Physical and theoretical aspects of a new vacuum arc controltechnology-self arc diffusion by electrode: SADE

Our new vacuum arc control technology SADE doubles the high current interruption capability of our conventional axial magnetic field technology. First, we describe the vacuum arc motion behavior recorded by a high speed charge-coupled device video camera. This arc behavior is closely related to axial magnetic field intensity. In particular, it depends on the profile of the externally generated axial magnetic field. The anode spot is likely to move to the highest magnetic field intensity. Second, we describe analytical results for concentration of vacuum arc at the anode side contact surface. This analysis implies the possibility of an ideal magnetic field profile and intensity for vacuum arc control. Finally, we describe experimental results for vacuum arc control compared with the physical and theoretical results mentioned above, and we show a practical electrode configuration for vacuum interrupters and its application in a high current interruption experiment     

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

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

The Investigation of Copper-Chromium Contacts in Vacuum Interrpters Subjected to an Axial Magnetic Field

The characteristics of a vacuum arc between CuCr contacts under an axial magnetic field have been investigated. Test samples were made of CuCr contacts, and the arcing voltage was measured. The arcing voltage of the CuCr contacts under an axial magnetic field is lower than that of pure copper contacts by 10-20 V. From the measurement of the post-arc current and interruption test results, it was found that the arc concentrates and a part of the electrode melts at a rather low current, but that the electrode melting does not affect the interrupting capability. Also, the insulation characteristics were measured. With respect to voltage conditioning, high-current conditioning improved the breakdown voltage by 50 percent for a 20-mm gap and by 100 percent for a 3-mm gap. These test results show that CuCr contacts, used with axial magnetic fields, are promising for use in high-voltage and high-power vacuum interrupters.     

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

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

Characteristics of a vacuum switching contact based on bipolaraxial magnetic field

Axial magnetic field (AMF) contacts can increase the interruption capability of vacuum interrupters. Depending on the design, the principles of the local axial field arrangement are different. For unipolar arrangements the direction of the axial magnetic field is the same within the whole contact area. For bipolar arrangements, the polarity of the field changes once. In this paper investigations have been carried out to characterize a bipolar AMF contact system and to test its interruption performance. The influence of the bipolar AMF on the arc development and the thermal stress is described by high speed camera and contact surface pictures. In addition, three-dimensional AMF simulations have been performed by means of a finite-element program to estimate the influence of slots within the contact plates on the AMF performance. The high interruption capability of the bipolar AMF contact system has been verified in different test laboratories up to 12 kV/80 kA_RMS (symmetrical) and 36 kV/40 kA_RMS (including 40% DC) by three and single phase tests. The investigations are completed by measuring the post-arc current and the shield-potential during recovery period, both describing the switching behavior of the contact system     

Vacuum interrupter design based on arc magnetic field interactionfor horseshoe electrode

Computational methods for evaluating the complex magnetic field structure in axial magnetic field type vacuum interrupters based on horseshoe electrode geometry are demonstrated. Arcing behavior can only be understood by incorporating an arcing model. An essentially low-current arcing model is capable of predicting the high-current arcing behavior and can be used to optimize the contact structure     

Experimental Investigation of Anode Activities in High-Current Vacuum Arcs

It is well known that the melting of electrodes (mainly anode melting) in vacuum arc can increase the metal vapor density around current zero and even lead to interruption failure. In order to clarify the anode activities and their influence on arc appearance and interruption capacity, series experiments of cup-shaped axial magnetic field copper electrodes were conducted. Obvious anode melting was detected; the liquid copper flowed on the contact plate of anode and formed a clockwise swirl flow. The appearance of anode melting is likely to correlate to the transition of arc mode from high-current diffuse mode to high-current diffuse column mode. The melting of anode was severer than cathode and was influenced by the distribution of cathode spots. Various kinds of copper particles at macroscopic level can be seen in arc column. Even at the interruption limit, the majority of melted copper of anode sputtered out of gap in form of liquid droplets or was pressed into the cup of anode, the copper vapor evaporated into arc column only accounted for a few portion and no obvious anode jets was found due to large plasma pressure in arc column.      

The Current Distribution and the Magnetic Pressure Profile in a Vacuum Arc Subject to an Axial Magnetic Field

The steady-state electric-current distribution and the magnetic pressure in a uniform conducting medium, flowing in a cylindrical configuration between two circular electrodes, was determined by solving the magnetic field transport equation with a superimposed axial magnetic field. This medium models the interelectrode plasma of the diffuse mode metal vapor vacuum arc. The results show the following. a) The electric current and the flux of the poloidal magnetic field are constricted at the anode side of the flowing plasma. Most of the constriction takes place within a boundary layer, with a characteristic length of 1/Rme, where Rme is the magnetic-Reynolds number for axial electron flow. b) The electric-current constriction inversely depends on K?, where K? is the azimuthal surface current density which produces the axial magnetic field. c) The magnetic-pressure profile shows a radial pinch force in most of the interelectrode region, but in the anode boundary layer it is axially directed, thus retarding the plasma flow. d) The peak of the magnetic pressure is at the anode, and its amplitude directly depends on K?. As K? increases, the peak location moves toward the anode center.     

Characteristics of the statistical distribution of the current transmitted by a cathode spot of a vacuum arc in variously oriented magnetic fields

The influence of variously oriented uniform magnetic fields on the cathodic attachment of a low-current vacuum arc with electrodes made of oxygen-free copper and CuCr30 composition is studied. It is found that, if the current is fixed, cathode spots in the arc attachment are distributed by the normal law in the entire range of variation of the amplitude of magnetic induction vector B and angle α between this vector and the normal to the cathode surface. The parameters of the distribution depend on the magnetic field and cathode material. The magnetic field dependence is appreciable only when angle α exceeds some critical value α* (α* ≈ 30° and ≈45° for cathodes made of copper and CuCr30, respectively). At α > α*, the parameters of the distribution become strongly dependent on α, while the B dependence remains weak. Only when α → π/2 does the field amplitude have a pronounced effect on the parameters of the distribution. From the obtained results, we determine the statistical characteristics of the distribution of the mean current transmitted by a cathode spot in variously oriented magnetic fields. The found relationships make it possible to explain the peculiarities of the structure of the cathodic attachment of the high-current vacuum arc stabilized by an external axial magnetic field.     

Contact temperature and erosion in high-current diffuse vacuum arcson axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I_P, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I_P. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion     

Vacuum arc investigation of dual-part cathode electrodes

The basic characteristics of a vacuum arc are investigated with a special electrode whose cathode consists of two half-disks. Pure copper is used for one part of the cathode, while copper, chromium, silver, and titanium are used for the other part. The tested arc current value is less than 4000 A, and the flux density of the axial magnetic field applied to a vacuum arc is less than 0.015 T. Experimental results show that the arc voltages of dual-part cathode electrodes are much lower than those of the individual pure metals, and that current sharing between the two parts is roughly determined by the arc current-voltage characteristics of the metals. The arc voltage of the dual-part cathode electrode is extremely low when the current is less than 1000 A     

The High Current Metal Vapor Arc Column between Separating Electrodes

We observed metal vapor arcs between separating electrodes in a demountable vacuum chamber using high speed photography. The peak values of the ac arc current half-wave ranged from 5 kA to 67 kA. Determination of the arc appearance as functions of arc current and electrode gap revealed that the arc can assume various types of columnar forms when the current at the instant of electrode separation exceeds 7 kA. The duration of the columnar arcing forms is influenced by axial magnetic fields differently for different field strengths. The graphical representation of the results allows prediction of the most probable arc appearance for a given set of operating parameters. A qualitative explanation of the various arc appearances on the basis of balances between magnetic and kinetic pressures is provided.     

Cathode processes in free burning and stabilized by axial magneticfield vacuum arcs

Collective behavior of the cathode spots (CS) has been investigated in free burning and stabilized by axial magnetic field (AMF) vacuum arcs. Experiments carried out proved previously discovered phenomenon of CS group formation in free burning arc to be a general phenomenon for a short high-current vacuum arc. The dependency of CS group size in the developed are on arc current for different contact materials has been analyzed. Application of AMF with even relatively low intensity strongly affects on cathode processes. In short arcs, it hinders formation of the CS group and consequently reduces thermal stress applied to the electrodes. It has been revealed that high current vacuum arc under the action of AMF can exist only at current densities exceeding certain minimal value that depends on AMF intensity, contact gap, and does not depend on current itself. The dependency of this minimal (or normal) current density on AMF intensity has been studied for short and long vacuum arcs. A qualitative model of the cathode spot dynamics has also been proposed     

Basic Characteristics of Vacuum Arcs Subjected to a Magnetic Field Parallel to Their Positive Columns

As vacuum arcs subjected to a magnetic field parallel to their positive column (an axial magnetic field) spread uniformly over all the electrodes and burn in the interelectrode region, arc voltages of these arcs are low and quiescent. When the magnetic field strength decreases, however, the arc voltage develops a large noise component and electrode melting occurs. Experiments were conducted to investigate the condition of these transition phenomena. As a result of these experiments, it was found that these two phenomena do not always occur simultaneously and that a new explanation for the mechanism of anode spot formation should be considered.     

Axial magnetic field contacts with nonuniform distributed axial magnetic fields

It is well known that axial magnetic fields (AMFs) can keep vacuum arc in diffuse mode at high current. According to our recent research and other published papers, it has been found that vacuum arc can be maintained in high-current diffuse mode at much higher current if nonuniform AMF is applied, that the axial magnetic field is higher at contact periphery than at center. The influence of spatial distribution of AMF on vacuum arc is mainly studied in this paper. Furthermore, two types of AMF contacts with new structures to generate nonuniform AMF are presented.     

The Gamitron - A High Power Crossed-Field Switch Tube for HVDC Interruption

A high power crossed-field discharge device has been developed for use as a high voltage direct current interrupter. This device operates at low pressure (0.05 Torr), conducting current at a fixed voltage (~500 V) only in the presence of a weak magnetic field (~100 G) which is substantially perpendicular to the electric field between the electrodes. When the magnetic field is removed, ionization ceases and current interruption results. Physical phenomena occurring in this device have been investigated, including the glow-to-arc transition, gas cleanup, and high voltage breakdown. Based on the results of these investigations, switch tubes have been developed and successfully tested at the 2 kA, 100 kV level, with recovery rates in excess of 2 kV/?s. The availability of such devices will make possible many applications including HVDC circuit breakers, ac current limiters, and practical inductive energy storage.     

The Interaction of Vacuum Arc Ion Currents with Axia I Magnetic Fields

We summarize a series of experiments in which we measured the distribution of ion currents leaving the interelectrode region of a vacuum arc with Cu electrodes. Ion currents were collected by an arrangement of cylindrical collectors surrounding the arcing space. A Helmholtz coil arrangement surrounding the arcing chamber generated the axial magnetic field. Arc currents ranged from 70 to 2400 A dc.     

Cathode spot dynamics on pure metals and composite materials inhigh-current vacuum arcs

An investigation has been carried out of cathode spot dynamics in a triggered vacuum arc in a demountable chamber. A rectangular current pulse of 1-5 kA, 1-5 ms has been used. Sufficient statistics were collected. The expansion of a cathode spot ring on a clean, pure metal surface was corroborated to be a retrograde movement in the self-magnetic field which obeys the same law as the movement of a single spot in an external magnetic field. The influence of a contact gap of 0.5-8 mm and current on the dynamics of cathode spots was investigated. The gap dependence of the proportional coefficient between the spot velocity and magnetic field in the case of a pure copper cathode was obtained. A phenomenon was discovered, where a group of cathode spots form in the short arcs on the CuCr cathodes after a transition diffuse arc stage. The follow-up investigation revealed that a close interrelation exists between the cathode and anode processes in short arcs. This interrelation is responsible for the appearance of the discovered phenomenon. Short-circuit performance tests conducted for a commercial vacuum interrupter proved cathode spot group formation to be responsible for the interruption failure at short contact gaps     

The influence of unipolar axial magnetic field on the behavior of vacuum arcs

The behavior of vacuum arcs under the influence of unipolar axial magnetic field (AMF) has been investigated. In experimental investigations, the vacuum arc mode is studied at different arc currents by using high-speed charge-coupled device (CCD) video images. In spite of the AMF, first sign of arc constriction appears at relatively small currents of about 15 kA (RMS). Three different arc modes were found. Based on generalized Ohm's law, the current density distribution in the vacuum arc with unipolar axial magnetic field is computed using three-dimensional finite-element method (FEM) software. The calculated current distribution is confirmed by the vacuum arc appearance taken from CCD video film. The distribution of AMF can be optimized by such experiments and theoretical analysis.