Low-current Vacuum Arc Research Articles

Methodology for Testing Selected Parameters of Low-Current Vacuum Electric Arc

This article presents the author’s methodology for testing selected parameters of a low-current vacuum arc, implemented using an innovative test stand based on a vacuum discharge chamber with a contact system mounted inside. In order to verify the validity of the adopted research methodology, as well as the correctness of the operation of the developed laboratory bench, measurements and calculations were made, among other things, of the energy and burning time of the vacuum arc, depending on selected factors, such as pressure and the delay time of the contact opening, calculated from the “passage through zero” of the sinusoid of the current flowing through the system. The tests were performed at 230 V and a current of 5 A for two pressure values: p1 = 1.00 × 105 Pa (atmospheric pressure) and p2 = 4.00 × 10−3 Pa (high vacuum environment). It was found that the vacuum insulation technique allows a significant reduction in the value of the arc energy and the burning time of the arc. It was also observed that in the case of a high vacuum environment, the ignition of the vacuum arc occurs after a time equal to about 3 ms from the “passage through zero” of the current flowing through the system. Below this value, the phenomenon did not occur. The results obtained provide an opportunity for the design and manufacturing of vacuum switchgear, where there is the prospect of reducing the negative effects associated with the arc burning process in the contact gap.

Investigation of the behaviours of cathode spot current density on plasma parameters in low-current vacuum arc

Investigation of the behaviours of cathode spot current density on plasma parameters in low-current vacuum arc

Analysis of the Thermal Conductivity Mechanism Affecting on the Plasma Parameters in Instability Low Current Vacuum Arc

Analysis of the Thermal Conductivity Mechanism Affecting on the Plasma Parameters in Instability Low Current Vacuum Arc

Application of an Artificial Neural Networks Model for Prediction of Instability Phenomena in Low Current Vacuum Arc by Cathode Spot Model

Application of an Artificial Neural Networks Model for Prediction of Instability Phenomena in Low Current Vacuum Arc by Cathode Spot Model

Experimental investigation on the characteristic and voltage of low-current vacuum arc in vacuum OLTC

Due to vacuum interrupter's excellent interruption performance, an increasing number of on-load tap-changers (OLTCs) have chosen it as a load changeover switch in recent years. The stability of the arcing process is crucial for the interruption since the vacuum interrupter in OLTC must break current frequently with the requirement of long electrical life. In this paper, experiments on butt contacts and axial magnetic field (AMF) contacts applied in OLTC were carried out in a demountable vacuum chamber, and the arcing process was investigated by a high-speed camera. Investigations were conducted on the effects of opening speed, opening phase, current amplitude, and AMF on arc characteristics, e.g., arc expansion and arc voltage. The experiment findings demonstrated that expansion and instability of arc increased with higher opening speed, number of cathode spots moving to the inclined surface decreased with shorter arcing time, and that cathode spots' propensity to form group cathode spots increased with higher current. AMF lessened the impact of high opening speed and high current on the arcing process. The investigation carried out in this paper helps to better understand the characteristics of arc evolution and the influence of some key factors, which may serve as a guide for improving the design and vacuum arc control of vacuum OLTCs.

An Analysis of Sheath Development of Cathode During the Instability Stage of Vacuum Arc

The purpose of this article was to investigate the dynamic evolution laws of the plasma between the gap during the instability phenomena in low-current vacuum arc using the simulation date comparison with the experimental data. The instability phenomena are characterized by high-frequency peaks of the arc voltage which are prior to the actual current chopping. Those peaks, which reach a level of more than ten times the normal arc voltage, have typical correspondence with the fluctuations of arc current and will result in a sudden chopping of the current through interaction with the elements of circuit. The fluctuations of arc voltage were classified into the types of long pulses and short pulses because of different characteristics on the duration time and oscillation morphology, which can summarize the peaks on the instability phenomena. The concepts of prearc cathode sheath and the cathode-emission intensity are proposed to reasonably explain the difference and connection between these two spikes. The distribution and motion of particles in the gap at different cathode-emission intensity, which demonstrating the development direction of the prearc cathode sheath and the relationship with the cathode-emission intensity, are simulated by the particle in cell-Monte Carlo collision (PIC-MCC). As a result, the relationship in the development of the prearc cathode sheath, the voltage spikes, and the intensity of cathode emission is summarized.

Optical Investigation of Excited Species Distribution in Low-Current Vacuum Discharges

It is well-known that when high currents flow through a vacuum gap, anode spots are formed on the anode surface. However, anode flare phenomena are also observed in low-current vacuum discharges. The objective of this article is to experimentally observe the spatial distribution of excited Cu atoms and ions generated from a single cathode spot (CS) and to interpret the mechanism of the anode flare at low currents (40–80 A). Optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) techniques have been used for investigating low-current vacuum arc discharge confined to axial magnetic fields (AMFs). The effects of AMFs and arc currents on the axial distribution of the radiation intensity were investigated for different copper ionization states (Cu I and Cu II). The highest luminance was found near the electrodes for both excited neutral copper atoms and ions, and only weak spectral lines of excited Ni atoms were detected in the near-anode region. With increasing the external AMF, a bright “spot” appeared near the anode surface. In the visible light range, excited atoms were the main contributor to anode flare in low-current vacuum discharge. As measured by LIF, the density of copper atoms near the anode increased from 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}$ </tex-math></inline-formula> to 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}\,\,\text{m}^{-3}$ </tex-math></inline-formula> when the applied magnetic fields increased from 31 to 74 mT. The anode flare contains far more excited atoms from the cathode material than the anode material. They are the results of cathodic plasma expansion to reach the anode after charge neutralization and reflection on the anode surface. For the single CS, the observed fact that a tiny number of atoms leave the anode surface and participate in the anode flare is more likely the result of a sputtering effect rather than thermal evaporation by cathode electron stream.

Numerical simulation of low-current vacuum arc jet considering anode evaporation in different axial magnetic fields

As the main source of the vacuum arc plasma, cathode spots (CSs) play an important role on the behaviors of the vacuum arc. Their characteristics are affected by many factors, especially by the magnetic field. In this paper, the characteristics of the plasma jet from a single CS in vacuum arc under external axial magnetic field (AMF) are studied. A multi-species magneto-hydro-dynamic (MHD) model is established to describe the vacuum arc. The anode temperature is calculated by the anode activity model based on the energy flux obtained from the MHD model. The simulation results indicate that the external AMF has a significant effect on the characteristic of the plasma jet. When the external AMF is high enough, a bright spot appears on the anode surface. This is because with a higher AMF, the contraction of the diffused arc becomes more obvious, leading to a higher energy flux to the anode and thus a higher anode temperature. Then more secondary plasma can be generated near the anode, and the brightness of the ‘anode spot’ increases. During this process, the arc appearance gradually changes from a cone to a dumbbell shape. In this condition, the arc is in the diffuse mode. The appearance of the plasma jet calculated in the model is consistent with the experimental results.

Numerical and Experimental Investigation of the Thermal Conductivity Affecting on Current Chopping and Transient Recovery Voltage in Low Current Vacuum Arc for Ag-Pd Compound Cathode Using Cathode Spot Model

This analysis describes the physical mechanism of the effect of thermal conductivity on instability phenomena using the cathode spot model. An analysis of stable arc current is performed by changing the thermal conductivity of the Silver cathode by compound material. It has been found out that the minimum stability arc is inversely proportionate to the thermal conductivity. These results have confirmed that stable arc current has been approved by the cathode spot model, and it can be used to design an amount of weight percentage of the compound cathode for reducing the chopping current. Moreover, the minimum stable arc current decreases with the decrease of thermal conductivity. These results reflect directly on the current chopping level. These show that if the chopping current is reduced, then the peak of transient recovery voltage will be reduced. It has been found out that the instability arc current is dependent on cathode characteristics. This is a significant result for the development of cathode materials for low-surge vacuum interrupters. Finally, it can be concluded that the results obtained in this study clearly demonstrate the physical mechanism of the current instability and transient recovery voltage occurrence with reducing cathode thermal conductivity.

Study of formation mechanism of double metal plasma jets in a low-current pulsed vacuum arc discharge

Plasma jet formation was studied in a vacuum arc configured with a conical cathode located inside a hollow cylindrical anode. The outside of this anode was insulated, except at a 0.4 mm diameter micropore. The grounded vacuum chamber also served as an anode. The hollow cylindrical anode was connected to the grounded chamber anode through a resistor R. 170 A 14 μs arcs were excited by a pulse generator comprised of a series connection of a 0.1 F capacitor bank charged to 12 kV, a 200 μH inductor, and a 30 Ω resistor. Two plasma jets formed during the arc discharge, one originating from the cathode tip and the other through the micropore. It was found that when R was increased from 0 Ω to 1 MΩ, (1) the plasma jet originating at the micropore weakened, (2) the peak current to the hollow cylindrical anode decreased from 73 A to 0, and (3) the peak arc current decreased from 176 A to 150 A. Plasma jet velocities for R = 0 were inferred from the difference between electron current peak times at two probes, to be 9 km/s for the plasma jet from the cathode and 27 km/s for the plasma jet at the micropore.

Three-dimensional hybrid simulation of single cathode spot vacuum arc plasma jet under axial magnetic field

Vacuum arc is a special metal vapor discharge phenomenon, because its discharge medium totally comes from the evaporation and ionization of electrode materials. In the case of low current, the vacuum arc is completely composed of plasma jets emitted from discrete cathode spots on the cathode surface and the current carried by each spot depends on the cathode material. When the arc current exceeds a certain value, a certain number of cathode spot plasma jets will appear. Vacuum arcs play a very important role in some industrial applications such as vacuum circuit breakers, vacuum coatings and electric thrusters. As an important plasma control method, the external axial magnetic field (AMF) has an important influence on the macroscopic morphology and microscopic parameter distribution of the vacuum arc. Various studies of vacuum arc under AMF have been carried out and some progress has been made. However, the existing literature about the simulation research of vacuum arc is mostly concentrated in the case of large current, and less attention is paid to the case of small current. The reason is that the traditional methods, magneto-hydrodynamics or particle-in-cell, are limited by either accuracy or efficiency, and cannot be effectively applied to the low current vacuum arc plasma jet simulations. In this paper, we develop a fully three-dimensional hybrid plasma simulation algorithm to study the single cathode spot vacuum arc plasma jet under AMF. In this model, ions are modelled as particles while electrons are treated as massless fluid, and the self-generated magnetic field is also considered. To simplify the condition, the cathode spot in our model only exists as a plasma jet source, thus the detailed mechanism of producing plasmas is neglected. And the movement of the cathode spot is not considered either. The results show that the single cathode spot plasma jet diffuses into the interelectrode in a cone shape after leaving the cathode spot, and the ion density drops rapidly from cathode to anode. Under the simulation conditions in this paper (&lt;i&gt;I&lt;/i&gt; ≤ 150 A), the self-generated magnetic field will not have a significant influence on the plasma jet itself in the case of low current. The external AMF has a compressive effect on the diffusion of the vacuum arc plasma jet. Under the AMF, the radial movement of the ions is suppressed, and the decrease of the ion radial velocity leads to a smaller diffusion radius of the jet. This compression effect of the AMF on the plasma jet is related to both the intensity of the external AMF and the magnitude of the arc current. In the case of a constant arc current magnitude, the compression effect gradually increases as the value of the AMF intensity gradually increases; in the case of a constant value of the external AMF, the compression effect gradually decreases as the current gradually becomes larger.

A magnetohydrodynamic model of a low current vacuum arc jet with a deuterium-impregnated Zr cathode

This work investigates the characteristics of the vacuum arc jet (VAJ) with deuterium-impregnated Zr electrodes. The magnetohydrodynamic model is used considering different components including ions, electrons, and atoms. The radius of the arc root is assumed to be 1–1.5 mm, and thus, the cathode spots and the cathode spot mixing zone are not studied. The density distributions of different components are calculated to obtain the average charge number. Simulation results are discussed to analyze some experimental results. The simulation results show that atom density decreases rapidly near the cathode side. With a more focused beam, the electron temperature increases significantly, leading to more highly charged ions and a higher average charge number. Compared with pure Zr electrodes, deuterium-impregnated electrodes can emit light D ions that can increase the ion velocity and decrease the electron temperature. Therefore, the average charge number of Zr is lower. With more deuterium in the cathode, the arc voltage is lower. However, the arc column cannot change the fraction of deuterium in the VAJ, indicating that the high deuterium fraction in the experiments is caused by the deuterium emitted from the adjacent area of the cathode spots.

Effects of Homopolar Magnetic Fields on Low-Current DC Vacuum Arcs

This article deals with the effects of homopolar magnetic fields on the lifetime of low-current dc vacuum arcs. A homopolar magnetic field is a distribution with the axially symmetric radially oriented field in the contact gap. It is produced when two dipole magnets are kept with the same poles facing each other. Switching experiments are conducted using an off-the-shelf ac vacuum contactor, and the ring magnets are used to set up a homopolar magnetic field in the contact gap. The arcing times are measured for currents varying from 1 to 30 A with voltages up to 100 V and magnetic surface strengths from 0.34 to 0.43 T. The tests are performed with the fixed contact of the vacuum contactor connected to positive as well as negative polarity. It is observed that the arcing time values are lesser in the presence of magnetic field with the fixed contact at the negative polarity due to asymmetry in the placement of the ring magnets. Finite element method (FEM) simulations are performed to examine the magnetic field distribution in the contact gap. The ratio of radial-to-axial magnetic field components in the contact gap decreases with increasing strength of magnets in the present experimental arrangement. It correlates well with the observed arcing time trends against the variation in the strength of magnets and the polarity of the fixed contact. The present article shows the potential of homopolar magnetic field in the development of dc vacuum contactors and interrupters without the need to create an artificial current zero using forced commutation.

Dependence of the plasma composition in the low-current vacuum arc discharge on CuCr cathode on the current value and surface conditions

The dependence of the ion flux mass-charge composition from the vacuum arc plasma with the CuCr cathode on a value of the discharge current was investigated via the Thomson spectrometer with automatic signal registration and analysis. The microsecond arc was formed in a millimeter gap between CuCr cathode and copper anode under high vacuum conditions. The arc current was changed within the range from a few to tens of ampere. At the highest currents the plasma composition corresponded the elemental composition of the cathode surface. The fraction of the chromium ions significantly decreased with the arc current decreasing below 30 A. There was noticeable increasing of the fraction of the chromium ions in the plasma at the discharge currents near the threshold values for the cathode materials. The effect appeared on coarse-grained non-melted cathode surfaces, and vanished after a multiple arcing.

A Simple Model of Distribution of Current Over Cathodes of Vacuum Circuit Breakers

There are several hundreds of spots operating simultaneously on cathodes of vacuum arcs in high-power vacuum circuit breakers. In this work, the spot distribution along the contact surface is simulated by means of an approach that is based on the concept of surface density of spots and represents a natural alternative to tracing individual spots. An equation governing the evolution of the surface density of the spots or, equivalently, the distribution of macroscopic (averaged over individual spots) current density over the cathode is obtained by generalizing the concept of random walk of a single cathode spot in low-current vacuum arcs. The model relies on empirical parameters characterizing individual spots (the diffusion coefficient of the random motion of cathode spots and the velocity of drift superimposed over the random motion), which may be taken from experiments with low-current arcs, and does not involve adjustable parameters. The model is simple and physically transparent and correctly reproduces the trends observed in the experiments under conditions where the cathode arc attachment is diffuse. The distribution of the macroscopic current density on the cathode, given by the model, represents the boundary condition that is required for existing numerical models of vacuum arcs in high-power vacuum circuit breakers.

Analysis of Instability Phenomena at Current Interruption in Vacuum Arc Discharge Compared With Silver or Copper Electrode

Vacuum circuit breakers have an issue of instability arc current and chopping phenomena when the current is cut off from the circuit. To clarify the effect of the instability low-current vacuum arc for circuit breaker development, the instability mechanism of low-current vacuum arc for various parameters was proposed to verify consequent affecting circuit breaker capacity in this paper. This paper used the cathode spot model neglect the transition region between the collisionless sheath and collisional plasma region. The eight equations were used for solving the dependent variable of plasma parameters for both silver and copper electrode materials. The low ionization was supposed in the cathode spot model. The results showed that the changes in the work function and ion current fraction at ambient temperature did not have the main influence on the arc stability limit current both in the silver electrode and copper electrode. On the contrary, the changes in the heat conductivity at ambient temperature affecting the arc stability limit current significantly both in the silver and copper electrodes. Furthermore, the results were found out that the cathode spot can be confirmed unstable current phenomena when the current below 19 A for copper cathode and below 19.4 A for the silver cathode. These phenomena can be explained that the reversed electrons flow from the plasma side to sheath region affecting the electric field at cathode spot become negative zone; thus, the starting noise on the current trace appears before chopping current.

Experimental investigation on the characteristics of the plasma jet of a low-current vacuum arc in axial magnetic fields

In this paper, the characteristics of the plasma jet of a low-current vacuum arc with a single cathode spot (CS) in an external axial magnetic field (AMF) up to 150 mT is investigated experimentally, at a constant arc current ranging from 20 A to 60 A. The experiments are conducted with Cu butt contacts in a demountable vacuum chamber. Images of the plasma jets are photographed with a high-speed digital camera with an exposure time of 2 μs. The uniform constant AMF (Bn) within the inter-contacts region is supplied by Nd–Fe–B permanent magnets. The influence of the external AMF on the shape of the jet near the anode surface as well as in the arc column is mainly investigated. A luminous ‘spot’ is observed on the anode surface facing the position of the CS under a relatively strong AMF. The mechanism of the appearance of the luminous ‘spot’ is proposed to be connected to the secondary plasma originating from the anode. Moreover, with the increase in the strength of the AMF, the spreading angle of the cone-shaped plasma jet in the arc–column region decreases gradually. The plasma jet, subjected to a relatively strong AMF (120 mT and 150 mT), becomes cylindrical in shape in the arc–column region and conical in shape in the near-electrode regions. The overall geometry of the plasma jet looks like a dumbbell.

Pre-Explosion Phenomena Beneath the Plasma of a Vacuum Arc Cathode Spot

A 2-D hydrodynamic model has been developed that describes the pre-explosion processes in a microprotrusion of a vacuum arc cathode based on a self-consistent calculation of the electric potential drop in the near-cathode region. The model includes a calculation of the cathode temperature in view of the surface heat fluxes carried by electrons and ions during the interaction of the cathode surface with the cathode spot plasma and the Joule heating of the cathode. The near-cathode space charge sheath is considered in the 1-D local Bohm approximation. It has been shown that the heat flux from a cathode plasma having parameters characteristic of low-current vacuum arcs can induce thermal instability (thermal runaway) in a cathode microprotrusion and heat it to a critical temperature within some tens of nanoseconds. Comparative analysis of the volumetric Joule mechanism and the surface electron-plasma mechanism underlying the development of thermal instabilities in a cathode has been performed in one numerical experiment. It has been shown that the instability induced by the surface mechanism can arise at lower densities of the cathode spot plasma and its growth rate is lower compared with the instability induced by the Joule mechanism.

Erosion behaviour of a Ti3SiC2 cathode under low-current vacuum arc

In this article, the arc erosion behaviour of high-purity Ti3SiC2 in vacuum was investigated by x-ray diffraction, scanning electron microscope, energy dispersive x-ray spectroscopy, and micro-Raman spectroscopy. From the results obtained, Ti3SiC2 is unstable due to the high energy intensity and high temperature of the vacuum arc. The dissociation of Ti3SiC2 takes place at the sample surface, resulting in the formation of solid TiCx and gaseous Si. TiCx is ejected from cathode to the surface of anode while Si is evaporated to the vacuum chamber. The micro-Raman results reveal that small amounts of carbon appeared as a by-product of the dissociation of Ti3SiC2, indicating that the Ti–C bonding is broken down under the vacuum arc.

Numerical Simulation of Low-Current Vacuum Arc Plasma

Three-dimensional magnetohydrodynamic(MHD) model of vacuum arc was built based on two-fluid model of ion and electron and Maxwell equation. In the MHD model mass conservation equation, momentum conservation equations, energy conservation equations of ion and electron, electric potential equations and magnetic equations were considered. With the aid of Computational Fluid Dynamics(CFD) software FLUENT and Re-development by visual C++, the important plasma parameters of low current vacuum arc , such as axial current density , ion temperature, electron temperature, mach number, were analyzed. The simulation results shown that the distribution of plasma parameters is consistent with that of the 2D axis-symmetric model.