kA Rms Research Articles

Effect of six pure metals cathode on constricted characteristics of high-current vacuum arcs subject to axial magnetic field

The objective of this paper is to quantitatively determine the effect of the six pure metals cathode on the constricted characteristics of high-current vacuum arcs subject to axial magnetic field (AMF). To measure the current density in anode surface, a butt-type split-contact with four axisymmetrical areas was selected as the anode, with the material of CuCr25. A plate contact was selected as cathode, under six pure metals material, including Cu, W, Mo, Fe, Cr, Al, respectively. The experimental arc current was 10 kA (rms) at 50 Hz. The opening velocity was 2.0 m s−1. The currents of the four areas in the anode surface were measured using four Rogowski coils situated outside the vacuum chamber. An external uniform AMF of 74 mT was applied during the experiment. The observed arc modes were recorded by a high-speed CCD video camera. The experimental results indicate that, in the diffuse arc mode, the cathode materials effect the constricted characteristics of the vacuum arcs. The vacuum arc column with the cathode of W and Mo constricted more severe than that in Cu, Fe, Cr and Al. For the cathode of Cu, Fe, Cr and Al, the uneven distribution of anode current density was mainly at the previous stage before the current peak. After the current peak, the difference of the average current density between the central area to other three periphery areas reduced continually. However, for the cathode of W and Mo, the uneven distribution of anode current density maintained during the whole arcing time. Moreover, the influence of the six pure cathode contacts on the constricted characteristics mainly focused on the spreading angle of the plasma jet and the numbers of the jet sources in the cathode surface. With a greater number of cathode spots, the distribution of the arc current density should be more uniform.

Experimental Investigation on Triggered Vacuum Arc and Erosion Behavior Under Different Contact Materials

In order to improve the breaking capacity of contacts, it is very important to find a better contact material. In this paper, the triggering experiments of plate contact with different materials are studied to investigate the characteristics of vacuum arc. The current range is from 1 to 9 kA (rms). The gap distance is selected as 3 and 5 mm. The arc appearance and the behavior of cathode spots are captured by high-speed camera with an exposure time of $2~\mu \text{s}$ . The surface topography of contacts after experiments is evaluated by scanning electron microscope. The arc current and arc voltage are measured at the same time by Rogowski coil and high-voltage probe. The diameter of the contact is 25 mm. There are seven kinds of materials (CuCr10, CuCr25, CuCr30, CuCr40, WCu10, WCu30, and CuW–WC) conducted in this paper. Compared with the experimental results, the arc of WCu alloy distributes more evenly on the contact plane with many bright cathode spots. The small gap distance makes the burning point more fixed. The arc of CuW–WC is unstable. During the arc trigger moment to the arc stable burning stage, the radial diffusion velocity of WCu10 cathode spot is the lowest with about 25% of CuCr10. CuCr10 alloy has the worst melting resistance. WCu alloy has good melting resistance and bad toughness. The toughness of CuW–WC is even worse. Under the low current, CuCr30 might be preferred for vacuum interrupters from the cathode spots’ radial diffusion speed, arc appearance, and erosion result. When the current increasing, the arc can easily spread to the entire surface of WCu contact, and WCu10 would be the prefer material with the diffusion arc and good erosion resistance.

Simulation results of influence of constricted arc column on anode deformation and melting pool swirl in vacuum arcs with AMF contacts

In the process of vacuum arc breaking, the energy injected into the anode will cause anode melting, evaporation, and deformation, resulting in the formation of the anode melting pool. The anode activities have great influence on the arc behavior. When the arc current is large enough, even the influence of axial magnetic field is considered, the arc column still is in contraction state, which means the arc burns only on a part of the electrode. In this paper, the model of anode melting pool deformation and rotation is used, and the model includes anode melting and solidification module, magneto-hydro-dynamic module of the anode melting pool, the volume of fraction method, and the current continuity equation. In this paper, the diffuse arc area is selected as 100%, 75%, and 50%, respectively. The anode temperature and deformation, the anode melting layer thickness, and the rotational velocity of the anode melting pool are obtained. The results show that when the current is at 17.5 kA (rms) and the diffuse arc area is 100%, the anode's maximum temperature is 1477 K and the crater depth is 0.83 mm. But when the diffuse arc areas are 75% and 50%, the anode's maximum temperatures reach 1500 K and 1761 K, and the crater depths reach 1.2 mm and 3 mm, respectively. Arc contraction will lead to more serious anode deformation. A similar result is obtained when the simulation current is 12.5 kA. Under the similar situation, the simulation results in the crater depth, the residual melt layer thickness, the rotational speed of the melting pool, and the maximum temperature of the anode at current zero are in good agreement with the experimental results.

Surface Temperature Analysis of Transversal Magnetic Field Contacts Using a Thermography Camera

Some vacuum interrupters (VIs) are equipped with transversal magnetic field contacts. A transversal directed Lorentz force makes the arc move circularly. The local surface temperature of the arc root is usually above the melting point at high current. The temperature of the heated and molten contact surface of a VI can be measured using a high-speed thermography camera. We present a methodology for determining the surface temperature. For this purpose, the emissivity of the contact material must be known. For Cu, we determined the emissivity at $\varepsilon _{\mathrm {liq}} = 0.053$ (liquid) and $\varepsilon _{\mathrm {sol}} = 0.035$ (solid) for a spectral wavelength range of 1.5–1.7 $\mu \text{m}$ . Switching tests were performed using spiral contacts (CuCr 75/25) with a diameter of 68 mm and a 50-Hz half-wave sinusoidal current of 19 kA (rms). We examined the temperature of cooling anode spots of two different switching tests with various arc burning times and arc diameter. During the cooling phase of the anode spots, we observed a plateau in temperature at both measurements with different slope and duration. The plateaus started at the liquidus temperature of 1903 K of CuCr 75/25.

Effect of an axial magnetic field and arc current on the anode current density in diffuse vacuum arcs

The objective of this paper is to measure the effect of an axial magnetic field (AMF) BAMF and arc current on the anode current density in diffuse vacuum arcs. The experimental geometry included a split anode and a butt-type cathode, both with a diameter of 60 mm. The anode surface was divided into a central area and three symmetrically disposed peripheral annular areas. The central area of the split anode had a diameter of 20 mm. The contact material was CuCr25 (25% Cr). The arc current IARC ranged from 4 to 14 kA (rms) at 50 Hz. The opening velocity was 2.4 m/s. The currents of the four areas on the anode contact were measured using four Rogowski coils situated outside the vacuum chamber. An external uniform AMF BAMF ranging from 0 to 110 mT was applied during the experiment. The observed arc modes were recorded by a high-speed charge-coupled device video camera. The experimental results quantitatively reveal that the current density distribution on the anode surface in the diffuse arc mode was not uniform but concentrated in the central area. The current density in the central anode area at the current peak JPeakArea I decreased with increasing BAMF following a power law. For BAMF of 0–110 mT and IARC of 4–14 kA, JPeakArea I = (2.2 IARC + 0.069 IARC2) BAMF−0.22, where JPeakArea I is in A/mm2, BAMF is in mT, and IARC is in kA. Moreover, the current distribution was uneven in the three peripheral areas.

Experimental investigation and numerical simulation of triggered vacuum arc behavior under TMF/RMF-AMF contact

A series of triggering experiments was carried out to investigate the characteristics of vacuum arc controlled by TMF/RMF-AMF contacts. During all the experiments, the current ranged from 5–20 kA (rms) and both the arc appearance and behavior of cathode spots were captured by high-speed camera with corresponding arc current and arc voltage. A 3D steady magnetohydrodynamics (MHD) model was built to simulate and analyze the vacuum arc behavior under TMF/RMF-AMF contacts, and arc plasma parameters were calculated based on the above model. The experimental results showed that arc deflection was visible under both low and high current. Under high current, arc core formed, which meant the arc contracted significantly. In addition, the anode became much more active under high current. The behavior of the cathode spots showed that they split themselves into other new cathode spots. Under high current, the bulk of the spots rotated along a clockwise direction on a transverse magnetic field (TMF) plate, which caused much noise and oscillation in the arc voltage. The simulation results show that ions are likely to gather on the branches of the TMF plate on the anode plane, as a result of the effects between the electromagnetic force and pressure gradient of the arc plasma. The current contracts in the center of the TMF plate on the cathode which was due to the thin connecting rod there. The anode contraction of the current is caused by the Hall effect. Ions move along a clockwise direction on the TMF plate, which is driven by Ampere force. The current contraction resulted in significant melting in the center of the cathode surface while the other region suffered from uniform melting. The melting caused by the anode contraction is more significant than that of the cathode.

Modeling of the anode surface deformation in high-current vacuum arcs with AMF contacts

A high-current vacuum arc subjected to an axial magnetic field is maintained in a diffuse status. With an increase in arc current, the energy carried by the arc column to the anode becomes larger and finally leads to the anode temperature exceeding the melting point of the anode material. When the anode melting pool is formed, and the rotational plasma of the arc column delivers its momentum to the melting pool, the anode melting pool starts to rotate and also flow outwards along the radial direction, which has been photographed by some researchers using high-speed cameras. In this paper, the anode temperature and melting status is calculated using the melting and solidification model. The swirl flow of the anode melting pool and deformation of the anode is calculated using the magneto-hydrodynamic (MHD) model with the volume of fraction (VOF) method. All the models are transient 2D axial-rotational symmetric models. The influence of the impaction force of the arc plasma, electromagnetic force, viscosity force, and surface tension of the liquid metal are all considered in the model. The heat flux density injected into the anode and the arc pressure are obtained from the 3D numerical simulation of the high-current vacuum arc using the MHD model, which gives more realistic parameters for the anode simulation. Simulation results show that the depth of the anode melting pool increases with an increase in the arc current. Some droplets sputter out from the anode surface, which is caused by the inertial centrifugal force of the rotational melting pool and strong plasma pressure. Compared with the previous anode melting model without consideration of anode deformation, when the deformation and swirl flow of the anode melting pool are considered, the anode temperature is relatively lower, and just a little more than the melting point of Cu. This is because of liquid droplets sputtering out of the anode surface taking much of the energy away from the anode surface. The azimuthal velocity of the anode melting pool for arc current 12.5 kA root-mean-square (rms) is larger than that for 17.5 kA (rms), which is likely to be caused by the thinner liquid layer, and also a smaller melting pool mass of 12.5 kA.

Anode Current Density Distribution Measurements for Different Vacuum Arc Modes Subjected to Axial Magnetic Field

We report our measurements of anode current density distributions for different modes in vacuum arc discharge using a split-anode and a cup-shaped axial magnetic field cathode configuration system. The modes investigated included the diffuse arc and anode spot. The anode surface was divided into four regions: one central region and three symmetrical-disposed peripheral annular-arc regions. Three different-sized central regions were selected with diameters of 12, 18, and 20 mm. The contact material was CuCr25 (25% Cr). The arc current in the tests ranged from 6 to 14 kA (rms) at 50 Hz. The opening velocities were 1.8 and 2.4 m/s, respectively. The currents of the four areas on the anode contact were measured by four Rogowski coils situated outside the vacuum chamber. The experimental results show the current density distribution in different arc modes. For the diffuse arc mode with arc currents of 6 and 8 kA (rms), the average current density in the central region at current peak ranged from 8.5 to 15.9 A/mm $^{2}$ . The anode spot current density ranged from 31.9 to 37.7 A/mm $^{2}$ , estimated from the current density in the eroded region caused by the anode spots covering the entire central region of diameters 12 and 18 mm, at currents of 12 and 14 kA, respectively, regarded as the lower limit current density of the anode spot. Moreover, the dependence of the current density distribution on opening velocity in diffuse arcs was slight. For the anode spots, the current density distribution with higher opening velocity was more concentrated.

Anode Erosion Pattern Caused by Blowing Effect in Constricted Vacuum Arcs Subjected to Axial Magnetic Field

The objective of this paper is to investigate the anode erosion pattern caused by the blowing effect in constricted vacuum arcs under an axial magnetic field (AMF). The blowing effect was reflected from the microstructure of the melt layer on the anode. A pair of 42-mm-diameter AMF contacts were installed inside a detachable vacuum chamber. The contact material was CuCr25 (25% Cr). The arc modes were observed with a high-speed charge-coupled device video camera. The anode surface temperature after current zero was measured with a two-color pyrometer. The microstructure of the cross section of the anode surface melt layer was observed by scanning electron microscopy. The experimental arc currents increased from 10.5 to 17.8 kA (rms) with an interval of 2 kA. The results show that the anode erosion pattern in constricted arcs is caused by an interactive process of thermal and mechanical effects. The constricted arc columns melted the anode surface material and blew the molten material from the center to the peripheral region. The redistribution of the anode molten layer caused by this blowing effect formed two erosion regions on the anode surface. The depth of the melt layer in the central region was less than 100 $\mu \text{m}$ , but in the peripheral region it was in the range of 100–400 $\mu \text{m}$ . In these different erosion regions of the anode melt layer, there was considerable variation in both size and shape of the Cr sphere. Moreover, the thermal and mechanical effects of the constricted arc redistributed the Cr content of the molten layer on the anode surface. The Cr content in the central region (Erosion Region I) increased to 31% and in the periphery region (Erosion Region II) it decreased to 18%, respectively, from 25% before the experiments.

Magneto-thermal Coupling Modeling for the Stability Analysis of a 110 kV/3 kA High Temperature Superconducting Cable under Fault Current Conditions

In the power grid, the cold dielectric hightemperature superconducting (CD HTS) cable frequently subjects to overcurrents caused by the shortcircuit fault. During the fault, a large amount of Joule heat is generated. The heat causes the temperature of the HTS cable to rise and leads to the transition of the HTS cable from superconducting state to normal state which would influence the current carrying capacity of the HTS cable seriously. An algorithm has been proposed to calculate the transient current distributions of superconducting layers in fault conditions. This method couples an equivalent circuit model and a thermal model. So the temperature distributions in HTS cable is obtained based on the heat generated by the fault current. Finally, the current and temperature distributions of a 110 kV/3 kA CD HTS cable are investigated with a fault current impact of 25 kA rms for 3 s. The results show that the currents of the conducting layers decrease while the temperature increases in the HTS cable and the former withstands most of the fault current impacts. The current in the former of the HTS cable changes from zero to about 90 % of the fault current. A current diversion effect in the HTS cable is obvious. The conclusions provide the basis for the protection strategy of superconducting cable considering fault current impact and will be a benefit to the stable operation of a superconducting transmission grid.

Non-Intrusive Magneto-Optic Detecting System for Investigations of Air Switching Arcs

In current investigations of electric arc plasmas, experiments based on modern testing technology play an important role. To enrich the testing methods and contribute to the understanding and grasping of the inherent mechanism of air switching arcs, in this paper, a non-intrusive detecting system is described that combines the magneto-optic imaging (MOI) technique with the solution to inverse electromagnetic problems. The detecting system works in a sequence of main steps as follows: MOI of the variation of the arc flux density over a plane, magnetic field information extracted from the magneto-optic (MO) images, arc current density distribution and spatial pattern reconstruction by inverting the resulting field data. Correspondingly, in the system, an MOI set-up is designed based on the Faraday effect and the polarization properties of light, and an intelligent inversion algorithm is proposed that involves simulated annealing (SA). Experiments were carried out for high current (2 kA RMS) discharge cases in a typical low-voltage switchgear. The results show that the MO detection system possesses the advantages of visualization, high resolution and response, and electrical insulation, which provides a novel diagnostics tool for further studies of the arc.

Current Margin of 275 KV Class HTS Power Cable With Joint Against Fault Current

The current margin of a jointed model cable of the 275 kV class high-temperature superconductor (HTS) power cable was measured under fault condition. The cable-to-cable joint is necessary to connect long continuous sections of the power cable. At the joint the superconductor is prone to degradation due to an excessive temperature rise caused by the current passing through normal conducting zones of the joint. The jointed, and for the comparison the joint-less cables were subjected to a short-circuited fault current of 63 kA rms for 0.6 s. In the results, the allowable current margins were about the same for both model cables. The jointed cable degraded at the joint area from the result of the magneto-optical (MO) image. The numerical simulations were performed to clarify current distribution and temperature rise at the cable joint area. The temperature rise as a function of the joint gap was also calculated. The experimental and simulation results were showed in this paper.

The Influence of Axial Magnetic Field Distribution on High-Current Vacuum Arc

The influence of the distribution of axial magnetic fields (AMFs) was investigated experimentally with three pairs of specially designed testing electrodes generating a conventional bell-shaped AMF profile and different saddle-shaped AMF distributions. The characteristics of high-current vacuum arcs (HCVAs) up to 20 kA (rms), e.g., the shape of the arc column and the distribution of cathode spots, were investigated with the aid of a high-speed digital camera with an exposure time of 2 mus. Experimental results further manifested the distinct influence of AMF distribution on the characteristics of HCVAs. Saddle-shaped AMFs could resist the constriction of HCVAs more efficiently than traditional bell-shaped AMFs. Furthermore, cathode spots in saddle-shaped AMFs distributed more uniformly than that in bell-shaped AMFs. It was found that AMFs in the central region of saddle-shaped AMFs should not be too weak. Otherwise, cathode spots would be out of control in the central region as observed in experiments. The ldquooptimalrdquo minimal AMF in the central region was estimated to be about 3-4 mT/kA.

Development of 1 m HTS conductor using YBCO on textured metal substrate

We fabricated 1 m high temperature superconducting conductor (HTS conductor) using YBa 2Cu 3O 7−x coated conductors (YBCO tapes) on textured metal substrates, which are expected to be lower in cost than YBCO tapes using ion-beam assisted deposition. Those substrate and intermediate layers were manufactured by Furukawa Electric, and YBCO and a protective layer were applied to the intermediate layer by Chubu Electric Power. Before fabricating the conductor, a 0.1 mm thick copper tape was soldered to the YBCO tape, and 10 mm wide YBCO tape was divided into three strips by a YAG laser. To have sufficient current capacity for 1 kA, a two-layer conductor was fabricated, and its critical current ( I c) was 1976 A, but the magnetic properties of the textured metal substrates affected the increase in AC loss. In a low current region, the AC loss in this conductor was much higher than the Norris strip model, but approached the Norris strip model in the high current region because the magnetization was almost saturated. Low AC loss of 0.144 W/m at 1 kA rms was achieved even though the conductor had a small outer diameter of 20 mm and was composed of YBCO tapes with magnetic substrates.

Design study of a HTS cable in Yokohama project

A new high temperature superconducting (HTS) cable project supported by Ministry of Economy, Trade and Industry (METI) and New Energy and Industrial Technology Development Organization (NEDO) has started in Japan. Target of this project is to demonstrate the reliable and stable operate of a 66 kV, 200 MVA HTS cable in the grid. Tokyo Electric Power Company (TEPCO) provides the real grid and studies the impact of connecting the HTS cable to the existing conventional facilities in Yokohama. Sumitomo Electric Industries, Ltd. (SEI) manufactures the HTS cable, terminations and joint. Mayekawa Corporation provides a cooling system. One of the technical targets in the project is to reduce the AC loss of superconducting cable. For this purpose, a new type DI-BSCCO, which is a production name of BSCCO wires made by SEI, with twisted superconducting filaments is planed to be applied in the cable. A short cable core using the new wires shows less AC loss than 1 W/m/ph at 2 kA rms. Another important point is to design a cable system to withstand a large over current at a short circuit failure. At a preliminary test with a short cable, it was recognized that the cable had no degradation of conductor I c after applying the fault current of 31.5 kA for 2 s, which are required for 66 kV line. This report shows details of these experiment results and design of the cable.

Over-current characteristics of YBCO superconducting cable

To achieve large current capacity and high mechanical flexibility, YBa 2Cu 3O 7 (YBCO) superconducting cables consist of a number of YBCO coated conductors that are assembled and wound spirally on a Cu former. In practical applications, superconducting cables are vulnerable to short-circuit fault currents that are 10–30 times greater than the operating current. Therefore, in order to ensure the stability of YBCO superconducting cables in such a situation and to protect them from the fault currents, it is important to investigate the redistribution of the transport current and electromagnetic coupling between the conductor layer, shield layer, and Cu former. In this study, we carried out experiments on a 10-m-long YBCO model cable, which was manufactured by Furukawa Electric. An over-current with a peak of 31.8 kA rms and a duration of 2.02 s was applied to the model cable. We performed numerical simulations using a novel computer program developed using the 3D finite element method to elucidate the electromagnetic and thermal behavior of the YBCO model cable in the presence of an over-current.

Numerical study on self-field losses of 30 m BSCCO HTS transmission cable

The self-field losses of the one phase of high- T C superconducting (HTS) transmission cable are calculated by the electric circuit (EC) model. The one phase of HTS cable is constructed by the former of fine-strands copper rod, HTS conductor with four superconducting layers, the insulation made by polypropylene laminated paper, and HTS shielding with two superconducting layers, which was fabricated by Sumitomo Electric Industries (SEI). The length of the cable is 30 m. Each HTS layer comprises BSCCO tapes. The current-dependent resistance of HTS layers in EC model is estimated on the base of Norris expressions for ellipse. The calculated losses are compared with the experimental results measured by 4-terminal method by SEI. The calculation of alternating current (AC) losses, a summation of the self-field losses in HTS layers and the eddy-current losses in the former, is almost equal to the measurement at wide transport-current range below the lowest value of the layer critical current. This result indicates that the numerical calculation by EC model is quite reliable. The minimum AC loss is also calculated by obtaining the optimum helical-pitch lengths of HTS layers at transporting 1 kA rms. The minimum loss is 36% lower than the loss of HTS cable designed by SEI at the transport current value. In HTS cable with the optimum helical-pitch lengths, the calculation of the layer currents are not uniform in HTS conductor but are almost uniform in HTS shielding, which is contradict to SEI’s one. It is considered that the numerical calculation by EC model is useful to obtain the optimum helical-pitch lengths in HTS cable with the minimum AC loss.

Experimental investigation on the motion of cathode spots in removing oxide film on metal surface by vacuum arc

The motion of vacuum arc cathode spots has a very important influence on the efficiency of removing the oxide film on the metal surface. In this paper, the characteristics of cathode spot motion are investigated experimentally. Experiments were conducted in a detachable vacuum chamber with ac (50 Hz) arc current of 1 kA (rms). A stainless steel plate covered by an oxide layer was used as the cathode. The motion of cathode spots during the descaling process was photographed by a high-speed digital camera with an exposure time of 2 µs. Experimental results indicate that the motion of cathode spots is influenced by the interaction among individual cathode jets and the position of the anode as well as the surface condition. The waveform of arc voltage is also influenced by the motion of cathode spots.

Simulation of the Thermal Process of Anode in Drawn Vacuum Arc

In this paper, a transient 2-D finite-element model was developed to simulate the anode activities of drawn vacuum arc. Some important thermal processes of anode, e.g., the temperature distribution, melt, and evaporation, have been investigated for copper vacuum arc of 27.5 kA (rms) with different separation current. The dimension of eroded region on anode due to melt and evaporation was simulated and compared with other experiments.

Self-field losses in 1 m HTS conductor consisted of YBCO tapes

The self-field losses of 1 m HTS conductor consisted of YBCO tapes are calculated by the electric-circuit (EC) model. The conductor has two superconducting layers using 32 tapes, which is fabricated by Furukawa Electric Corporation (FEC). The current-dependent resistance of HTS layers in EC model is estimated on the base of Norris expressions for ellipse and thin strip, respectively. Numerically calculated losses are compared with experimental results measured by 4-terminal method by FEC. The calculations based on the Norris expression for thin strip are almost equal to the measurements at wide transport-current range below the I C of conductor. The calculation of layer-currents indicates that the currents passing through inner and outer layers are slightly different. Uniform layer’s current distribution causes the minimum self-field loss of HTS conductor. Minimization of self-field loss at transporting 1 kA rms is numerically executed by adjustment of helical pitches of HTS layers. As a result, the difference of layer currents is improved. However, the obtained minimum loss is 0.30 W/m that is not so different from the original loss of 0.32 W/m in HTS conductor on the FEC’s design of helical pitches.