Air Gap Discharge Research Articles

Study on the damage characteristics of high-temperature superconducting cable insulation under air gap discharge

This study explores the impact of small air gaps in high-temperature superconducting cables on the insulating material polypropylene-laminated paper (PPLP), and the aging rules and mechanisms of the insulating material during practical uses. An air gap discharge test platform was built to simulate air gap fault defects of superconducting cables in the real operating environment. Hierarchical clustering method was used to divide the gap discharge process of defect model into four stages. Insulation damage assessment was conducted on the intermediate layer PP of the superconducting insulation material PPLP at different discharge stages, revealing surface changes and periodic alterations in dielectric properties. The morphological features, roughness, infrared spectra, dielectric loss, surface resistivity, and other phase characteristics of the superconducting insulation layer material were analyzed at different stages of air gap defects. Molecular group cracking in PP was attributed to the bond breakage on the main chain. These findings provide insights into high-temperature superconducting cable insulation under air gap discharge and provide a guideline for practical applications in semi-conductive industries.

Development characteristics of rod-plate Gap streamer-leader system at high altitude areas

Abstract Air density is an important factors affecting the morphology characteristics of long air gap discharge channels. Analyzing characteristic parameters, such as the propagation velocity and angle of the streamer-leader system at high altitude areas is indispensable in studying the discharge mechanism of long gaps. In this paper, optical images and optical power of the discharge channel were captured. The propagation velocity of the streamer-leader system and angle distribution characteristics of the streamer region were obtained.In accordance with the findings, the downward velocity of the streamer-leader system at low pressure ranges from 1×104 to 5×104 m/s. Simultaneously, the velocity decreases slightly under the voltage with a low rise rate, and it is often accompanied by the phenomenon of leader re-luminescence. In addition, the optical images were processed by the pseudo-color algorithm, and the angle of the streamer region of the downward leader head was obtained between 50.3° and 71.6°. The angle of the streamer region is inversely correlated with the voltage rise rate but positively correlated with the gap distance. The results obtained in this paper have academic significance for improving the theoretical system of long gap discharge in high-altitude extreme environments.

Observation and analysis of positive leader re-illumination in a 10 m ultra-high voltage transmission line gap under switching impulse voltages

The leader propagation is one of the most important stages in long air gap discharge. The mechanism behind leader re-illumination remains unclear. In high humidity conditions (20.0–30.1 g/m³), we have conducted experiments of long sparks in a 10 m ultra-high voltage (UHV) transmission line gap under switching impulse voltages. The positive leaders predominantly propagate discontinuously, with almost no significantly continuous propagation occurring. The leader channels are intensely luminous and each elongation segment is straight, with streamers resembling the “branch type” which differs from the “diffuse type” streamers at the front of continuous propagation leaders. The distribution of the propagation velocities is highly random (3.7–18.4 cm/μs), and the average velocity (9.2 cm/μs) significantly exceeds that of continuous propagation (1.5–2.0 cm/μs). Analysis suggests that the current-velocity models suitable for continuous leader propagation do not align well with the experimental data in re-illumination mode. Based on the discharge current waveforms and optical images, it is speculated that the newly elongated leader in re-illumination mode does not evolve gradually from the stem (about 1 cm) but rather evolves overall from a thermal channel much longer than stem.

Investigation of Upward Leader Development Characteristics by Two Kinds of Charge Density Models in Wind Turbines

ABSTRACTThe upward discharge is a significant factor threatening the operation of wind farms. This paper investigates the inception and development of upward leaders of wind turbines' tip, where blade rotation prevents charge accumulation, using charge density models from laboratory discharges and artificially triggered lightning. The results indicate that larger return stroke currents create higher spatial potentials, and slower downward leader speeds allow for longer development time, favoring the development of upward leaders. As wind turbine sizes increase, the rotation of the blade tips resembles artificially triggered lightning, and the long air gap discharge model may underestimate the development of upward leaders. Calculations from the artificially triggered lightning model reveal improved upward leader formation when further laterally away from the downward leader within a certain distance, elucidating why multiple upward leaders are frequently observed around turbines. The striking distance and attractive radius are largely determined by the development of upward leaders. The absence of corona charge shielding at blade tips means upward leader inception is independent of downward leader velocity, emphasizing downward leaders' significant influence on turbines. Thus, the formula for estimating a wind turbine's lightning incidence requires integrating the return stroke current and velocity of the downward leader.

Ultra-high temporal resolution images of a homogeneous electric field short air gap negative streamer at overvoltage and atmospheric pressure

Air gap discharge is one of the most basic scientific problems in the field of high-voltage engineering. The homogeneous electric field 1.5 mm air gap negative streamer at overvoltage and atmospheric pressure is observed by a high-speed 4-channel framing camera. The ultra-high temporal resolution images of a single negative stream are captured (the exposure time is 5 ns, and the inter-frame delay is no more than 0.1 ns). It is observed that the negative streamer formed in the middle of the air gap and grew bidirectionally towards both electrodes. At the same time, the electrical measurement is also carried out.

Simulation of breakdown voltage associated with phase-to-phase air gap for ultra high voltage transmission lines under switching impulse voltage

Abstract Knowledge of the breakdown voltage of air gaps when subjected to positive switching impulse voltage is fundamental to the design of insulation coordination for power transmission systems. However, the length of phase-to-phase air gaps in ultra-high voltage power transmission lines has reached more than 30 m. It is very difficult, costly, and impossible to conduct full-scale test experiments to obtain the breakdown voltage of these gaps. Here, the numerical simulation method based on the self-consistent leader inception and propagation model was first used to simulate the air gap discharge process at four different scales and to obtain the gap breakdown voltage. The simulated data was compared with the measured data to verify the validity of the model. Then, the numerical simulation method was used to simulate the discharge process of phase-to-phase air gap for ultra-high voltage transmission lines with 30.3 m in length. The current and the development trajectory of the leader tip were obtained for the discharge process. More importantly, the breakdown voltage, which could not be obtained by full-scale testing, was obtained by simulation.

Propagation of nanosecond discharge in an air gap containing a water droplet: modelling and comparison with time-resolved images

The plasma-water interface is a complex medium characterized by interesting physical and chemical phenomena useful for many applications such as water processing or material synthesis. In this context, optimizing the transport of reactive species from plasma to water is crucial, and it may be achieved by increasing the surface-to-volume ratio of the processed object. Herein, we study the characteristics of a streamer produced by nanosecond discharge in air gap with a droplet of deionized water. The discharge is characterized experimentally by electrical measurements as well as by 1 ns-intergated ICCD images. To report plasma properties that are not accessible through experiment, such as the spatio-temporal evolution of electron density, electric field, and space charge density, a 2D fluid model is developed and adapted to the experimental geometry. Due to the fast propagation of the ionization front, the droplet is considered as a solid dielectric. The model solves Poisson’s equation as well as the drift-diffusion equation for electrons, positive ions, and negative ions. The utilized transport coefficients are tabulated as a function of the reduced electric field. Helmholtz equations are also included in the model to account for photoionization. The electron impact ionization source obtained from the model is compared to experimental 1 ns-integrated ICCD images, and a good agreement is observed. Finally, the model is used to investigate the influence of droplet dielectric permittivity and wetting angle (the angle between a liquid surface and a solid surface) on the properties of the discharge. Overall, the data reported herein demonstrate that the model can be used to investigate plasma properties under different conditions.

Atmospheric pressure uniform dielectric barrier discharge (DBD) in a wide air gap initiated from a narrow starting point

Generating a uniform non-equilibrium plasma in atmospheric pressure air has always been a challenge. It is believed that the maximum spacing for generating a uniform non-equilibrium plasma in atmospheric pressure air, whether using AC or nanosecond pulse drive, is 4 mm. Discharges are always non-uniform when the spacing is greater than 4 mm. In this paper, we propose a new type of dielectric barrier discharge structure to address this challenge. The left end of the structure rapidly increases the discharge spacing from 0.5 mm to 6 mm, while the right side of the main discharge gap maintains a uniform spacing of 6 mm. Nanosecond pulse voltage is used to drive the plasma, an ICCD camera is used to capture the image of the plasma during a discharge pulse cycle, which indicates that a uniform plasma within the 6 mm spacing of the main discharge gap is generated. Upon further reducing the ICCD camera’s exposure time to 20 ns, it is revealed that the uniform plasma is formed due to the rapid propagation of the plasma from left to right at a speed of order of 105 m s−1. Due to the small transverse component of the external electric field, this rapid propagation behavior cannot be due to the external electric field. Therefore, this paper further proposes the hypothesis of electric dipole formation leading to this fast propagation. The hypothesis suggests that the charge separation on the surface of the anode forms an electric dipole, which generates a local discharge at its right end. This local discharge further triggers the discharge in the main gap, and the main gap discharge, in turn, forms a dipole due to charge separation again, by repeating this cycle, the plasma propagates rapidly to the right. Further analysis demonstrates that this dipole can indeed produce a strong electric field of up to 41 kV cm−1 at its right end, which is sufficient to induce a local discharge. Moreover, under such a strong electric field, the electron migration rate can indeed reach 105 m s−1. These findings support the plausibility of this hypothesis.

Nanosecond photography observation of the discharge process across the 220 kV/500 kV insulators in lightning impulse

Long air gap discharge is the basis for the study of lightning protection and external insulation design of high voltage lines. However, there is a lack of detailed and comprehensive observation of the long air gap discharge process at the ns-level, as well as higher voltage insulator discharge observations. In this paper, the lightning impulse discharge/flashover tests are conducted on 220 kV glass insulators and 500 kV glass/composite insulators. Firstly, the U50% test is performed to determine the U50% value of each insulator under standard lightning impulse, and 1.1U50%, slightly higher than U50%, is applied to ensure the breakdown of the insulator. Subsequently, emICCD is utilized with a very short exposure time (minimum 10 ns) to obtain repeated single shots of discharge images under different delays, exposure times and light intensity gain. These images were spliced together based on time sequence to create a continuous discharge process. The discharge processes of 220 kV glass insulator and 500 kV glass/composite insulator are compared and analyzed. The results show that in the early stage of discharge, the discharge processes at the low voltage end of both 220 kV and 500 kV glass insulators are more intense. Additionally, the downward leader is faster than the upward leader in the discharge process of 500 kV composite insulators.

Temperature evolution characteristics of stem root during dark period in positive long spark discharge

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.

Research on DC arc fault detection in PV systems based on adjacent multi-segment spectral similarity and adaptive threshold model

This paper proposes a practical adaptive detection method of series DC arc that could be more adaptable to PV systems’ intricate and complex environment than conventional detecting methods. Firstly, The discharge in a short air gap with unsymmetrical dielectric-covered electrodes will bring additional noise interferences to the current of the circuit where the arc is located and give rise to the alternation of the spectrum of the noise current. By analyzing the current noise spectrum before and after the occurrence of a DC arc fault, it was found that the adjacent multi-segment spectral similarity (AMSSS) characteristic could identify the event of the mark. Secondly, an algorithm architecture using principal component analysis(PCA) of AMSSS and integrated φ-statistic was used to determine an adaptive threshold model. Finally, to validate the fidelity and resilience of the pattern, a photovoltaic plant platform containing 20 PV modules and one three-phase inverter was constructed to acquire relevant data. Then, validation of the adaptive thresholding model was implemented by including several sets of tests generated under different conditions, such as normal, shading, inverter start-up, crosstalk, and arcing conditions. According to the comparative tests, We tentatively conclude that the adaptive model has a relatively strong arc detection capability and high environmental adaptability and could be applied to natural PV systems.

Thermal dynamics of leader decay and reactivation in long air gap discharges

In this study, we present a comprehensive investigation of positive leader discharges, with the aim of enhancing our understanding of leader decay and reactivation. Our approach involved a detailed experimental and computational analysis of the phenomena. Specifically, we employed a time-resolved quantitative Schlieren platform, which provided us with high spatial resolution (60.0 μm pixel−1) and short exposure times (0.37 μs frame−1), allowing us to capture the 2D spatial–temporal evolution of gas temperature in positive leaders with a gap length of up to three meters. In addition, we employed a detailed thermal-hydrodynamic model coupled with a comprehensive kinetic scheme, consisting of 28 chemical species and 125 chemical reactions. Our simulations showed good agreement with the measured mean gas temperature and expansion rate of thermal radius. We conducted experiments under the same applied conditions to obtain both stable and decaying leaders. Our results show that once a positive leader starts to decay, the temperature drops below 3000 K. At the same time, both the electric field and conductivity decrease significantly compared to a stable leader. In addition, before the temperature drops below 2000 K and transforms into an aborted leader, a decaying leader might be reactivated.

Multi-perspective observation of streamer discharge morphology

As the initial discharge, streamer is the key to explore the long air gap discharge. At present, the experimental research on streamer discharge focus on observation from a single perspective. In order to understand streamer discharge more accurately, it is necessary to conduct photographic observation from multiple perspectives. For this reason, the dual-camera synchronous observation experiment platform was used in this paper to observe the positive streamer discharge in the rod-plate gap from two perspectives under the conditions of different voltage amplitude and electrode size. The morphological characteristics of the discharge from the two perspectives were basically the same. The length and diameter of the streamer discharge increase with the increase of voltage amplitude and the decrease of the curvature of the electrode. It is found that the length and diameter of the streamer measured in the left view are basically higher than those measured in the right view, reflecting the inaccuracy of the streamer discharge observed in the two-dimensional streamer image.

Ionization Activity Detected During Dark Periods in Long Air Positive Sparks

AbstractThe dark period in long air gap discharges has been assumed in the literature as the time between consecutive streamer current pulses when ionization and luminosity are absent. These dark periods are also present in natural lightning, in processes such as the inception and propagation of upward positive leaders, the development of needles, as well as transient luminous events in the upper atmosphere. Only recently, faint luminosity has been observed during dark periods, challenging this assumption. This paper aims to study any possible electrical activity during dark periods by means of experiments supported by computer simulation. Therefore, an experimental platform, including low‐current measurements, Schlieren and standard photography as well as ultraviolet (UV)‐photon detection was used to observe the electrical‐optical‐thermal characteristics of the dark period. A complementary numerical model was used to estimate the streamer space charge spatial distribution and its drift during dark periods. It is found that faint UV and visible light during the dark period is emitted exactly at the location of the low‐density streamer stem channel. This process is accompanied by the generation of an electronic current in the order of hundred microamps to milliamps. The simulation results show that this ionization activity occurs due to strong reduced electric fields in the residual stem channel above 112 Td, which is mainly determined by a combination of applied voltage, space charge distribution, and localized heating. Thus, the presented results show that there is indeed ionization activity during dark periods in long air gaps, which maintains a continuous glow‐like discharge.

Discharge Characteristics and Numerical Simulation of the Oil–Gas Surface under DC Voltage

Low insulation strength at the oil–gas surface due to oil leakage and partial discharge of oil-immersed power equipment is a major threat to the safe and reliable operation of power systems. This paper investigates the initiation and development of the oil–gas surface discharge. The oil–gas surface discharge test platform was established, and discharge tests were carried out at different gap distances (1–2.5 mm). By coupling the electric field and flow field, the multi-layer dielectric discharge streamer model was built, and the characteristics of charge and electric field distribution at different gap distances were studied. The test results show that the liquid surface between the electrodes rises during the discharge process. Furthermore, the surface discharge voltage exceeds the air gap discharge voltage. With the simulation analysis, the oil–gas surface discharge is a typical streamer development process. Under 50 kV applied voltage and 2.5 mm gap distance, the average development speed of the streamer is 12.5 km/s. The larger the gap distance is, the greater the average streamer development speed is. The recording and numerical simulation of the discharge process are of great significance for exploring the mechanism of oil–gas surface discharge, optimizing the discharge process, and diagnosing partial discharges.

Effects of rod radius and voltage on streamer discharge in a short air gap

Streamer discharge is the inaugural stage of gas discharge, and the average electron energy directly determines the electron collision reaction rate, which is a key parameter for studying streamer discharge. Therefore, taking into account the average electron energy, this work establishes a fluid chemical reaction model to simulate and study the course of evolution of a streamer discharge in a 5 mm rod–plate gap, considering 12 particles and 27 chemical reactions. It introduces the electron energy drift diffusion equation into the control equation, and analyzes the temporal and spatial changes of average electron energy, electric field intensity and electron density with change in rod radius and voltage. The effects of voltage and rod radius on the course of streamer discharge can be reflected more comprehensively by combining the average electron energies. Three different values of 0.3 mm, 0.4 mm and 0.5 mm are set for the rod radius, and three different values of 5 kV, 6 kV and 7 kV are set for the voltage. The influence of an excitation reaction on the streamer discharge is studied. The findings indicate that, as voltage raises, the streamer head’s electron density, electric field and average electron energy all rise, and the streamer develops more quickly. When the rod radius increases, the electron density, electric field and average electron energy of the streamer head all decrease, and the streamer’s evolution slows down. When an excitation reaction is added to the model, the average electron energy, the magnitude of the electric field and the density of electrons decrease, and the evolution of the streamer slows down. An increase in average electron energy will lead to an increase in electric field strength and electron density, and the development of the streamer will be faster.

The contribution of femtosecond laser filaments to positive and negative breakdown discharge in a long air gap

Discharges of different polarities develop in different modes, resulting in different guiding effects by femtosecond laser filaments. Knowledge of the contribution of laser filaments to positive and negative discharges is the basis of the laser-guided long-air-gap discharge technique. This study presents a direct comparison of the inception, propagation, and breakdown characteristics of discharges of both polarities. Long-air-gap discharge experiments under the switching impulses of both polarities are carried out under the same experimental conditions. Discharge modes and phases are also considered. The statistical results show that positive discharge inception voltages are transformed from a Weibull distribution into an exponential distribution under the influence of laser filaments, but there is little effect on the negative discharge inception voltage. The guidance probability of a positive discharge reaches 15% at most during the dark period stage, leading to little effect on the breakdown discharge probability. However, for negative discharges, the guidance probability can exceed 95%. An investigation of the filament contributions to both polarity discharges shows that the different migration directions of photoelectrons lead to a difference in the effects of laser filaments on inception voltages, and the difference in the connection of the two discharge passages leads to a difference in the guidance probability. Through the results of a simulation model, it is speculated that the connection condition for positive discharges is that the positive leader overlaps with the laser filaments, and, for negative discharges, the rod electrode is connected to the laser filaments through bi-directional discharge propagation.

Data used in the manuscript of Thermal dynamics of leader decay and reactivation in long air gap discharges

Data used in the manuscript of Thermal dynamics of leader decay and reactivation in long air gap discharges

Experimental Platform for Measuring Electrical Discharges in Long Air Gaps With Floating Objects and Its Applications Under Lightning Impulse

Long air gaps containing floating objects are common gaps in power transmission and substation projects. Investigating the discharge mechanism of gaps helps improve the power grid operational reliability. This article presents an experimental platform for measuring the electrical discharges in long air gaps with floating objects, which can measure the applied voltage and the discharge current, and capture the images of the discharge process. By analyzing these measured data, the breakdown voltage characteristics of the gap can be explained. In a long air gap with a floating conductive object under lightning impulse, as the floating object moves from the high voltage (HV) electrode to the ground potential electrode, the breakdown voltage first decreases and then increases, producing a minimum breakdown voltage area (MBVA). In this article, the experimental platform was used to explain the formation of MBVA as an application example. The results show that the formation of MBVA is related to the timing relationship between the breakdown of sub-gap 1 and the discharge in sub-gap 2 entering the final jump. The minimum breakdown voltage occurs at the shortest sub-gap 1 ensuring that the discharge in sub-gap 2 enters the final jump just as sub-gap 1 is broken down.

Study on the Electron Density Measurement of 1 m Rod-plate Gap Discharge Process under the Lightning Impulse

Observation and measurement of the microscopic parameters in the long air gap discharge process play an important role in studying its physical mechanism and the discharge simulation model. In this paper, we combined an electron-multiplying intensified charge-coupled device (EMICCD) camera and a spectrometer, and built a lightning discharge spectrum observation platform of spatiotemporal distribution for the 1 m rod-plane air gap with nanosecond exposure time. The spatiotemporal distributions of the spectral information of the 1 m air gap discharge under positive and negative lightning impulses were obtained. Based on the atomic emission spectroscopy diagnostic method, N and O atomic spectral lines were used to calculate the corresponding electron density at different positions and time points during the breakdown discharges. The testing results were compared with other literature and the differences also were discussed.