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Gas temperature plays an important role in the characterization of plasma parameters. The temperature evolution of stem roots during the dark period is at the heart of our understanding of the physical mechanism of streamer to the leader transition process in a long spark discharge. The quantitative schlieren system with high spatiotemporal resolution was designed to conduct positive leader discharge experiments with a 1.0 m rod-plate gap at atmospheric pressure, and the amplitude and rise time of positive impulse voltage waveforms were 380 kV and 200 μs, respectively. The time-resolved quantitative schlieren images of the discharge channel near the electrode tip can be captured to gather the temperature data of stem roots for case study. Further statistical tests revealed that due to the dispersion of the first streamer discharge, there were three evolutionary trends of gas temperature at the axis of the stem root in the early dark period. Interestingly, the gas temperature at the axis of the stem root was maintained at 1000–1200 K in regardless of the evolutionary trend in the early dark period. In addition, the statistical results indicate that there is a significant positive correlation between the curvature radius of the discharge electrode tip and the critical charge value Qcrit of the first streamer discharge, which leads to a transition in evolutionary trends of gas temperature at the axis of the stem root. This research has important guiding significance for understanding the physical mechanism of leader inception and the whole process modeling of long air gap discharge. [ABSTRACT FROM AUTHOR]

An interpretable convolutional neural network model is proposed for the classification of very low frequency and low frequency lightning electric field waveforms. This model adopts multi‐scale convolutional kernels and shortcut connections to enhance the ability of lightning waveform classification. Based on the data recorded from five provinces in China, the proposed model achieves an accuracy of 98.56% for a four‐type classification task including return strokes, the intra‐cloud lightning, preliminary breakdown, and narrow bipolar events. The proposed model is validated with another open‐source data set from Argentina with an accuracy of 98.45%, which shows good robustness. To ensure the classification, the features learned by the model are visualized. The class activation mapping (CAM) method is adopted to visualize the class‐specific contribution of different waveform parts by using the feature maps of the final convolutional layer. It is highlighted by the CAM method that the proposed model focuses on waveform parts that align with those areas of interests identified by human experts. The high‐contribution waveform parts are furtherly analyzed, which indicate that the proposed model possesses the capability to associate waveform features with the corresponding lightning discharge processes. Plain Language Summary: Electromagnetic waveforms in very low frequency and low frequency bands are usually used to detect and locate lightning activities. Traditional waveform classification methods have difficulties in distinguishing multiple types of lightning waveforms. Although machine learning models have great potential in multi‐type waveform classification tasks, these models rely on the features proposed by human experts and cannot capture the features of different scales in lightning waveforms. To this end, this paper proposes an improved convolution neural network model, which incorporates modifications to the model structure to better suit the lightning waveform classification task. The data set for model training comes from five provinces in China and contains different meteorological conditions. The proposed model achieves a classification accuracy of 98.56% on this data set and 98.45% on an open‐source data set from Argentina. Meanwhile, the classification process is interpretable by visualizing the convolution outputs. The analysis of the visualization results shows that the high performance of the proposed model is reliable for its ability to focus on waveform parts that align with areas of interests identified by human experts. A closer inspection of these waveform parts suggest that the proposed model possesses the capability to associate waveform features with the corresponding lightning discharge processes. Key Points: The proposed model achieves an accuracy of 98.56% for a four‐type lightning waveform classification task and shows good robustnessThe model is interpretable by visualizing the contribution of different waveform parts to the classification resultsThe proposed model aligns with human‐expert classifications and suggests potential to link waveform features with lightning processes [ABSTRACT FROM AUTHOR]

This paper presents an original investigation into the axial evolution of streamer stem during a dark period in long spark discharge. To obtain thermodynamic morphology and temperature distribution of stems, we set up a quantitative schlieren system with the temporal and spatial resolutions of 0.37 μs and 31 μm/pixel, respectively. The quantitative schlieren observation experiments of positive leader discharge with a 1.0 m rod-plate gap were carried out, and the time-resolved quantitative schlieren images were captured. Furthermore, the temperature distribution of stems and its morphology evolution in the axial direction during a dark period were obtained. Due to the dispersion of first streamer discharge, the gas temperature in stem roots shows two evolutionary trends, namely, rising and falling. It was found that the gas temperature in stem decreased along the axis with the increase in the distance from stem root, and the gas temperature of a thermal thin channel was between 400 and 800 K. There is a significant dependency between axial development parameters of thermal thin channels and the first streamer discharge parameters. The phenomenon of channel abrupt elongation triggered by secondary streamer discharge was observed by the schlieren system, and the influence of characteristic parameters on the inception of secondary streamer was statistically analyzed. The ion current waveform in leader relaxation phase was measured, and it is clarified that the generation mechanism of thermal thin channels is due to the energy transfer between positive ions and neutral particles, which finally leads to the increase in gas temperature in the channels. [ABSTRACT FROM AUTHOR]

With the rapid development of semiconductor devices, researchers have proposed different types of high-frequency pulse voltage generators based on power electronic converters to meet the requirements of microorganism inactivation, water pollution purification, and piezoelectric and dielectric electroactive polymer drives. The transient modeling of these high-voltage converters aids in understanding the electrical characteristics of the devices and optimizing their performances. This article analyzes the electrical behavior of a semiconductor switching device (i.e., an insulated gate bipolar transistor) used during the switching transient of the converter and constructs an electromagnetic transient model of the flyback converter that considers both the parasitic elements and the ferromagnetic losses of the transformer. The accuracy of the proposed model is then verified by comparing calculation results obtained using the model with experimental measurement results. [ABSTRACT FROM AUTHOR]

Lightning puncture damage on the wind turbine blades can pose a severe threat to the safe operation of large-capacity wind turbines. We investigated its formation by performing long air gap discharge experiments under positive and negative downward leaders. Upward streamers were observed to emit from the sample blade before its electrical breakdown. To explain this phenomenon, a numerical model considering the streamer discharge in air, charge transport in GFRP laminate and surface charge accumulation on air-solid interfaces was established. It is found that streamer discharges can be triggered inside the blade hollow to generate space and surface charges, which strengthen the electric field on the outer blade surface and contribute to the upward streamer initiation. The upward streamer development leads to the accumulation of surface charges with opposite polarities on the two sides of GFRP laminate, which causes the electric field on the GFRP laminate to exceed its electrical breakdown threshold. The electrical breakdown, which occurs before the thermal effect of lightning current appears, is the initial stage of lightning puncture damage of blades. The electrical discharge inside the blade hollow and the surface charge accumulation on the GFRP blade should be suppressed to prevent the puncture damage of blades. [ABSTRACT FROM AUTHOR]

We provide the clearest experimental evidence to date that positive leader velocity decreases with the decline of air density in the troposphere. The experiments are conducted at altitudes of 4,300 m in Tibet and 53 m in Beijing using the long air gap discharge technique. Rigorous comparative studies are performed using synchronized measurement of positive leader velocity and current under different air densities and similar temperature and humidity conditions. The effect of reduced air density on positive leader velocity is analyzed with a detailed thermal‐hydrodynamic model. It is found that the widely used similarity law setting initial leader radius inversely proportional to air density does not hold in estimating positive leader speeds in the troposphere. Plain Language Summary: The effect of reduced air density on leader discharge is a fundamental concern in several subjects such as lightning physics, atmospheric electricity, and electrical engineering. Increasing observation results have shown that lightning leaders seem to propagate slower at higher altitudes. To rigorously investigate the effect of reduced air density on positive leader velocity, electrical discharge experiments were performed at an altitude of 4,300 m in Tibet under the basis of excluding other influencing factors, such as discharge current and other atmospheric conditions. By comparing measurements near sea level and at high altitudes under similar temperature and humidity conditions, we provide reliable evidence to confirm that the positive leader velocity decreases with the decline of air density. Based on a thermal‐hydrodynamic model, it is found that the leader velocity calculated with the use of similarity law deviates significantly from the experimental measurements. A revised scaling formula for the initial conditions of leader stem at reduced air density is proposed. Our work can motivate scientists to revisit the use of similarity law in analyzing lightning discharges and upper‐atmospheric discharges such as gigantic jets. Key Points: We report a rigorous experimental study confirming that positive leaders propagate slower at higher altitudesBased on an improved thermal‐hydrodynamic model, the effect of air density on leader velocity is discussedOur study indicates that the similarity law for leader radius needs to be revised for electrical discharges in the troposphere [ABSTRACT FROM AUTHOR]

SF 6 /N 2 mixtures is considered as a practical alternative proposal of SF 6 in GIS and GIL. Simulating particle transportations and chemical reactions during the positive streamer propagation in SF 6 /N 2 mixtures assists in the insulation performance optimization of SF 6 /N 2 mixtures. This paper presents a fluid dynamic model coupled with the chemical reaction system for the positive streamer propagation in SF 6 /N 2 mixtures. The Townsend coefficients of the 20%SF 6 /80%N 2 mixture derived from the proposed chemical reaction system are in accordance with the measurement results. Based on this model, the propagation of the positive streamer in a 1 mm needle-plate gap in the 20%SF 6 /80%N 2 mixture is simulated. Simulated results indicate that there exists a negative space charge region in the streamer channel. It can be attributed to the increasing attachment rates of SF 6 yielding SF 5 − and SF 6 −, and the reduction of the ionization rates of N 2 and SF 6 in the streamer channel. Besides, it is noted that the ionization activities of N 2 and SF 6 at the streamer head are equally important to support the continuous propagation of the positive streamer. The ionization of N 2 in the streamer channel serves as an important factor to supply electrons and avoid the accumulation of the negative space charge. [ABSTRACT FROM AUTHOR]

Electrical discharges in long air gaps, referred to as 'leader discharges', usually propagate with transient thermal plasma channels. The gas temperature evolution of the channels plays a key role in the inception and development of leader discharges. This paper is aimed at investigating the use of quantitative Schlieren techniques in temperature measurement of the leader discharge channels. To do this, a calibrated Toepler's lens-type Schlieren system was set up to measure the radial temperature profile of leader discharge channels. The measurements were compared with the simulation results from a detailed numerical model which describes the thermal-hydrodynamic properties of the channel. The comparison shows that although a good agreement is obtained when the leader is propagating in a stable manner, the measurements of the temperature evolution at the axis of leader channel deviate significantly from the numerical simulation during the leader initiation stage. Apart from the limited spatial and temporal resolution of the Schlieren system, the deviation is also caused by the non-isobaric gas heating during leader inception. Based on the theoretical analysis and numerical simulation, we proposed several suggestions to the application of quantitative Schlieren techniques to measure the temperature of leader channels. [ABSTRACT FROM AUTHOR]

The unified power flow controller (UPFC) is the third generation of flexible AC transmission systems. It can control the power flow and voltage of the power system quickly and flexibly that has an advantage in compensating the AC transmission system in real time. The world's first 500 kV UPFC project is under construction in Jiangsu Province of China. On the paralleled converter side, it is connected to the 500 kV Mudu substation by using a gas insulated transmission line, and the series converter side is connected to the 500 kV Meili substation through a 500 kV double-circuit overhead transmission line. In order to optimise insulation design of the UPFC system and ensure the safety operation of the power system, it is essential to analyse the characteristics of lightning overvoltage. Considering the invaded path of lightning surges in the UPFC system, a detailed model was established by using Electro-Magnetic Transient Program/Alternative Transients Program in this study. The lightning overvoltage level at key nodes of the UPFC system was presented. The check calculation for the configuration and protection level of ZnO arresters were also carried out. The obtained results can provide a reference for insulation coordination design of the 500 kV UPFC project. [ABSTRACT FROM AUTHOR]

The ±1100 kV Zhundong–Wannan ultra high-voltage direct current (UHVDC) transmission project is under construction in China, which has the highest operating voltage in the world. The length of the UHVDC overhead transmission line is greater than 3000 km. The electromagnetic transient process caused by grounding faults might have significantly impact on the overvoltage level of DC transmission line, which is crucial for the external insulation configuration of the overhead transmission line. This paper presents a detailed electromagnetic transient simulation model for the UHVDC transmission system using an open-loop extinction angle control principle. The simulation results demonstrate that the single pole grounding fault into a bipolar system causes the most serious damage. The maximum overvoltage is 1.628 p.u. and the 2% statistic overvoltage is 1.635 p.u., while the reference value is 1122 kV. The influence of DC filters for line overvoltage is also analysed here. Furthermore, results show that adding a no gap line arrester at the midpoint of transmission line can suppress line overvoltage, while the maximum overvoltage reduces by 0.138 p.u. The results obtained provides a reference for the optimisation of the external insulation configuration. [ABSTRACT FROM AUTHOR]

Sliding electrical contact between rail and armature in rail launchers is characterized by high speed and large current. Melt erosion is caused by the current concentration on the contact surface of armature. Because the current melt erosion (CME) was considered to be a main mechanism of armature-rail contact failure, it was experimentally and theoretically studied in the early years using current density of 30–40 kA/mm, which is the exact driving current of rail launches. However, the critical behavior at the onset of CME cannot be observed due to the serious melt erosion at such high current densities. In this paper, the CME of armatures has been experimentally studied in the current range of 10–20 kA/mm with a lab-scale rail launcher. A payload separated method was used to keep the recovered armatures intact. The critical process for the onset of melting was observed and the erosion spreading patterns on the contact surface was analyzed. It is found that the current melt-wave model postulated in early years cannot describe the development of CME in our experiments. The result shows that current erosion mostly occurs at the static and the low-velocity stage of armature. The CME begins at the point of maximum contact pressure provided by armature-rail interference fit, and then, the melt erosion spreads longitudinally and transversely. The current erosion is affected by both current distribution and the movement of liquid aluminum. In longitudinal direction, the flow of liquid aluminum results in erosion propagation to leading edges. The transverse width of erosion zone expands with increasing current magnitude along the edge of interference fit. [ABSTRACT FROM PUBLISHER]

Monolithic C-type armature is one of the most important armatures in electromagnetic railgun. Some key parameters of monolithic C-type armature in rectangular caliber railgun system—width of armature X, height of armature Y, thickness of armature’s throat d0 , length of armature’s tail d1 , thickness of trailing edge of armature’s tail d2 , and angle of armature’s tail $\theta $ —are studied in this paper. By introducing current carrier margin coefficient named $\alpha $ , ratio of armature thickness to armature width named $\beta $ , ratio of armature height to armature width named $\gamma $ , and ratio of length of armature’s tail to armature height named $\kappa $ , equations of some key parameters of monolithic C-type armature in rectangular caliber railgun system are proposed. Based on analysis of mass of armature, electromagnetic contact force between armature and rail, mechanical strength of armature’s tail, and inductance gradient, relationship of some key parameters of monolithic C-type armature in rectangular caliber railgun system are proposed. Conclusion can be drawn that increase of $\theta $ can reduce mass of armature and improve mechanical strength of armature’s tail, but electromagnetic contact pressure between armature and rail will be decreased. Decrease of $\gamma $ could be a considerable way to remarkably increase inductance gradient, and then increase launch system efficiency. [ABSTRACT FROM PUBLISHER]

By introducing current carrier margin coefficient named $\alpha $ , ratio of armature thickness to armature width named $\beta $ , and ratio of armature height to armature width named $\gamma $ , equations of some key parameters of rectangular caliber railgun system, which is related to linear current density, are proposed. Then, some key parameters for different caliber railgun system are studied. Based on the analysis of electromagnetic contact pressure between armature and rail, $+ di/dt$ skin effect, velocity skin effect, and melt wear of armature, it can be obtained that current density of rail/armature interface is directly proportional to the linear current density. Thus, in order to decrease rail/armature interface current density, it is necessary to reduce linear current density. It can also be obtained that current density of rail/armature interface in current ramp-up is inversely proportional to the root of current rise time. Thus, it is necessary to increase current rise time to decrease rail/armature interface current density. Average melt wear rate of armature is directly proportional to the square of linear current density. [ABSTRACT FROM PUBLISHER]

Experiments on leader development were carried out in 10 m and 6 m rod-plane gaps under positive impulse. Leader propagation was recorded by high speed CCD camera. Effect of branching on its channel tortuosity was studied based on these high speed photographs. Over 70% of turning points along the 2-D leader channel are accompanied with leader branching in the experiment. That is branching makes an important contribution to the channel tortuosity of positive leader. [ABSTRACT FROM AUTHOR]

Positive leader current spikes were observed in long air gap discharge. However, there has been no explanation on this phenomenon so far, and some leader advancement models can only produce the current without spikes. Therefore, it is necessary to figure out this phenomenon in order to deepen the understanding of leader discharge. In this paper, experiments of leader development were carried out in a 10 m rod-plane gap under positive switching impulse. The discharge current and high speed photographs of discharge process were recorded synchronously. Then the high speed photographs during the period when the leader current fluctuated were carefully analyzed. The experimental results showed that the leader current spikes were with leader branching. Finally, the effect of branching on the spikes of positive current was explained qualitatively and a hypothesis on leader branching was proposed. [ABSTRACT FROM AUTHOR]

In this paper, the effect of the corona space charge on the leader initiation is experimentally and theoretically investigated. The critical corona charge for the stem–leader transition is reviewed and a more proper charge criterion is recommended based on the experimental measurements and theoretical analysis. It is shown that the chocking effect of the corona space charge for the second corona inception should not be ignored, which decreases the electric field strength of the stem tip and hinders the initiation of the leader. A simple physical model is proposed to calculate the leader initiation time, which can consider the thermodynamic mechanism for stem–leader transition and the chocking effect of the corona space charge, and a good agreement with the experimental results was obtained. [ABSTRACT FROM PUBLISHER]

In this paper, the variation of the development of positive first corona space charge in long air gaps is investigated. A 1-m rod-plane air gap under different rising rates dU/dt of impulse voltages is reported. The current signals and still photographs are recorded during the first corona development. The relationship between the first corona charge Qi and the inception voltage Ui is presented. The physical mechanism of the influence of the first corona is provided. With the experiments and the single branch propagation model, a semi-empirical calcultion method for positive first corona charge under impulse in long air gaps is obtained through the calculation of the number of branches Nb. Using this method, the variations between Qi and Ui for different dU/dt and electric field distributions are calculated and compared with the reported experimental results. Reasonable agreement between the calculated and experimental results is obtained. It shows that this calculation method is suitable for predicting the positive first corona charge Qi and the discharge current at a given dU/dt and Ui. [ABSTRACT FROM PUBLISHER]

Research on the attachment processes in competition tests can provide insights into the influencing factors involved in the selection process that determines the lightning strike point. In this paper, a series of competition tests with a rod–rod gap configuration are designed and carried out. The test results indicate that the strike mechanism is decided by two discharge processes, namely the free development of the downward-propagating discharge and the interception process of the upward and downward discharges. If the first process prevails, the strike probabilities for different grounded rods will tend to be equal. For the second process, the strike probabilities reflect the different performances of the grounded rods for intercepting downward discharges. As the scale of the discharge gap increases, the development of the downward leader becomes more and more dominant in selecting the strike point. In natural lightning strikes, the scale of the downward leader is normally more than 90% of the discharge gap, hence the protective effect of lightning rods with different tips will not be significant for low rod heights and small lightning currents. To reflect the effect of the downward leader development on the choice of the strike point, a minimum discharge air gap of 10 m is suggested. [ABSTRACT FROM PUBLISHER]

In order to investigate the different features of the positive leader propagation in the rod–plane and inverted rod–plane gaps, the discharge experiments in a 3-m rod–plane gap and a 3-m inverted rod–plane gap were carried out under switching impulse voltage. The positive leader propagation process was recorded by using a high-speed CCD camera. Some aspects to describe the positive leader propagation characteristics of the rod–plane and inverted rod–plane gaps are obtained and compared in this paper, including the critical corona radius, the average continuous leader inception voltage, the axial leader velocity, and the final jump length. The possible reason resulting in those differences is also discussed here. [ABSTRACT FROM PUBLISHER]

A novel observation method is proposed to study the streamer-leader propagation processes of long air-gap positive discharges. Experiments of 3-12 m air-gap discharges have been conducted in this paper. The observed images could clearly and particularly describe the characteristics of the positive streamer-leader propagation processes. Some important phenomena of the propagation processes are indicated and discussed, such as the leader's velocity that varied with the time, the different lengths of inception leaders, the leader's branching phenomena, the continuous streamer-leader propagation processes simultaneously with the voltage's descent, and the upward leader's propagation processes. [ABSTRACT FROM AUTHOR]

Investigation of positive streamer-leader propagation under slow front impulse voltages can play an important role in the quantitative research of positive upward lightning. In this work, we performed a large-scale investigation into leader development in a 10-m rod–plane gap under a long front positive impulse. To describe the leader propagation under slow front impulse voltages, we recorded the leader propagation with a high-speed charge coupled device (CCD) camera. It is important to figure out this phenomenon to deepen our understanding of leader discharge. The observation results showed that the leader mechanism is a very complex physical phenomenon; it could be categorized into two types of leader process, namely, continuous and the discontinuous leader streamer-leader propagation. Furthermore, we studied the continuous leader development parameters, including two-dimensional (2-D) leader length, injected charge, and final jump stage, as well as leader velocity for rod–plane configuration. We observed that the discontinuous leader makes an important contribution to the appearance of channel re-illuminations of the positive leader. To clarify the above doubts under long front cases, we carried out extensive experiments in this study. The comparative study shows better results in terms of standard switch impulse and long front positive impulse. Finally, the results are presented with a view toward improving our understanding of propagation mechanisms related to restrike phenomena, which are rarely reported. [ABSTRACT FROM AUTHOR]

The main form of electrical discharge in long air gaps is broken down by leader mechanism. The leader development in rod-rod gaps under positive switching impulse was investigated by using two high-speed CCD cameras. The clearer and more particular colorized physical morphology of the leader propagation process than the traditional image converter cameras was recorded and the leader development parameters were also obtained. As a comparative study, an improved simplified model for rod-rod configuration was performed to simulate the leader development. The numerical simulations of leader development characteristics were in a close agreement with that of experimental measurements. [ABSTRACT FROM PUBLISHER]

The leader development in rod–rod gap under negative switching impulse was investigated by using two high-speed cameras. The clearer and more particular colorized physical morphology of the streamer-leader system than the traditional image converter cameras were recorded. On the basis of high time resolution images, the leader development parameters such as negative leader step length, final jump length and positive upward leader velocity and so on for rod–rod configuration are reported in the paper. [ABSTRACT FROM AUTHOR]