Prebreakdown Currents Research Articles

High-Voltage Nanosecond Discharge as a Means of Fast Energy Switching

The formation of a nanosecond discharge with the use of a Hamamatsu streak-camera and with simultaneously wideband (10 GHz) measurement of voltage and displacement current caused by a streamer in one pulse has been studied. Nanosecond voltage pulses of various amplitudes (16, 20, and 27 kV) were applied across a point-to-plane gap (8.5 mm) filled with air at various pressures (13, 25, 50, 100, and 200 kPa). It was found that the voltage across the gap drops as soon as a streamer appears in the vicinity of the pointed electrode. At the same time, a pre-breakdown current begins to flow. The magnitude of the pre-breakdown current, as well as the voltage drop, is determined by the rate of formation of dense plasma and, accordingly, by the rate of redistribution of the electric field in the gap. The streamer velocity determines the rise time and amplitude of the current. The higher the streamer velocity, the shorter the rise time and the higher the amplitude of the pre-breakdown current. The propagation of a backward and third ionization waves was observed both with the streak camera and by measuring the displacement current. As they propagate, the discharge current increases to its amplitude value.

Prebreakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 1: behaviour at positive polarity

This study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under positive polarity stress. It is of interest to improve knowledge on how functional properties of dielectric liquids vary, as new brands arrive in the market, and existing standards are unsuited for documenting the dielectric functional parameters of these new liquids. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with a 0.5 μs rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. There are clear differences in streamer stopping lengths and mode occurrence and mode velocities between these liquids. The differences seem to be influenced by molecular sizes governing evaporation energy for streamer formation and by concentration of aromatics which can be coupled to electron avalanche processes in the streamer heads.

Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

This study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under negative polarity stress. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with 0.5 μs rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. Compared to positive polarity, there are less differences in streamer behaviour in the oils examined under negative polarity. Breakdown voltages and acceleration voltages are higher for negative streamers than for positive ones, while their propagation velocities are lower. While propagation modes for positive voltages are quite distinct, the mode changes for negative ones are more gradual. The behaviour of both positive and negative streamers is in line with the hypothesis that the propagation is governed by electron avalanches and quantum chemical properties of liquid components.

Role of Prebreakdown Currents in a Static Breakdown of a Two-Sectioned Cold-Cathode Thyratron

The data on prebreakdown currents and static breakdown voltages measured in two-sectioned sealed-off thyratron with a cold cathode TPI1-10k/50 are presented. The temporal behavior of the anode voltage and of the voltage at the separate sections has been investigated at the prebreakdown and breakdown stages. It is demonstrated that the prebreakdown current presented in the separate sections causes redistribution of the anode voltage between the sections. Due to this effect, a maximum thyratron breakdown voltage can be obtained. The other method to increase the breakdown voltage is based on the forced distribution of the anode voltage between the sections using a capacitive divider. Special features of transition from the prebreakdown to breakdown current are discussed for different thyratron-switching circuits.

Relationship between the electric field integral value and vacuum breakdown characteristics in small electrode with multi‐apertures

For a critical plasma test equipment JT‐60SA, vacuum electric breakdown around apertures is a serious problem for developing a high‐energy and high‐power negative ion source accelerator. To improve the high voltage performance of the accelerator, we investigated the relation of a breakdown electric field in a vacuum with the electric field integral value of electrode's surface for multi‐apertures electrode. The results of breakdown tests for various types of multi‐aperture electrode show that the electric field integration ES had a relation with the vacuum breakdown field and field emission current characteristics. The reduction in the ES with the relaxation of electric fields improved the breakdown performance. Moreover, the low value of ES decreased the field enhancement factor ‘β’ and the emission area ‘A’ of field electron emission from the electrode. Therefore, we found that the breakdown electric field can be improved by reducing the prebreakdown current that flows before the occurrence of breakdown by reducing the ES. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Роль предпробойных токов в механизме статического пробоя двухсекционного тиратрона с холодным катодом

Роль предпробойных токов в механизме статического пробоя двухсекционного тиратрона с холодным катодом

Research on the Prebreakdown Current of the Laser-Triggered Vacuum Switch

The prebreakdown current flowing in a laser-triggered vacuum switch (LTVS), investigated in this paper, is below the threshold voltage, where the switch becomes conducting. The laser irradiates in a short time the sintered KCl and Ti target of the vacuum switch and produces an initial plasma. The prebreakdown currents are measured in two different electrode geometries (planar and multirod) at both polarities and analyzed with respect to the ion and electron interactions with the electrodes. It is found that the peak value of the prebreakdown current increases with the applied voltage across the LTVS until 60 to 80 V, where it turns instantly into the conducting state. The multirod and planar LTVSs show different prebreakdown currents at different polarities, which is related to the condition of becoming conducting.

Prebreakdown Currents in a Sealed-off Two-Section Cold-Cathode Thyratron and Methods for Increasing the Breakdown Voltage

The data on the measured static breakdown voltages and pre-breakdown currents flowing in the main gap of a two-section, sealed-off thyratron with a cold-cathode TPI1-10k/50 are presented. It is shown that pre-breakdown current in the lower section appears at lower voltages than in the upper section. A few methods are proposed for increasing the thyratron breakdown voltage via re-distribution of the voltage between the sections using capacitive and resistive voltage dividers. It is demonstrated that either the self-capacitance of one of the sections or the resistance due to the pre-breakdown current flow could serve as one of the arms of the divider.

恒定电压下变压器油隙放电特性

以变压器油作为研究对象，采用有限元方法(finite element method, FEM)对应用场致电离机理建立的用于表述液体电介质流注放电过程中载流子的产生及输运特性的偏微分方程(partial differential equations, PDE)进行求解分析，获得直流电压下变压器油隙放电过程空间电荷动力学特性。数值模拟结果表明，场致电离导致液体击穿的根本原因是油隙中自由载流子的产生、迁移形成了预击穿电流，由此产生的焦耳热使得油隙中形成低密度区，利于流注发展。根据这些现象得出的结论能够很容易理解液体电介质的击穿机理，有利于液体电介质在电力系统领域得到更合理的应用。 The transformer oil was taken as the object of study, and the finite element method (FEM) was employed to analyze and solved the partial differential equations (PDE) which was built by field ionization for describing the generation and transport properties of free charge carriers. The results showing that the dynamic characteristics of space charges in the liquid-gap discharge under DC were gained. According to the simulating results, the fundamental causes of the dielectric liquids breakdown by the electric field dependent molecular ionization are described as following; the pre-breakdown current is firstly formed by the generation and migration of the charge carriers in the oil gap, then the joule heat generated from it makes liquid-gap to form the low density region in the liquid, which facilitate initiated little streamer emergence as avalanche in gas discharge, meanwhile, the streamer is continuously evolving. Then these conclusions enable ones easily understand breakdown mechanism of the liquid dielectrics that are reasonably applied to power system fields.

Characteristics of long air gap discharge current subjected to switching impulse

Measuring the pre-breakdown current of long sparks in air is important for investigating the discharge mechanism. Since the breakdown of long air gaps is conducted by a series of streamer-leader processes, the corresponding current signals cover a bandwidth of 0 to more than 20 MHz. Measurement accuracy of the current from the high voltage side is affected by the displacement current and impulse electromagnetic interference. In this paper, a coaxial current sensor with a DC bandwidth of 74.45 MHz is developed. A displacement current-restrained electrode structure is proposed to reduce the equivalent capacitance between the current sensor and the ground over 30 times. Combined with the digital optical fiber synchronous acquisition unit, a current measurement system for long air gap discharge is established. For the purpose of the UHV system's external insulation optimization design, the discharge current waveform of a 6 m rod-plane air gap under positive switching impulse voltage with 250 μs and 1000 μs time to crest is obtained. Discharge images and stressed voltage are combined to analyze the continuous feature of a current waveform under critical time to crest impulse and discontinuous feature under long front duration impulse. For the purposes of a lightning protection study, the current waveform of a 10 m rod-plane air gap is subjected to negative switching impulse. Finally, the pulse characteristics of the current corresponding to the single channel and branching stepped negative leader are discussed.

Prebreakdown current and DC breakdown strength of alumina-filled poly(ethylene-co-butyl acrylate) nanocomposites: Part II - prebreakdown currents

Pre-breakdown currents through the poly (ethylene-co-butyl acrylate) (EBA) nanocomposites filled with spherical alumina particles were studied as a function of particle coating and moisture content. The nanocomposites were prepared from a mixture of EBA formulations (13 wt% butyl acrylate groups content) and alumina powder. Two kinds of alumina nanoparticles were used: NA (25 nm in diameter) and ND (45 nm in diameter). The particles were either unmodified or surface-treated with aminopropyltriethoxy silane or octyltriethoxy silane. Two different relative humidities of air were used for conditioning the samples prior to testing: 0 % RH (dry case) and 86 % RH (samples were saturated with moisture). The pre-breakdown current measurements were performed with a DC ramp of 1.2 kV/s. Twenty samples were tested for each material formulation - ten of which were tested in dry conditions and ten were tested after conditioning in humid environment. Results were compared to the reference unfilled EBA samples. The measured currents were analyzed under the assumption of space charge limited conduction. As expected, conditioning materials in humid environment caused an increase in pre-breakdown currents. This effect was more pronounced for nanocomposite materials, while the reference unfilled material was least affected.

Deviations from the Paschen's law at short gap distances from 100 nm to 10 μm in air and nitrogen

Deviations from the Paschen's law in air and nitrogen are investigated for gaps from 100 nm to 10 μm, by using a high precision electrode positioning system. The deviation is observed when electrode gaps are smaller than 4 μm at atmospheric pressure. At distances lower than 1 μm, a nearly constant average breakdown field of 350 V/μm is evidenced in both gases with Au and Ru electrodes. A metallic plasma initiated by field emission from the cathode can explain the reduction of the breakdown voltage at such low gaps. In ambient air, the existence of a pre-breakdown current is also evidenced, probably due to the presence of water adsorbed on electrodes.

Method of improving the dielectric strength of vacuum insulation

At low field strength insufficient for field emission of electrons, processes arise at defect sites on the surface of high-voltage electrodes that produce prebreakdown currents, initiating microdischarges, and eventually breakdown of the vacuum gap (vacuum arc). These processes may be suppressed by lowering the gas saturation of the electrodes. It is suggested that high-voltage electrodes be made of metallic beads, which are degassed much more rapidly than all-metal electrodes. This aids in improving the dielectric strength of the vacuum gap.

ANN and wavelet-based discrimination technique between discharge currents in transformer mineral oils

This paper is aimed at the analysis of positive pre-breakdown currents triggered in mineral transformer oil submitted to 50 Hz alternating overvoltages. Different shapes of streamer currents and electrical discharges have been recorded to develop a discrimination technique based on an Artificial Neural Network (ANN) and Wavelet analysis of these currents. This enables us to address a complementary diagnosis tool that can serve as an online transformer monitoring and protection.

Environment Exposure Tests of Electron-Emitting Film for Spacecraft Charging Mitigation

A new electron-emitting device operating in completely passive manner has been developed to prevent spacecraft charging and discharging. It is named as electron-emitting film (ELF) for spacecraft charging mitigation. This emitter (ELF) utilizes the field enhancement at the triple junction formed at the interface where metal and insulator are met and exposed to vacuum. ELF emits prebreakdown emission current that might lead to arcing at the triple junction. Hence, it balances the input and output currents to the spacecraft during substorm. After ensuring the robustness of this ELF against ground handling and in-orbit contamination, laboratory experiment confirmed continuous electron emission for 100 accumulated hours to assure the endurance. In this paper, its durability against high-energy electron and proton irradiation equivalent to ten solar years in GEO is reported. Effect of heat cycling (-150 °C-100 °C) and vacuum ultraviolet irradiation in GEO on this emitter is also completed. Postemissions of this ELF confirm the durability under those harsh space environments.

Peculiarities of formation and development of initial stages of an impulse breakdown in argon

The experimental results on the formation and development of initial stages of an impulse breakdown in atmospheric-pressure argon at townsend and streamer breakdown mechanisms for different initial conditions are presented. A streamer channel is shown to be initiated by bright luminescence formed at the point of critical avalanche amplification at different distances from the cathode depending on overvoltage. Prebreakdown currents are experimentally measured for the townsend and streamer breakdown mechanisms and peculiarities of spark channel formation for these mechanisms are studied.

Fabrication of gas ionization sensor using carbon nanotube arrays grown on porous silicon substrate

We fabricated gas sensors based on field ionization from multiwalled carbon nanotube (MWCNT) arrays grown on porous silicon templates. MWCNTs were grown through thermal chemical vapor deposition. We measured breakdown voltages, discharge and pre-discharge currents of the device for various gases in different concentrations. Our gas ionization sensors (GIS) presented good sensitivity, selectivity and short response time. The GISs based on porous substrates showed higher discharge current and good mechanical stability in comparison to those which were fabricated on polished silicon substrates. Additionally, we applied a high electric field to align CNTs. This increased the pre-breakdown current. Also, introducing humidity in dry air caused enhancement in pre-breakdown current and lowered breakdown voltage. We also used our GIS in monitoring gas mixtures.

600-V / 2-A Symmetrical Bi-Directional Power Flow Using Vertical-Channel JFETs Connected in Common Source Configuration

Bi-directional solid-state-circuit-breakers (SSCBs) are highly desirable in power-electronic fault-protection applications due to their high actuation speed and repeated fault isolation capability. Normally-on SiC vertical-channel JFETs (VJFETs) are excellent candidates for high power/temperature scalable SSCB applications as majority carrier devices with low conduction losses and stable +300°C thermal characteristics. 600-V / 2-A bi-directional power flow was demonstrated using two VJFETs connected back-to-back with their sources in common. The low VJFET pre-breakdown leakage currents and sharp onset of breakdown are critical in enabling bi-directional power flow. 0.1-cm2 low conduction-loss VJFETs were designed for efficient and reliable SSCB applications.

Dynamic Prebreakdown Current Measurement of Nanotips-Based Gas Ionization Sensor Application at Ambient Atmosphere

A gas ionization sensor (GIS) with nanotips as anode and tungsten needle as cathode was fabricated, which is working with moderate input voltage. Controlled dc voltage was applied, and the prebreakdown currents were measured. The sensor can be used to determine target gases at ambient atmosphere. A novel dynamic prebreakdown current measurement was exploited which can decrease the noise interference and improve the stability of the GIS. It is discussed in detail about the advantages of the dynamic prebreakdown current measurement and its influence factors. In addition, carbon nanotubes and zinc oxide nanorods were used as ionization tips, respectively. Their anti-oxidation performances were indicated by experiment.

A novel highly sensitive gas ionization sensor for ammonia detection

A novel highly sensitive gas ionization sensor for ammonia detection in ambient air is introduced in this paper. Carbon nanotubes (CNTs) grown on silicon substrate were incorporated to fabricate a gas ionization sensor. Application of a positive bias to the CNTs generates electric fields sufficiently to field-ionize passing gas-phase molecular and initiate a prebreakdown current. When the CNTs film is configured as the cathode, secondary electrons repelled away from the CNTs tips into the gap spacing make more ionizing collisions and also initiate a prebreakdown current. By monitoring the prebreakdown current, the gas ionization sensor was demonstrated to be capable of ionizing and detecting the ammonia and with a linear response over the entire range from 1 to 160 ppm ammonia in air. The sensitivity mechanism of the gas ionization sensor was also discussed in detail. The sensitivity and selectivity of the gas ionization sensor to the gases is not only dependent on its ionization energy but also its electric dipole moment. The novel CNTs-based gas ionization sensor described here exhibits high accuracy, repeatability and stability. The sensor is promising for use in various fields.