Influence of bubble size and flow velocity on AC electrical breakdown characteristics of LN2

To evaluate insulation design of high voltage for high T_c superconducting (HTS) power apparatus, the effect of bubble and electrode arrangements on the AC breakdown characteristics of LN₂ were investigated. Supposing that an outbreak of quench, the three electrode models were employed for breakdown voltage measurement in LN₂ with bubbles. Experimental results for various quench conditions revealed that the breakdown voltage of LN₂ with increasing the bubble size, flow velocity and electrode distance. Then, the relationship between the bubble conditions and the AC breakdown characteristics of LN₂ were clarified. Also, bubble movement phenomena were observed with an electrode model which consist of plane to cylindrical needle electrode immersed LN₂ for the simulation of the insulation environment in HTS pancake type coils and others of the quenching state.     

Breakdown voltage of glow discharge of pin electrodes

One of the shortcomings of glow discharge tube is its high breakdown voltage. In this paper, 2-pin electrode, 3-pin electrodes and 4-pin electrode are fabricated. The breakdown voltages of glow discharge of these electrodes have been measured within the pressure range of 0.4-2.5 kPa. The breakdown voltages vary obviously for the different kinds of electrodes, and the maximum difference of the breakdown voltages is about 208 V. Electric fields of the electrodes are calculated and the results consist with the experimental results. It is concluded that the breakdown voltage can be significantly reduced by adjusting electrode structure without changing the distance between electrode pins. The proposed electrode structure has potential applications in devices of glow discharge.     

Breakdown voltage of glow discharge of pin electrodes

One of the shortcomings of glow discharge tube is its high breakdown voltage. In this paper, 2-pin electrode, 3-pin electrodes and 4-pin electrode are fabricated. The breakdown voltages of glow discharge of these electrodes have been measured within the pressure range of 0.4–2.5 kPa. The breakdown voltages vary obviously for the different kinds of electrodes, and the maximum difference of the breakdown voltages is about 208 V. Electric fields of the electrodes are calculated and the results consist with the experimental results. It is concluded that the breakdown voltage can be significantly reduced by adjusting electrode structure without changing the distance between electrode pins. The proposed electrode structure has potential applications in devices of glow discharge.     

Effects of temperature and pressure change on positive corona and sparkover under direct voltage in short airgaps

Measurements are reported of breakdown voltage values and pre-breakdown voltage-current characteristics as a function of air density in a simple rod-plane electrode gap 50 mm in length. Pressure has been reduced and temperature raised from normal atmospheric values, independently of each other, to give the relative air densities in a common range less than unity. The effects of different diameters of rod have been studied. The results allow a comparison of the characteristics at equal relative air densities attained by either pressure or temperature adjustment. It is shown that at common relative air densities, breakdown is temperature- and geometry-dependent and that there is a temperature dependence of the pre-breakdown glow currents. The density-dependent relationship between the pre-breakdown glow discharge currents and breakdown voltages is discussed     

Conditions for complete self-healing breakdown in glow discharge polymerized styrene films

Breakdown patterns of glow discharge polymerized styrene film sandwiched between the top and base electrodes and fabricated on a glass substrate have been observed with a microscope. The breakdown patterns can be classified into three types: a broad ring of partly vanishing top electrode metal occuring around the breakdown hole in the dielectric (type A); a large hole made through the three layers (one dielectric and two metal layers), the dielectric film and the top electrode being folded back together at the periphery of the hole and the base electrode being melted over the hole area (type B); successive breakdown around the periphery of previous breakdown spots (type P). These patterns are interpreted in terms of the action of an arc across an extremely short gap as well as joule heating. Type A is preferable for attaining complete self-healing and the conditions that most favour the occurrence of type A holes are (a) the application of a positive voltage to the top electrode and (b) the top electrode being thin compared with the base electrode.     

Self-healing breakdown in NaCl epitaxial capacitors

The single-hole non-shortening breakdown characteristics of single-crystal films of NaCl were investigated. Thin film epitaxial Ag/NaCl/Ag capacitors were fabricated on heated NaCl substrates in a vacuum. The breakdown events were found to be uniformly distributed over the whole capacitor area. The three main features of the breakdown are as follows: (1) when breakdown occured at room temperature the voltage waveforms showed an abrupt voltage drop to zero indicating that a voltage threshold does not exist; (2) as the temperatures increases the voltage drop becomes erratic and the repetition frequency of the breakdowns increases; (3) when the base electrode is negatively based self-healing is promoted but when it is positively based the self-healing response deteriorates.     

Mechanical vs. electrical failure mechanisms in high voltage,high energy density multilayer ceramic capacitors

Causes of breakdown, both mechanical and electrical, in high voltage, high energy density, BaTiO₃ capacitors were studied. The flexural strength of the capacitors was 96 MPa. Failure was due to surface defects or pores close to the surfaces of the samples. The dielectric breakdown strength of the samples was 181 kV/cm. The causes of breakdown were either electrode end effects or pores between the dielectric and electrode layers. Weibull statistics were used to determine if there was a correlation between mechanical failure and dielectric breakdown. A strong correlation between the two types of failure was not found in the study, in contrast to earlier studies of single dielectric layer capacitor materials.     

Validity of the Space-Time Enlargement Law for vacuum breakdown

This paper investigates, theoretically and experimentally, the applicability of the Space-Time Enlargement Law to vacuum-insulated systems. A discussion on how characteristics of possible vacuum breakdown mechanisms determine the distribution function of the breakdown voltage random variable is presented. By superimposing effects of electrode surface enlargement and inter-electrode gap enlargement, expressions for the mean value and standard deviation of the breakdown voltage random variable are obtained. In the case of time extensions, the assumption of complete independence of consecutive discharge processes is discussed. Experimental testing of the Enlargement Law was performed on Rogowski type two-electrode systems, with different electrode surface areas, inter-electrode gaps and vacuum pressures. Measurements were conducted using industrial ac voltage, dc voltage with 50 V/s rate of rise, standard atmospheric pulse voltage (1,2/50 μs), and commutational pulse voltage (250/2500 μs). The final conclusion, based on the comparison of theoretical considerations and the experimental results, is that the Space-Time Enlargement Law can be applied in the design phase during the development of vacuum devices, with certain limitations, regardless of the type of the applied voltage.     

Boundary area between gas and vacuum breakdown mechanism

This paper considers the transition from the gas breakdown mechanism to the vacuum breakdown mechanism. Gas breakdown mechanisms at the points of Paschen minimum and on the left from it, as well as vacuum breakdown mechanisms and their dependence on experimental parameters have been analysed. In accordance with this analysis, an experiment with the following variable parameters has been conceived and carried out: type of gas (i.e. residual gas); gas pressure; inter-electrode gap; electrode material; electrode surface topography; type of voltage load; and position in the insulation system of an additional source of ion-electron pairs i.e. of the α-radiation Bragg peak. By a statistical analysis of experimentally obtained statistical samples of the random variable breakdown voltage (dc and impulse), it has been established that the transition from gas to vacuum breakdown mechanism takes place in a relatively broad region, in which the gas breakdown mechanism of an anomalous Paschen type and the avalanche mechanism of vacuum breakdown are present simultaneously.     

介电弹性材料电致动变形及失效规律实验研究

介电弹性功能材料(简称DE)是一种新型智能材料,可在电压驱动下产生厚度与面积变形。为了掌握其在电场作用下的变形以及失效规律,本文通过实验方法研究了在不同的预拉伸量、电极特性、电压频率等条件下,DE材料在电场作用下产生变形的最小电压以及发生电击穿时的失效电压的规律,并对实验现象进行了分析和解释,给出了DE材料维持正常工作的条件,为DE材料的工程应用提供了相应的准则。     

Methods of calculating the air-gap breakdown voltage in weakly and strongly nonuniform fields

The results of experimental study of streamer and spark processes under a standard lightning voltage pulse are shown for different degrees of electric-field nonuniformity in sphere–plane and sphere–sphere electrode systems. The possibility of introducing similarity relations for streamer processes is considered. The applicability of various numerical methods for determining the breakdown voltage is analyzed.     

Positive corona progression over profiled insulator surfaces near the breakdown condition

Experimental observations are presented of the development and propagation of positive impulse corona near the breakdown condition in a point plane gap in air at atmospheric pressure in the presence or absence of insulators having simple cylindrical and profiled surfaces. Optical techniques have been used and the effects of the insulator materials have been demonstrated. All the results with corona were obtained using an applied peak impulse voltage of 0.9U₅₀ in each case, where U₅₀ is the 50% breakdown voltage of the gap. Comparisons were made between air, polytetrafluoroethylene (PTFE) and porcelain surfaces. It has been demonstrated that corona development in the gap is dependent on the relative permittivity of the insulator material, the profiles and the proximity of the insulator to the high voltage point electrode. Simultaneous photomultiplier records and photographs taken on fast film with Quartz optics show details of corona development using the UV region of the spectrum.     

An investigation into the parameters affecting the breakdown voltage and inter-particle bonding in the electrical discharge compaction of metal powders

The aim of present investigation is to gain deeper understanding of breakdown behavior and inter-particle bonding by conducting experimental tests. This may lead to improve the state of compaction by relative arrangement of initial parameters to maintain uniform distribution of current density and producing compacts with sufficient mechanical strength. Experimental work was carried out using two different set-ups. The first arrangement was employed to provide steady-state alternating voltage. The effect of column geometry and particle size on breakdown voltage was investigated under this condition. The second set-up, capacitor discharge circuit, was used to provide impulse voltage. Under this condition, the influence of column geometry, particle size, application of axial pressure, evacuation of air, energy input, electrode material and configuration on breakdown voltage was studied. Also, scanning electron microscopy was employed to study the effect of different parameters on inter-particle bonding. The results of experiments conducted on the influence of each of the voltage and capacitance on the compaction properties are also discussed.     

Correlation of electrical breakdown of supercritical helium in short gaps with partial discharge in cable samples

As part of a project to develop a superconducting power cable at Brookhaven National Laboratory (BNL), many experiments have been conducted on short coaxial cable insulation samples lapped with various polymeric tapes. These experiments have shown the partial discharge inception voltage to be a function of a sample construction and the density of the helium impregnant. It was concluded that the partial discharge inception voltage in the butt gaps of lapped tape insulation is determined by the product of helium density and gap depth, which corresponds to the tape thickness. This means that the inception voltage obeys the similarity law. This conclusion was based on additional experiments performed to determine the breakdown voltage of helium in a short gap between both bare metal electrodes and plastic insulation coated electrodes. The effects of the condition of the electrode surfaces and the area of the electrodes were essential in determining the breakdown voltage in a short gap of high density cryogenic helium.     

Electrode erosion in a high-frequency spark discharge

The disperse composition of the products of electrode destruction in a high-frequency vacuum breakdown and the variation of the relative degree of plasma ionization as a function of the material of the electrodes and the high-frequency voltage applied to them are investigated.     

Influence of a DC field on the near-electrode zone of nonuniform microwave discharge in hydrogen

We investigate, by the emission spectroscopy, the influence of the DC electric field on the nearsur-face plasma of the electrode microwave hydrogen discharge (EMD) at pressures of 1 to 5 Torr. We obtain the DC current-voltage characteristics of the EMD, the strength values of the microwave field, and its spatial distributions in the EMD near-electrode zone. Under a positive voltage on the electrode as against the chamber, the variation in the discharge structure are minor, and, at the particular voltages depending on the pressure and the microwave power, the spatial charge layer breakdown takes place near the discharge chamber surface. Under a negative voltage on the electrode (the plasma cathode microwave mode), the discharge structure and dimensions vary and, at high currents, the discharge cancels.     

Investigation of partial discharge activity of single conducting particle in transformer oil under DC voltages using UHF technique

Converter transformers play an important role in the high-voltage DC power transmission system. The insulation of these transformers is stressed by both AC and DC voltages. Particle contamination is one of the major problems in insulation structures, as they generate partial discharges. A methodical experimental study was carried out to understand the partial discharge formation because of single macroscopic conducting particle movement in transformer oil at different temperatures under DC voltages using ultra-high-frequency (UHF) technique. It is observed that the single particle moves in the vertical electrode gap under the DC voltage, and generates UHF signal in the range of 1-3 GHz. At high electric fields, the levitated particle sometime floats in the oil medium and as and when the applied voltage is removed, the particle slowly drops down. When it comes in contact with ground electrode, it generates UHF signals with the frequency content lying in the range of 1-3 GHz. Oil breakdown tests were carried out (without any particle in the oil electrode gap) under DC voltages and fast Fourier transform (FFT) analysis of the signal generated by the UHF sensor because of breakdown indicates that the frequency content of the UHF sensor signal lies up to 1 GHz.     

Electrical breakdown mechanism of liquid nitrogen in the presence of thermally induced bubbles

An investigation of the electrical breakdown behaviour of liquid nitrogen in the presence of a local hot spot on an electrode has been performed to determine the breakdown mechanism of liquid coolant in cryoresistive or superconducting apparatus under quenching conditions. It has been found that the vapour bubbles are generated at a hot spot and the manner of the vapour generation and the vapour structure depend strongly on the ambient pressure, the heat transfer rate at a hot spot and the applied voltage. Electrical breakdown took place through various series of composites of liquid and gas phases, depending on the pressure and the heating power applied to a hot spot. Therefore, the breakdown characteristic of liquid nitrogen under hot spot conditions depended to a marked degree on these factors.     

Some particularities of development of high frequency capacitive discharge between a drop-jet electrolytic electrode and a flow-type electolytic cell

Particular results of the development and burning of the high frequency capacitive discharge (HFCD) between a drop-jet electrolytic electrode and a flow-type electric cell are presented. Significant current and voltage pulsations taking place after the electric breakdown in the process of the HFCD burning confirm the multichannel shape of the discharge burning at atmospheric pressure. Three modes of HFCD burning between the combinations of the drop-jet electrolyte and the surface of the flow-type electrolyte have been specified. An HFCD with glowing luminescence and “streamer discharges” at the copper tube surface when the electrolyte jet is fed under the pressure of 3 × 10³ Pa has been discovered.