Air Gap Discharge Research Articles

Effect of droplet deformation on discharge at icicle tip of ice-covered insulators during melting period

• The drop of melting water affects the gap discharge of ice-covered insulator. • The electric field of the gap varies with the deformation of the water droplets. • The effect of droplet deformation on gap discharge is related to icicle length. Existing studies show that icicle tip discharge of insulators is related to overhanging melting water droplets. However, the effect of dynamic deformation of droplets on the icicle tip discharge has not been studied. In this paper, the experiments of dynamic deformation of melting water droplets and discharge during melting period of composite insulators with a spacing of 70mm for big umbrella skirts are carried out. Furthermore, a fluid-electric field coupling simulation model under different icicle lengths corresponding to the experiment was established . The results show that the dynamic deformation of the melting water droplet increases the intensity of the air gap electric field and affects the discharge at the air gap. When the icicle length is 50 mm, the electric field intensity of the gap during the droplet breaking increases by more than 170%, and the air gap discharges at this time. When the icicle length increases to 60 mm, the length of the air gap decreases with the droplet deformation, and discharge occurs before the droplet is broken. When the icicle is almost bridged, the discharge phenomenon will be observed all the time at the air gap.

Dynamic development model for long gap discharge streamer-leader system based on fractal theory

Establishing a long air gap discharge model considering the streamer-leader transition and randomness of the discharge path is of great significance to improve the accuracy of discharge characteristic prediction and optimize external insulation design. Based on fractal theory and thermal ionization theory of streamer-leader transition, this work establishes a dynamic development model for the long air gap discharge streamer-leader system, which includes streamer inception, streamer development, leader inception, development of streamer-leader system and final jump. The positive discharge process of a 3 m rod plate is simulated to obtain the fractal distribution of the discharge path and the law of leader development for comparison with the discharge test results. The results show that the simulation model is similar to test results in the development characteristics of leader path distribution, each stage time and leader velocity. Finally, a simulation calculation of a 50% breakdown voltage of the rod-plate gap and ball-plate gap is carried out, with results fairly consistent with test data, proving the effectiveness and practicality of the model.

Schlieren techniques for observations of long positive sparks: Review and application

Understanding the mechanism of positive leader discharge is important in lightning protection engineering and the external insulation design in high voltage power transmission systems. During the propagation of a positive leader, some processes without light-emitting, for example, the insulation recovery process after the breakdown, cannot be observed by optical photography techniques. With the combination of the digital high-speed imaging system, the conventional Schlieren techniques offer new vistas in the long air gap discharge observation. The important features of high spatial resolution, high sensitivity, and easy arrangement make Schlieren techniques powerful and effective tools for characterising the discharge processes without light-emitting. This work presents a brief review of current Schlieren techniques and discusses the design of the Schlieren optics system for long spark observations. Several interesting phenomena discovered at different phases of long sparks with high-speed Schlieren techniques are also presented.

Distribution characteristics for initial time delay of streamer of shielding spheres under operating impulse voltage at high altitudes

With continuous promotion of construction of ultra high voltage DC projects in high-altitude areas, it is of important theoretical and practical engineering significance to study long gap discharge characteristics at high altitudes. As the initial and key stage for development of long air gap discharge, the initial stage of streamer reflects the randomness of long gap discharge to a certain extent, and time delay characteristics are important indicators to reflect the randomness. In this paper, we build a photoelectric detection system based on the photomultiplier tube and photoelectric integrated sensor, carry out an operating impulse discharge test of long gap of the shielding sphere-plate with different diameters at altitudes of 50 m and 2200 m respectively, obtain the space field strength, optical power, discharge current and voltage of the whole process of discharge development, and analyze the characteristics of boost time delay and statistical time delay of streamer based on the photoionization model and Weibull Distribution model. According to study results, the average value and dispersibility of initial time delay of streamer increase with the increase of altitude and electrode diameter; the boost time delay t 0 of streamer decreases with increase of altitude and increases with increase of electrode size; the statistical time delay t s of streamer can properly comply with Weibull distribution, and its characteristic parameter has a clear physical meaning. The dispersibility of statistical time delay will increase with altitude and electrode size, and compared with electrode size, altitude has a greater impact on the dispersibility of statistical time delay. The study results of this paper have important guiding value for selection and optimization on clear distance of the valve hall of a converter station in high altitude areas.

Positive corona streamer interaction with metalized dielectric: Possible mechanism of cathode destruction

This paper examines the effect of pulsed positive point-to-plane corona discharge in millimeter air gaps on the surface of a metalized dielectric. A footprint method was applied to reveal the streamer–surface interaction with Al and Zn thin films (20–50 nm) as a sensitive indicator. A thin metal film-dielectric substrate system was destructed at relatively low typical average currents of 20–50 μA during exposure times of 2–200 s. Destruction occurred in local zones with a size of several μm2 per one discharge pulse, which is substantially lower than the conventional streamer size of several tens of micrometers. An offered model of electro-thermal heating of the cathode layer shows that the dielectric surface temperature can achieve 1000 K and more during the single current pulse of submicrosecond duration. The indicated mechanism is possibly responsible for the effects of the discharge plasma interaction with low heat conductivity cathodes, including biological objects. Intensive heating of the cathode layer should be considered when modeling the streamer–cathode interaction.

Improved Electrogeometric Model of UHV Transmission Line Based on Long Gap Discharge and Its Application

Based on long air gap discharge test data and lightning return stroke observation data, an improved electrogeometric model (EGM) considering terrain conditions is established and verified to analyze the lightning shielding performance of UHV transmission lines. The striking distance formula is modified as rs = 0.13 (I2 + 40I)0.814. In this paper, the lightning shielding failure rate of three-phase conductors of EHV and UHV transmission lines calculated by the improved EGM model is consistent with the lightning observation data of actual transmission lines in Japan and the scaled lightning discharge simulation experimental results of UHV transmission lines in plains and mountainous areas of China, which verifies the applicability of the improved EGM model to evaluate the lightning shielding performance of large-scale UHV transmission lines. The improved EGM model is applied to evaluate the influence of tower type and slope steepness on the shielding failure tripping rate of UHV transmission lines. The shielding failure tripping rate of the SZ322 tower is higher than that of the SZT1 tower, and the shielding failure tripping rate of UHV transmission lines is greatly affected by slope gradient and increases with the increase of slope gradient.

Medium Voltage Converter Inductor Insulation Design Considering Grid Requirements

Medium voltage (MV) SiC devices facilitate direct (without 50-/60-Hz transformer) connection of the power electronics converter to the MV grid using simple topology. Compared to the insulation design of MV inductors that have been widely used in the power system, the insulation design of the MV converter filter inductors has new challenges. Particularly, the grid impact on insulation requirements on converter filter inductors is rarely discussed. This article introduces the insulation design of a grid-side filter inductor for a 13.8-kV power conditioning system converter. The grid insulation requirements, including the short-duration power frequency overvoltage and the lightning impulse voltage, are considered in the design. To meet the requirements, the layer-to-layer insulation, winding-to-core/ground insulation, the air gap discharge between the winding and the core, the local electric field, as well as the interturn transient voltage distribution are considered. The designed inductors are validated by the 46 kV (>1 min) dc hi-pot test, 12-kV ac partial discharge test, one-hour single-phase converter full rating test, as well as three-phase converter full rating test.

Numerical analysis of temperature characterization of air gap discharge in solid insulation under power frequency voltage

Air gap defects in solid insulation of power equipment can cause partial discharge. Temperature, as an important characterization of partial discharge, can be used as a criterion for the deterioration degree of air gap defects to some extent. In this paper, the temperature characterization of air gap discharge is studied. A plasma-kinetics-based dielectric barrier discharge model is constructed. The calculation method of air gap discharge heat source under power frequency voltage is presented. Moreover, the influence of voltage amplitude and gap thickness on temperature is studied. Obtained results indicate that under the power frequency voltage, Joule (Ohmic) heat contributes 90% to the heat source, and O4+ and O2+ and electron e are the main contributors to Joule (Ohmic) heat. O4+ is also the dominant ion in the mixed gas. As the discharge progressed, O4+ almost all clustered near the instantaneous cathode. The O2+ density generally depends on the electron density, but compared with the electron, the spatial distribution of O2+ density is slightly shifted to the instantaneous cathode. The discharge heat source is positively correlated with the air gap thickness and the voltage amplitude. With the increase in air gap thickness, the rising rate of heat sources will also increase. However, when the voltage is 10 kV, the heat source increases first and then decreases with the increase in the air gap thickness. The discharge does not occur when the air gap thickness reaches above 4 mm. This study provides theoretical support for identifying the deterioration degree of air gap defects from the view of temperature.

Positive Leaders Propagate Slower at Higher Altitudes: Experimental Evidence and Theoretical Explanation

AbstractWe 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.

Effect of Environmental Parameters on Streamer Discharge in Short Air Gap between Rod and Plate

Environmental meteorological parameters (temperature, humidity, air pressure, etc.) have an important impact on short gap streamer discharge. Based on the streamer theory and particle continuity equation of gas discharge, a short gap streamer discharge simulation model is established, and the influence of environmental parameters on the dynamic development process of streamer discharge is simulated and analyzed. The short gap air gap discharge characteristics were tested in a self-made gas discharge chamber. The results show that at low humidity (relative humidity <80%), with the increase of humidity, the electron density of the streamer channel increases, the field strength of the streamer head increases, the streamer development speed speeds up, and the gap breakdown voltage decrease; At higher humidity (relative humidity >80%), with the increase of humidity, the electron density of the streamer channel decreases, the field strength of the streamer head decreases, the development speed of the streamer slows down, and the gap breakdown voltage increase. With a decrease of gas pressure, the electron density of the streamer channel decreases, the field strength of the streamer head decreases, the development speed of the streamer increases, and the gap breakdown voltage decreases; The temperature change has little effect on the streamer development process.

ICCD Images of Leader Corona in a 4 m Rod-Plane Gap Discharge

As the featured discharge pattern and widely present in lightning discharge channels, leader corona system is crucial for continuous leader propagations in long air gap discharges and natural lightning, which had attracted numerous studies experimentally or theoretically. In the simulation of discharge process and lightning attachment calculations, the vertex angle of streamer zone ahead of leader channel is the crucial parameter, which could only be obtained by discharge experiments. As a result, morphological researches on leader corona system gave an intuitive insight on the streamer zone ahead of the leader channel and helped comprehensive theoretical modeling studies especially in the calculation of electric field. In order to obtain the accurate shape features and provide the calculation parameter for model modification, in this paper, experiments of 4-m long air gap discharges were carried out based on a four-frame PCO.dicam C4 ICCD camera. Clear images of leader corona were observed. Results showed that the corona zone was assumed as conical and the average angle at the vertex was 75.2° which was less than the first corona during the stable leader propagation stage. Influences of exposure time and leader branching phenomenon were investigated in this paper. It was concluded that the competitive relationship of the branched leaders shrank the vertex angle of the conical corona zone. Results in this paper could instruct the future theoretical model researches on leader corona system and help to modify the existing lightning attachment models like SLIM or leader process model.

On the electrical breakdown of GFRP wind turbine blades due to direct lightning strokes

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.

Optical Emission Spectroscopy of Lightning Impulse Discharges in Long Air Gaps with and without Water Mist

This work aims to study the various parameters of LI (lightning impulse) discharge channels in an air gap, i.e., the reactive radicals, electron density and channel temperature, and the influence of water mist on the reactive radicals, electron density and channel temperature of the discharge using optical emission spectroscopy. During a LI discharge in air, the gas in the air becomes highly ionized, and the spectra of which are similar to reference spectra of natural lightning. The electron density and channel temperature of the discharges in air under a LI voltage is ca. 3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">23</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and ca. 13,000 K, respectively, both of which are lower than those of natural lightning. Compared with LI discharges in air, more OH and OH <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> radicals are detected when the discharge takes place in a water mist-air mixture, indicating the partial decomposition of water molecules on the surface of water droplets during the LI discharge. The addition of water mist results in an 8.8-10.7% lower electron density than during LI discharges in air. However, the addition of water mist does not obviously influence the channel temperature of the air discharge.

Remote triggering of air-gap discharge by a femtosecond laser filament and postfilament at distances up to 80 m

We experimentally observed laser-induced remote high-voltage discharge triggering between two needle electrodes with half-a-cm spacing. The discharge was initiated by a 744-nm, 90-fs, 6-mJ laser pulse undergoing filamentation in air. For the direct voltage below the self-breakdown threshold, triggering of air-gap discharge was synchronized with a 10-Hz laser repetition rate and occurred between 40 and 80 m of the propagation path. No discharge guiding was observed. The experimentally registered and simulated remote triggering probability was above 80% in the range of 45–60 m from laser output and about 50% in the range of 60–80 m. The probability decreases as the postfilament hot spot diverges with a simultaneous increase in stochastic laser beam wandering.

Flashover discharges dynamic with continuous and discontinuous pollution layer under lightning impulse stress

The paper is aimed at the study of the dynamic of discharges propagating at the surface of flat PVC insulator (i) with uniform continuous pollution layer and (ii) with uniform discontinuous pollution layer of different conductivities, under lightning impulse voltage for both polarities. It is shown that the morphology of discharges, the current and the voltage as well as the discharge velocity depend on the polarity of voltage, the configuration and the conductivity of the pollution layer; and the behavior of discharges is similar to that of discharges in long air gaps.

Elongation and branching of stem channels produced by positive streamers in long air gaps

The elongation and branching of long positive spark discharges in the laboratory and in lightning have been attributed to the formation of thermalized channels inside a diffuse, glow-like streamer section at the leader head. It is experimentally shown here that the structured morphology of streamers produce low-density stem channels that elongate and branch well before a new leader channel section is formed. These non-thermalized stems are also shown to develop ahead of a developing leader channel. These findings are based on high-speed photography and Schlieren imaging used to visualize both the morphology of streamer filaments and stem channels. Numerical analysis is also performed to estimate the axial temperature and density of the stem channels. A stem-driven mechanism for the propagation and branching of positive long air gap discharges is proposed and discussed based on the presence of not-yet thermalized, low density channels formed by streamer ensembles at the leader head.

Observations of the Channel Illuminations During Dark Periods in Long Positive Sparks

AbstractNo observations show evidence of ionization in long air gap discharge during dark periods. However, for the first time, weak channel illuminations are observed during the so‐called dark periods in this paper. The length of these faint bright channels is almost constant during each dark period and is nearly equal to that of the remaining low‐density channels created by the previous discharge. The phenomenon, that is, the dark period is not really dark, is more likely to occur at the time when there is a greater change rate of the applied voltage. These findings are based on the high‐speed direct imaging and Schlieren photography together, in which the former is used to visualize the bright channels and the latter to detect the low‐density area.

Simulation Study on Discharge Process of Negative Polarity in UHV Tower-line Air Gap

Carrying out the research on the physical simulation model of the negative polarity discharge process in UHV tower-line air gap is of great significance to the lightning protection of UHV transmission lines. Based on the physical mechanism of negative discharge in long air gap, a physical simulation model including initial corona initiation and streamer development, leader initiation, streamer-leader system development, and final jump is established. The model considers the characteristics of corona initial development, the randomness of the leader path, and the negative development of the negative leader. Finite element software was used to calculate the background potential distribution of the gap, and the experimental observations were used to verify the model established in this paper. Finally, the model was used to simulate the discharge process of the negative polarity in tower-line air gap under actual size of tower window structure. The results show that the simulation calculations agree well with the test results.

Study on the Characteristics of UHV Negative Lightning Discharge in Tower-line Air Gap

Research on the characteristics of UHV negative lighting discharge process in tower-line air gap is of great significance to the lightning protection of UHV transmission lines. Based on the physical mechanism of negative discharge in long air gap, a simulation model of UHV negative lightning discharge is established. According to the UHV negative discharge model, the relationship between the development speed of the leader, lightning current wave front time, gap length and breakdown voltage in the physical process of lightning development is studied and analyzed. The analysis results discuss the influence of the development speed of the positive and negative polarity leader on the simulation calculation, and the relationship between wavefront time, gap length and breakdown voltage is obtained through fitting.

Shock wave propagation in long laboratory sparks under negative switching impulses

A series of 1.27 m air gap discharge tests under negative switching impulses were carried out to characterize shock wave expansion behavior along leader channels. A high-speed Schlieren system was used to identify shock wave propagation characteristics near the high-voltage electrode. According to the Schlieren images, three distinct types of shock waves were recognized during the leader propagation and after the air gap breakdown: (1) spherical shock wave (SSW), (2) cylindrical shock wave (CSW), and (3) bow shock wave (BSW). To the best of our knowledge, it is the first time to recognize SSWs at the point where the space leader meets the main channel and BSWs along the leader branch after the main leader bridges the air gap. According to the proposed velocity calculation method, the propagation velocity of SSW and CSW was calculated as a function of the shock radius, as well as the shock wave head of BSW. Moreover, a transient magnetohydrodynamics model was developed to predict shock wave front propagation characteristics of both straight and bending channels after breakdown. The predicted shock wave propagation velocity and shock radius showed good agreement with Schlieren images.