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     

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     

Neutral copper vapor density and electric recovery after forcedextinction of vacuum arcs

Dielectric recovery data were obtained for vacuum arcs between chromium copper butt contacts 30 mm in diameter and 2 mm apart. The 50-Hz arc current was forced to zero at its maximum of 200 A in about 1 μs. Following current zero, high-voltage pulses of a sufficient amplitude to always cause breakdown were applied to the gap. Gap recovery is characterized by the measured breakdown voltage as a function of time. Dielectric strength of the gap rises sharply within the first few microseconds after current zero, reaching its final value in about 10 μs. Neutral copper concentration in the center of the gap was measured by laser-induced fluorescence under conditions very similar to those of the recovery measurements. In contrast to the fast gap recovery, the copper vapor concentration does not change substantially during the first 100 μs from its value of 1.4×10¹⁸ m ^-3 near current zero. It is concluded that the neutral copper vapor concentration does not play a decisive role in gap recovery under these experimental conditions. This is corroborated by the fact that the mean free path for electron-impact ionization of copper atoms exceeds the gap length by four orders of magnitude     

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     

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.     

The Studies of a Vacuum Gap Breakdown after High-Current Arc Interruption with Increasing the Voltage

Vacuum-gap breakdown has been studied after high-current arc interruption with a subsequent increase in the transient recovery voltage across a gap. The effects of factors, such as the rate of the rise in the transient voltage, the potential of the shield that surrounds a discharge gap, and the arc burning time, have been determined. It has been revealed that opening the contacts earlier leads to the formation of an anode spot, which is the source of electrode material vapors into the discharge gap after current zero moment. Under the conditions of increasing voltage, this fact results in the breakdown. Too late opening leads to the breakdown of a short gap due to the high electric fields.     

Dielectric Recovery of an Axially Blown SF6-Arc after Current Zero: Part I?experimental Investigations

The results are presented of a model experiment to investigate the dielectric recovery of an axially blown sulphur hexafluoride (SF6) arc after current zero. With the aid of Schlieren pictures and interferometry, the temperature decay after current zero is observed up to the point of complete recovery of the gap. The dielectric recovery is directly measured by applying voltage pulses across the gap which causes breakdown at different times after current zero. Residual charges, which play a role in the early recovery phases, are detected using a specialiy developed technique. Variations of the shape of the voltage pulses and the geometry cause characteristic changes of the recovery, which support the interpretation of the experimental data.     

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     

Some interruption criteria for short high-frequency vacuum arcs

If the contacts of a vacuum interrupter open shortly before a current zero, the transient recovery voltage (TRV) can cause a reignition and reestablish the arc. When the current in a diffuse vacuum arc passes through zero, there is a distinct pause before the TRV builds up (approximately 40 ns for copper). During this pause the gap carries conduction current only with an ion component which depends on dI /dt, varying between 3 A for dI/dt=60 A/μs and 60 A for dI/dt=1235 A/μs. The ion current subsequently decays in tens or hundreds of nanoseconds. It can be distinguished from the displacement current at this time by varying dV/dt, keeping the other parameters constant. Among the interruption criteria for short high-frequency vacuum arcs, dI /dt prior to current zero and initial dV/dt are the most important. High values of dI/dt are more likely to precipitate reignitions, but breakdowns can occur after lower dI/dt's if the gap has been subjected to a high current for a relatively long time (>100 μs)     

Spatial and time characteristics of short gap, highdi/dt discharges

Under certain network conditions, vacuum circuit breakers may generate high-frequency currents. The quenching capability of vacuum circuit breakers for line-frequency currents and high-frequency currents plays an important role in the generation of unwanted voltage transients. This may occur when the gap distance at current zero is still too short to withstand the external voltage to the switch (TRV). The results of simulation calculations regarding these phenomena in the vicinity of current zero are described. Simulations are based on a detailed physical model, taking into consideration the basic conservation laws, the Maxwell equations, and the current continuity. The numerical solution takes into account the results of experimental streak photographs, revealing that the visible discharge covers only a small part of the contact diameter     

Influence of interrupted current amplitude on the post-arc currentand gap recovery after current zero-experiment and simulation

For commercial vacuum circuit interrupter valves with radial field contact, the post-arc current (PAC) waveform was measured after short circuit current interruption applying a transient recovery voltage (TRV) of about 15 kV/s (RRRV). Keeping the current decline di/dt at current zero constant, the power frequency (PF) current amplitude Iˆ_PF was varied from 0.5 up to two times of the rated short circuit current. Significant influence of Iˆ_PF on the gaps memory is shown in particular effecting the post-arc current duration which varies in a range between 2 and 8 μs. Based on the existing physical models an interpretation of the results is given. It is shown that the memory effect influences the gap recovery time. The experimental results were further used to determine the parameters of the sheath growth model described by Andrews and Varey (1971). The initial ion density is fitted according to the current ramp di/dt before current zero (CZ) and according to the measured Q_PF=∫i _PF×dt during the arcing phase. The results of two different test procedures were applied in order to separate the effect of current ramp di/dt and Q_PF=∫i_PF×dt. The influence of both parameters and of the rate of rise of the recovery voltage du/dt (RRRV) on the sheath edge velocity could be demonstrated by simulation     

Interrupting capacity of vacuum interrupters depending on thefrequency of current

Results of arc reignition voltages during current interruption of frequencies from 5.9 to 60 kHz by a short vacuum gap are presented. Measured arc reignition voltages depend on current amplitude and frequency, the values of preliminary arc current at the moment of switching on the HF current, and the discharge mode in the preceding current semiwave. Threshold amplitudes of the first semiwave of currents l_m1l and I_m1h as a function of frequency are determined. I_m1l and I_m1h divide current into three ranges to which different kinds of arc reignition voltage distribution correspond. Particularly large dispersion of reignition voltages takes place in the current range from I_m1l to I_m1h. The threshold current I_m1h is inversely proportional to frequency in the range from about 10 to 60 kHz, which is in agreement with the elaborated mathematical model     

Experimental Investigation of Low-Current Vacuum Arc Instabilities

The instabilities of 50-Hz low-current vacuum arcs and chopping phenomena were investigated in a test circuit where current was limited either by a resistor or an inductance. Employing a special test method which was developed for statistical evaluation, current zeros were measured oscillographically with a time sweep of 50 to 100 ns/div for durations of ? 60 div. The effects observed are quantitatively better to assess than with other techniques. It was found that each chopping process is initiated by a partial arc extinction (which as a rule occurs within less than 10 ns). One of the dominant factors of the whole process is the speed of recovery of the contact material. Copper contacts showed a much faster recovery and thus higher chopping currents than contacts of copper-tungsten. Furthermore, that current value at which the first instability occurs was measured as weli as the corresponding chopping current. Both values were evaluated statistically. In agreement with previous findings the occurrence of the first instability turned out to be independent of the capacitance parallel to the switching gap. However, the chopping current showed a significant dependence on the capacitance, as is well known from earlier investigations.     

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     

Influence of electrode separation on ion density in the vacuum arc

The density of singly ionized chromium shortly before and after forced extinction of vacuum arcs between chromium-copper electrodes was measured by laser-induced fluorescence for 2- and 10-mm contact gaps and currents between 200 A and 1 kA. In all cases studied, the ion density was constant before ramping down to the current and decayed exponentially after current zero. The ion density at current zero was found to be lower and to decay faster for a short gap than for a longer one, clearly indicating the effect of the contact separation on the charge carrier density. The variation of the time constant for the ion density decay with contact separation is closely analogous to the influence of contact separation on the recovery time of a switch gap. Furthermore, the recovery of dielectric strength of a chromium-copper gap proceeds on the same timescale as the decay of the density of singly ionized chromium. Both of these findings confirm that the ion density has a strong impact on the recovery of a vacuum gap     

Momentary interruption of current passing through zero in subnanosecond high-pressure gas-discharge switches

Data are reported for the performance of subnanosecond high-pressure gas-discharge switches aimed at generating high-voltage bipolar voltage pulses with a width of less than 1 ns, an amplitude of up to 200 kV, and a repetition rate of 100 Hz. Pulse formers, in which the polarity of the current passing through the switch does not change in the course of switching, seem to be the most appropriate. When the current changes sign, passing through zero, the effect of its momentary interruption is observed. The explanation of this effect is that, when the voltage across the gap reverses, a certain time interval is necessary for a cathode layer to form, which can transmit a high discharge current at a low voltage drop across the layer.     

Dielectric recovery of copper chromium vacuum interrupter contactsafter short-circuit interruption

The recovery of a vacuum interrupter gap after short-circuit interruption was measured by application of an overshooting transient recovery voltage (TRV) several tens of microseconds after current zero. Copper chromium contact materials were employed varying in composition (25 and 50% chromium content), gas content, and production method. The gap failure was either pure dielectric or it was dominated by a significant postarc current. Therefore, postarc current phenomena were experimentally investigated focused on the relationship among the postarc current, the power frequency current amplitude, and the gap length. It was found that two postarc current maxima exist: the first strongly dependent on the power frequency current, and the second on the field strength. A correlation among postarc current facilitated failures, the ultimately dielectric recovery, and the erosion rate of the material was found. Strong indication is given that all of these effects are dominated by the metal vapor pressure rise given by the constricted rotating arc. A significant influence of the material properties can be drawn from these experiments, allowing a good estimation of the capability for short-circuit current interruption, thus providing a useful tool for material development     

重频条件下电容器充电电源谐振电路的稳定

重频条件下电容器充电电源谐振电路由于谐振电容剩余电压的存在从而产生异常振荡,进而引发开关过流导致电源故障。针对这一问题,在分析谐振电路工作原理基础上,提出了在每个充电周期结束后,通过控制电源自身的部分开关导通,从而释放谐振电容剩余电压的解决方法,不仅可以让谐振电路趋于稳定,还避免了添加泄放电路的缺点,其控制方法也简单通用。对800 V/6 A的充电电源进行了电路仿真和实验验证,仿真和实验结果均表明,本文提出的方法可以在充电周期结束后将谐振电容上的剩余电压迅速归零,谐振电流也趋于稳定,有效抑制了谐振电路的异常振荡,从而验证了方法的有效性和实用性。     

Triggered discharges with high arc voltages in a vacuum interrupter

Studies of nonsustained disruptive discharges (NSDDs), isolated cases of which can occur in vacuum interrupters, indicate lateral discharges between the cathode and shield, which can initiate a brief discharge between the contacts. To facilitate the study of such discharges, the sample discharges were triggered by a surface discharge induced by a spark gap, built into the side of the cathode, and observed with a high-speed film camera and image-converter camera. The tests showed a cathode spot after igniting. The emitted electrons first charge the shield negatively and then are directed toward the anode. The discharge burns at a high voltage, with current ranging from 10 to 100 A. After a period of up to 400 μs, the current demand increases abruptly; an arc discharge occurs between the contacts and discharges the capacitances near the switch. The contact gap undergoes a rapid dielectric recovery, and the restored voltage is maintained. These types of discharge were also observed with NSDDs; thus it can be assumed that the triggered discharges studied correspond to the NSDD type     

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.