Measurements and modeling in the current zero region of vacuumcircuit breakers for high current interruption

Post-arc current and voltage measurements with high time resolution were carried out with industrial vacuum circuit breakers for investigating their current zero conditions and current interruption. Based on experimental results, plasma parameters were estimated by means of a modified “continuous transition model.” Dielectric effects in the post-arc current zero region were detected and modeled. At lower current amplitudes (Iˆ<16 kA), typical post-arc currents were obtained. At higher current amplitudes, the probability of higher post-arc current charges increases without the occurrence of breakdowns. It has been shown that charge generation takes place in the post-arc current region     

Simulations of V-I characteristics in a plasmafocus fun

The voltage and current characteristics in a plasma focus gun are simulated to yield the current efficiency and the rundown velocity of the current sheath. Since the discharge circuit is strongly influenced by the rundown dynamics of the current sheath, the simple snowplow model, which is modified to include the time-varying current and mass efficiencies, us used. The computer simulations are carried out for two separate experiments using deuterium and argon gas. A steady state of constant rundown velocity and saturated sheath current for both gases is correlated to the maximum holding voltage of the glass insulator. As much as 27% of the total discharge current for the deuterium gas and 33% for argon gas stay behind as leakage currents around the glass insulator when the radial compression phase begins     

Plasma density decay of vacuum discharges after current zero

In vacuum circuit breakers the post-arc current caused by the remaining ions and electrons in the contact gap is an indication of the residual ionization and its decay. It coincides with the formation of a positive space charge sheath in front of the new cathode, which grows toward the new anode. In a vacuum test chamber an arc (1.5-15 kA RMS) is drawn between high current electrodes of the spiral type. At different times after current zero a transient recovery voltage is applied across a separate pair of high voltage electrodes. In contrast to real circuit breakers, where the transient recovery voltage reappears between the arcing contacts, this separation allows the study of residual plasma free from the thermal stress and melting on the contact surfaces. From the post-arc current across these electrodes, in comparison with a mathematical model of sheath growth, the density of the charge carriers can be evaluated. Such values and their temporal decay are presented     

Measurement and Statistical Simulation of Multiple Reignitions in Vacuum Switches

When the gap length of a vacuum switch at current zero is too small, successive reignitions and extinctions may occur under certain circuit conditions leading to overvoltages. This process of "virtual chopping" is more likely to happen when the contacts accidentally open shortly before current zero. In order to determine the values of the overvoltage and its probability when opening the contacts randomly, a computer program was developed which simulates the transient recovery voltage (TRV) in a single-phase circuit and determines the highest voltage for each switching operation. The moment of contact opening is simulated by pseudorandom numbers. Input data of the calculation are the circuit parameters, the reignition voltage of the gap versus time after current zero, and its capability of quenching the high-frequency current oscillation occurring after a reignition. These recovery data of the opening gap were determined experimentally for some contact materials using a special switching device in a synthetic test circuit with current injection.     

Interaction between Vacuum Arcs and Transverse Magnetic Fields with Application to Current Limitation

The interaction between diffuse vacuum arcs and magnetic fields applied transverse to the electrode axis has been investigated both theoretically and experimentally. For arc currents < 6 kA, Hall electric fields, generated by the interaction, bow the plasma out of contact with the anode and raise the arc voltage. In the presence of a parallel capacitor, the arc current falls to zero and the arc is extinguished. For arc currents of 6 to 15 kA, arc extinction can be achieved with an oscillatory magnetic field; during such extinctions the arc voltage remains in phase with the magnitude of the field. Arc extinction via magnetic field/vacuum arc interaction could have applications to ac-current limiters and dc breakers. The fault current limiter application is discussed in this paper.     

Electric power switches

A review is given of the physics and engineering behavior of arcs in vacuum and axial gas blast under the conditions found in high-voltage circuit breakers. The topics included are: the zero energy switch; current limiting switches; arc control; the gas-blast circuit breakers; the low-current gas-blast arc; arcing with ablation; interruption dynamics; arc modeling; vacuum as a switching element; the vacuum arc; the constricted arc; and the properties of SF₆     

Use of Axial Magnetic Fields to Improve High-Current Vacuum Interrupters

When an axial magnetic field is applied to a vacuum arc, the arc tends to be stabilized in its diffuse mode. A minimum arc voltage is found for a certain magnetic field. In this condition, interrupting current is significantly increased, and it is nearly proportional to the diameter of electrodes. About ten years ago, a practical axial magnetic field electrode was developed for vacuum circuit breakers. Since then, through various improvements in its structure, this electrode has been refined for practical application in vacuum circuit breaker interrupters. The application has successfully covered not only medium-voltage circuit breakers, but also high-voltage (84 kV), dc high-voltage, and high-current circuit breakers. In this paper, ten years experience in this area is described.     

An experimental investigation on high-frequency vacuum arcinterruption at small gap length

An experimental study of high-frequency (126, 230 kHz) vacuum arc interruption behavior and the voltage escalation processes at a small gap length (⩽1 mm) for three contact materials (Cu, CuCr, and CuTeSe) is discussed. Two experimental methods have been used: current injection in a low-voltage circuit and in a 10-kV AC circuit. Experimental results of the high-frequency current interruption ability and the dielectric breakdown voltage are presented. Three kinds of breakdown are distinguished: the reignition voltage, the breakdown voltage, and the cold breakdown voltage. It has been found that the interruption ability is directly related to the reignition voltage     

Influence of bias magnetic field on the vacuum arc in double‐break vacuum circuit breakers

When double‐break vacuum circuit breakers (VCBs) interrupt the fault current, the series arc will generate their individual magnetic fields in different breaks. The magnetic field in one break will influence the arc in another break if the magnetic field is strong enough or the two breaks are very close. In this case, an interactive magnetic field effect happens. This field is also called the bias magnetic field (BMF). BMF can cause anode erosion and affect the performance at current zero. The distribution of BMF and the optimal configuration of the double‐break VCBs were obtained by the electromagnetic field simulation using the Ansoft Maxwell software. Based on the simulated magnetic field data, in the experiments, the interaction between the series vacuum arcs in double‐break VCBs was equivalent to the interaction between a single vacuum arc and the magnetic field generated by a Helmholtz coil. A high‐speed CMOS camera was used to record the trajectory of the vacuum arc plasma under different BMFs with different types of contacts. The results show the BMF can increase the arc voltage, and the arc becomes unstable. When the BMF becomes stronger, the arc voltage increases, and the arc becomes more unstable. In addition, for different types of contacts, the development process of the arc and the influence level are different under the same BMF. For a Wan‐type transverse magnetic field (TMF) contact or strong BMF, metal sputtering is evident and anode erosion becomes serious. For a cup‐type axial magnetic field (AMF) contact, the influence of BMF on the series arc plasma in double‐break VCBs is less than that of the Wan‐type TMF contact. The results of this work may be helpful for the design of compact double‐break VCBs.     

Current Chopping Introduced by Arc Collapse

The intensive cooling by a moving gas can cause violent elongations and curls of the circuit breaker arc, especially when small currents are interrupted. During the elongation the arc voltage increases rapidly. This introduces a breakdown across a smaller distance by short circuiting a part of the arc. Such a breakdown is called here arc colapse. The abrupt decrease in resistance and arc voltage may give rise to an oscillating discharge of the circuit breaker parallel capacitance into the arc path. This oscillation can force the main current to zero and thus cause current chopping. In this paper this kind of current chopping is studied and compared with chopping by instability oscillation. It is theoretically explained why these independent origins for chopping produce the same chopping levels.     

Events Associated with Zero Current Passage during the Rapid Commutation of a Vacuum Arc

Events associated with the rapid commutation of the current in a diffuse vacuum arc are explored experimentally and analytically in the short interval during which the current is brought to zero and the recovery voltage is established across the residual plasma. A brief pause of the order of 100 ns is observed between the passage of current through zero and the point where the recovery voltage starts to rise. Current zero is interpreted as the instant when the ion and electron currents are equal. During the subsequent pause the electrons are brought to rest and only then does a positive ion sheath develop, allowing voltage to build up across it. The charge remaining in the gap at current zero is observed to depend on the initial steady-state current and the rate at which the current is ramped to zero. Qualitative agreement is obtained by analysis, but the decay of the charge residue appears to be more rapid than the analysis predicts.     

Some Aspects of Current Interruption Physics in High Voltage Circuit Breakers

Recent results of experimental and theoretical investigations on current interruption processes are presented with a focus on SF₆ high voltage circuit breakers. Various aspects of thermal interruption at the zero crossing of the current are shown, including the scatter and the distribution of arc voltage shortly before CZ and the role of turbulence. The thermal interruption capability of air and CO₂ are compared to that of SF₆. Investigations on the dielectric recovery are shown for SF₆ and CO₂. The breakdown voltage during the dielectric recovery can be described by simple streamer and leader inception models. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reignition phenomena after high-frequency current interruption withshort vacuum gaps

Based on a large number of measurements of high-frequency (HF) current interruptions in vacuum at small contact gaps (⩽600 μm), the statistical reignition behavior of vacuum switching devices after a HF current zero is investigated. Three types of reignitions can be classified. Statistical evaluation of post-arc current measurements for different parameters at current zero and different HF current ignition processes gives information about the stress of the gap as a result of the transient recovery voltage after a HF current zero (HF-TRV) and the accumulated post-arc charge. Comparing post-arc current values at the beginning of the HF-TRV and at the moment of reignition reveals a production of charge carriers during the recovery interval. Possible reasons for the different types of reignitions are discussed     

A Model for DC Interruption in Diffuse Vacuum Arcs

A theoretical model for current interruption in a diffuse vacuum arc with dc commutation is described. Before current zero the interelectrode plasma is modeled as an ion-neutral fluid through which electrons are flowing. After current zero a positive ion sheath grows into the plasma from the former anode, driven by the transient recovery voltage. Using the basic laws of conservation, the decay of the plasma during commutation is evaluated numerically, enabling the post-arc current, the electric field at the former anode, and the power input to this electrode after current zero to be calculated. For copper electrodes, with a commutation time of 30 ?s, the ion density and velocity at current zero are 23 percent and 35 percent of their respective steady state values. The calculated post-arc currents of tens of amps are in good agreement with experimental data. The post-arc data generated with this model can be used to study reignition mechanisms and the interrupting capability of different contact materials.     

Recent technical developments in high-voltage and high-power vacuumcircuit breakers

Explanations are given regarding recent technical topics in the development of vacuum circuit breakers. The area-effect concept is introduced to aid in insulation designs of vacuum interrupters. By using Cu-Cr prepared through a special method, the voltage performance of vacuum interrupters has been remarkably improved. Through the development of axial magnetic field electrodes, large-current interruption has been made possible. The design concept for determining contact radii has been established. A computer simulation method for generating switching overvoltage is described, thereby indicating the possibility of realizing large-current interrupting vacuum interrupters with low switching overvoltages     

Townsend-type breakdown as a criterion for high-frequency vacuumarc interruption at submillimeter gaps

In vacuum circuit breakers, multiple reignitions give rise to HF current arcing (≈500 A; ≈200 kHz). Due to the small contact distance and the very large current gradient, at every arc-current zero pressures of several tens of millibars can be expected. Very soon thereafter (≈30 ns) this gap is dielectrically stressed by the first component of the restriking voltage (≈10 MHz), originating from parasitic impedances. The combination of the associated high electric field and the relatively high neutral density may cause Townsend-type breakdown, leading to another half-sine of continued arcing. Both dielectric stress and residual neutral density are expressed as a function of di/dt, yielding values of interruptible di/dt as a function of the contact distance with the Townsend breakdown criterion. Comparison with experiments shows fair agreement in the range of di/dt of 100-1000 A/μs and distances of 0.1-0.5 mm for two different circuits     

真空断路器弧后残余等离子体的探针诊断方法

真空断路器开断过程中弧后残余等离子体是表征其开断性能的重要参量。基于探针电子饱和区域工作原理,提出了一种真空电弧弧后残余等离子体电子密度测量方法,分析了其结构和工作原理。设计了探针诊断系统的探针结构和控制系统,基于可拆卸真空腔体进行了残余等离子体电子密度的单探针测量实验,采用高速相机观测电弧发展演变过程,研究了电流大小、触头结构等参数对残余等离子体衰减过程的影响。通过前人其他诊断方法对比验证了该测量方法的有效性,为后续真空断路器弧后微观特性研究提供了一种低成本、有效的诊断方法。     

Experimente zum Abreißstrom des Vakuumbogens

The theory of the chopping current of a vacuum arc developed by Ecker assumes that in the local circuit vacuum arc — wire inductivity — capacity connected in parallel oscillations arise on account of the falling characteristic of the vacuum arc at small currents. These oscillations cause the vacuum arc to chop. If this theory is correct, the chopping current can be reduced by damping these oscillations. The aim of this paper is to proof this consideration experimentally, and it appears that a strong reduction of chopping current can be obtained. The chopping current never falls short of a lower limit, which is probably caused by the available voltage.

     

Metallurgical aspects of contact materials for vacuum switchingdevices

Both the metallurgical aspects of contact materials for vacuum switching devices and their practical applications for important electrical load cases (circuit breaker, contactor, and load switch) are discussed. Because a vacuum contact material has to fulfil conflicting requirements, an optimization of the contact material with regard to the type of application is always necessary. The selection procedure for the different types of contact materials is discussed on the basis of experimental results. Cu/Cr-type contacts yield very good values for the breaking capacity of the circuit breakers used in the medium-voltage range. Optimization must attempt to improve the welding behavior and ability to withstand a high voltage after making short-circuit conditions. W/Cu-type contacts show a long service life in high-voltage contactors. Current chopping behavior, however, has to be optimized. WC/Ag-type contacts are a good choice for low-voltage contactors. The service life and breaking capacity, however, are not always satisfactory and need optimization     

Separation of spiral contacts and the motion of vacuum arcs at highAC currents

A framing camera is used to photograph the vacuum arc between separating spiral-petal vacuum interrupter contacts. The rupture of the molten bridge between the contacts first leads to a high-pressure, transient arc column. This arc motion can become constricted for several milliseconds before it goes diffuse as the current decreases to zero. The current through the spiral contacts produces a magnetic field perpendicular to the arc column, which forces the arc to move outward and run along the periphery of the petals. Several vacuum arc modes occur during the half-cycle of high current arcing. Movies, gap-current curves, and arc voltage traces are used to study the development of the arc motion and how it is affected by the contact structure. This information is used to generate arc appearance diagrams in which the arc form and motion are correlated to instantaneous values of current and gap for a wide range of peak currents. Appearance diagrams are shown for two ranges of opening delay from current onset