristics of the process from the partial discharge (PD) inception to the breakdown of surface discharge under constant AC voltage, a 126 kV GIS test platform was built and long-term uninterrupted detection was carried out in this study. By processing the discharge data of the entire process, distribution characteristics of phase-resolved PD and N–V spectra and the trend of discharge repetition rate, discharge amplitude (maximum, mean, standard deviation) and discharge time interval (maximum, mean, standard deviation) were obtained. The results indicate that: there are two typical stages in the entire process of surface discharge under constant voltage and both discharge characteristics and risk of breakdown of those two stages are different obviously. Furthermore, based on the electro-chemical aging theory and space charge effects, the long-intermittent characteristics of solid insulation discharge are explained.