Anode thermal process in high current vacuum arc under low frequency AC discharge condition

Vacuum arcs are well influenced by a nearby ambient magnetic field. This effect may cause a burn-out of the shield in a vacuum interrupter or even cause a failure interruption. The objective of this paper is to determine an influence of a nearby ambient magnetic field, induced from a conductor parallel assembled to a vacuum interrupter, on the high-current vacuum arc characteristics in the VI. A copper rod conductor is assembled in parallel but electrically connected in series with a test vacuum interrupter. The distance between the conductor rod axis and the test vacuum interrupter axis was controlled as 60 mm, 100 mm and 140 mm, respectively. In the vacuum interrupter, cup type axial magnetic field electrodes were used, which has a contact material CuCr30 and the contact diameter was 42 mm. Contact gap was set to 6mm and 12 mm, respectively, in order to compare an influence of the ambient magnetic field on the high-current vacuum arc behaviors. Arc current is provided by a single-frequency LC discharging circuit, which provides an arcing time of 10.4 ms. And the arc current increased from 3.1 kA to 24.8 kA, which stepped by 3.5 kA. Experimental results showed that with a stronger ambient magnetic field, the arcing energy variation was higher and the arc voltage noise lasted a longer duration. As the ambient magnetic field increased from the minimum case (No copper rod) to the case of axial distance 140 mm, 100 mm and the strongest case 60mm axial distance, the arc energy variation range increased from 0.66 kJ to 1.25 kJ, 1.36 kJ and 1.78 kJ, respectively, at 10.3 kA with the 12 mm contact gap distance. Correspondingly, the arc energy variation range increased from 0.82 kJ to 0.75 kJ, 0.84 kJ and 1.07 kJ at 17.8 kA with the 6mm contact gap distance. And the arc-voltage noise duration increased from 2.6 ms to 4.5 ms at 18.1 kA with the 12 mm contact gap distance, and the duration increased from 2 ms to 3.8 ms at 17.8 kA with the 6mm contact gap distance.

The objective of this paper is to compare the electron temperature and electron density in diffuse vacuum arc mode and anode spot mode. The experiment was based on the butt type copper electrodes, with contact diameter D=40mm, and gap distance g=10mm. The vacuum arc was ignited by a needle tungsten electrode, which was embedded in the cathode. A high-voltage pulse with a rise time of 8ns, and aptitude of 24kV was applied on the needle electrode. The arc current varied from 2.8kA to 7.5kA (rms). The arc current was measured by Hall sensor and the arc voltage was measured by Tektronix P6015A, respectively. The arcing process was recorded by a high-speed charge-coupled device (CCD) video camera. The emission intensities of the interelectrode plasma of the vacuum arc were collected by an optics spectrograph. The Bolzman plot method was used to calculate the electron temperature and the Saha equation was used to determine the electron density of diffuse vacuum arc mode and anode spot mode. The electron temperature was 1.09eV in diffuse arc mode and it was 1.22eV in anode spot mode. The electron density was increased from 7.6 × 1015/cm3 to 2.1 × 1015/cm3 when the arc mode changed from diffuse arc mode to anode spot mode.

The vacuum or metal-vapour arc between butt electrodes of 6?75mm diameter, at separations up to 40mm, has been studied experimentally throughout a range of sinusoidal current loops up to 100kA peak. The appearance and properties of the copper-vapour discharge are shown to vary widely over this current range. The well known diffuse low-voltage vacuum arc which exists at a few hundred amperes develops into a constricted higher-voltage form as the current is increased, and, during 50Hz loops in excess of about 10KA peak, the arc produces grossly melted electrode spots at temperatures which are shown to be close to the boiling point of copper. High-pressure magnetically constricted arc columns develop from these large electrode spots, producing a discharge which bears many resemblances to a high-pressure high-current arc. Theoretical interpretations of these findings are presented in the companion paper.

This paper discusses the issues of a possible change of the type of cathode attachment of high-current vacuum arcs (HCVA) with an average cathode current density of more than 105 A/cm2. This type of HCVA is used as pumping plasma gun in experiments with plasma puff z-pinches. These experiments showed that the measured linear mass of the HCVA plasma jet is much higher (by a factor of 10 or more) than the expected mass, which can be obtained from the assumption that cathode attachment occurs only through a multitude of cathode spots emitting supersonic plasma jets. It is shown that in HCVA of the type under consideration, at some time instant there are two types of cathode attachments - cathode spots and thermionic erosion attachment (TEA). It can be said that HCVA of this type have a hybrid cathodic attachment. Unlike cathode spots, TEA produces a subsonic plasma flow, which contributes to an increase in the linear mass of the HCVA plasma jet.

Anode melting and crater formation significantly affect interruption of high-current vacuum arcs. The primary objective of this paper is to theoretically investigate the mechanism of anode surface crater formation, caused by the combined effect of surface heating during the vacuum arc and pressure exerted on the molten surface by ions and electrons from the arc plasma. A model of fluid flow and heat transfer in the arc anode is developed and combined with a magnetohydrodynamics model of the vacuum arc plasma. Crater formation is observed in simulation for a peak arcing current higher than 15 kA on 40 mm diam. Cu electrodes spaced 10 mm apart. The flow of liquid metal starts after 4 or 5 ms of arcing, and the maximum velocities are 0.95 m/s and 1.39 m/s for 20 kA and 25 kA arcs, respectively. This flow redistributes thermal energy, and the maximum temperature of the anode surface does not remain in the center. Moreover, the condition for the liquid droplet formation on the anode surfaces is developed. The solidification process after current zero is also analyzed. The solidification time has been found to be more than 3 ms after 25 kA arcing. The long solidification time and sharp features on crater rims induce Taylor cone formation.

The capabilities and advantages of vacuum interrupters have been widely recognized on switching and controlling the fault currents in medium voltage level, and there have been doing various research and developments for using vacuum interrupters in transmission and distribution lines due to its environmental friendliness. Axial magnetic field electrodes are widely used in vacuum interrupters because the axial magnetic field can keep vacuum arc in a diffuse arc mode at high current level. It can be created by the design of electrodes itself or by external coils of a vacuum bottle where current flows. The interruption capacity of the vacuum interrupter can be increased by the magnitude of the field since the arc gets a diffused mode when the field between a contact gap is enough sufficiently. In this study, three-dimensional coupled-fields analysis of high-current vacuum arcs with an axial magnetic field which is produced by a cup-type electrode itself was conducted. We used a commercial multiphysics package, ANSYS, for coupling the fields which are connected with electromagnetics and hydrodynamics sequentially. It was found that arc constriction with the magnitude of applied currents could be predicted well with the temperature distribution of a contact gap.

To improve the limiting current interruption capability and minimizing vacuum interrupter with axial magnetic field (AMF) electrodes, it is significant to investigate the vacuum arc behaviours between the contacts. AMF distributions of the slot type electrodes were studied by both numerical analysis and experiments. Furthermore, the behaviours of vacuum arcs for different parameters of the slot type AMF electrodes were investigated by using high-speed CCD camera. The influences of gap distance, contact diameter and phase shift time between AMF and arc current on the vacuum arc were investigated. The results provide a reference for research and development of vacuum interrupters with slot type or other types of AMF electrode.

Generation of multiply charged metal ions in a high current short pulse vacuum arc

Anode activity is critical in high-current vacuum arcs in vacuum interrupters. If the anode temperature gets too high, an evaporation of metal vapor from an anode melting pool may lead to a formation of anode spots, which would lead to a failure of current interruption of vacuum interrupters. The input energy from an arc column to an anode surface is adjusted by an anode sheath. The objective of this paper is to develop a numerical simulation model of the anode sheath of vacuum arc by using a Particle-in-Cell method. A low-current case and a high-current case of arcing are simulated. The physical parameters of electrons and ions are obtained from magnetohydrodynamic simulations, and are set at the simulation boundary of the current model. The sheath thickness is 2∼4 Debye length, and the anode potential fall is about 1.5∼3 V for both case. It is found that the acceleration of ions by electric field in the anode sheath is more significant for the high-current case. This also adds to the energy flux into the anode surface.

The effect of the angular distribution of cathode ions on the parameters of a vacuum arc is investigated. It is shown that, at current densities characteristic of a high-current arc, collisions of flows of fast ions from different cathode spots may result in a high value of the ion temperature at the cathode plasma boundary. The boundary in current density of the region of a stable current flow as a function of the width of angular distribution of cathode ions is determined. It is found that, in the region of supersonic flow of ions, the permissible value of the current density drops rapidly as the angular distribution width increases, and in the subsonic region, it rises rapidly. An expression is obtained for the maximum possible ion sound velocity and for the equivalent critical ion velocity. Two- and three-fluid hydrodynamic models are used to perform calculations that illustrate characteristic distributions of parameters in the discharge gap. Analysis of the calculation results makes it possible to discuss the characteristic features of current flow in a vacuum discharge.

This work investigates the interaction between high current vacuum arc (HCVA) and the macroparticle (MP) generated from the cathode spots (CS) in the small-size vacuum trigatron. Magnetohydrodynamic (MHD) model is used to describe the arc column. MPs are added into MHD model, which are described by discrete phase model (DPM). The control equations of MPs include charging equation, motion equation and heat transfer equation, in which the evaporation of MPs is also considered. Based on the above model, the effects of MP diameter and arc current on the characteristics of MPs are studied. Simulation results show that MPs leave the interelectrode gap with a deflection angle larger than the initial angle under the influence of plasma. The motion of MPs is similar to that of projectiles. MPs with smaller diameter are influenced by the arc plasma more significantly. MPs can act as a source of the plasma, but these influences are not obvious in HCVA. What may be more harmful to the trigatron is the charging process of MPs during post-arc process.

Large-capacity vacuum generator circuit breakers (GCBs) are essential for the safe operation of power systems, yet extremely high short-circuit currents and steep transient recovery voltage (TRV) rise rates pose challenges to current interruption. In this work, an experimental platform for high-current vacuum arcing and post-arc transient dielectric recovery was established. High-current tests with the peak current of 100 kA were conducted using 80 mm diameter axial magnetic field contacts, and the evolution of arc morphology was recorded by a high-speed camera. Based on arc motion and volt-ampere characteristics, the arcing process was divided into five evolutionary stages. Repetitive tests under identical conditions revealed, for the first time, the existence of arcing uncertainty. The influence of arc morphology evolution on arcing uncertainty at different stages was clarified, showing that the vacuum arc possesses an inherent self-regulating capability that gradually transforms highly uncertain states into less uncertain ones. In addition, an arcing memory effect was identified. The influences of arcing energy and arc morphology on post-arc transient dielectric recovery were elucidated, demonstrating that arcing uncertainty is the key factor responsible for its dispersion. Finally, design considerations for the large-capacity GCB were proposed from the perspectives of arc behavior control and safe post-arc interruption, providing a basis for the engineering application.

Phenomena in a molten bridge between electrical contacts at the stage preceding vacuum arc ignition and their influence on the arc duration are investigated using a mathematical model and experimental data. Two models of a bridge are considered and compared. The first model is based on the representation of a bridge as a melting filament between contact surfaces, while the second one describes bridging as extension of a liquid metal drop between contacts. The initial conditions for the temperature fields of electrical contacts at bridging start are formulated taking into account heat transfer in solid contacts before bridging. Evolution of a bridge during contact opening, its length, radius, time and coordinates of bridge rupture at boiling temperature are evaluated depending on electrical current, opening velocity and parameters of contact materials. The above mentioned mathematical model enables estimatation of the density of metal vapors in contact gap just after bridge explosion, which is responsible for vacuum arc ignition and maintenance at the initial stage of arc burning. It was found that bridge duration and anode arc duration is interrelated. There exists a critical contact gap, which becomes too large to support the power capacity required for the maintenance of extending zones of anode melting and evaporation. If the length of a bridge exceeds this critical gap, then the anode arc phase could be not appearing at all. On the contrary, if the bridge length is negligible or if the bridge does not appear at all, then duration of anode arc phase is maximal.

In this paper, a new type of contact structure for vacuum interrupters, sinusoidal surface contact, is proposed, and its vacuum arc characteristics at intermediate frequency are studied. Then the arc mode evolution, arc voltage characteristics, arc energy, breaking performance, the ablation of anode contact and arc product condensation ability of the sinusoidal curved contact and the plate contact with the same contact diameter (41mm) and the material are compared and analyzed under small open distance. The correlations between the arc energy and the amplitude and frequency of the current and the influence of the arc energy on the breaking capacity are achieved. The results showed that the sinusoidal curved contact has strong application potential by its lower arc energy, lower arc voltage noise and arc voltage peak, lighter ablation on the surface of the anode contact and splash, stronger breaking capacity.

We experimentally investigated the influence of contact contour on breakdown characteristics in vacuum under uniform field. Four vacuum interrupters were used and their contact contours were designed to obtain an effective area (Seff) from 211 mm2 to 550 mm2. The contact parameters were contact diameter 42.8 mm, contact gap 6 mm, contact thickness 16 mm and contact material CuCr50. Basic impulse level (BIL) voltage (1.2 × 50 ?s) was applied. Experimental results revealed that the effective area Seff had a significant influence on the conditioning process. The larger the effective area was, the slower the conditioning process was. When the breakdown voltage saturated, the breakdown probability distribution followed a Weibull distribution. The shape parameter that represented the breakdown voltage scatter was constant, independent of the effective area. The effective area (Seff) had no significant influence on the breakdown voltage under a uniform field in vacuum.