Search

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 \textm . 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. [ABSTRACT FROM PUBLISHER]

A new two-contact system with double-transversal magnetic field (TMF) structure is investigated regarding its potential for high current interruption. It consists of two concentric TMF contacts, in which the inner contact has a disk-shaped structure (spiral contact) and the outer contact a cup-shaped structure. This contact assembly gives several advantages over the use of single-TMF contacts, as the constricted high-current arc might either split between both inner and outer contacts or commute from the inner to the outer contact and continue its rotation. In both scenarios, we expect a better distribution of the arc energy over a larger contact surface. B-field simulation and Lorentz force calculation are carried out to investigate whether two constricted arcs, with one burning on the inner contact and the other on the outer contact, can coexist given their mutual attraction. Single-phase high current interruption tests were carried out to compare the performance of this new double-TMF contact system with that of a standard spiral-type single-TMF one. These tests have revealed superior performance of the new double-TMF contact system, which could interrupt current amplitude at least 20% higher than the one interrupted by the single-TMF contact system. [ABSTRACT FROM PUBLISHER]

This paper consists of investigating the vacuum arc behavior during the high-current interruption process with a new innovative coaxial double-contact system. The structure of the so-called TMF-AMF double-contact offers the advanatage of low total resistance for nominal current conduction as in standard transverse magnetic field (TMF) contacts and similar vacuum arc control as in axial magnetic field (AMF) contacts. For an optimized contact's geometry, finite element method B-field simulations were performed to evaluate the effect of geometric parameters on the axial B-field strength and distribution and mechanical simulations to evaluate the closing forces distribution. The arc dynamics for two distinct cases, where the arc ignition takes place between the inner contacts, and between the outer contacts, are investigated experimentally. The arc appearance extracted from the high-speed movie is correlated with the arc voltage to explain the mechanisms of arc commutation to fully diffuse mode. It has been shown that the commutation to fully diffuse arc takes place in all cases but with a shorter commutation time with the second case. The benefit of using the present TMF–AMF contact system for high-current interruption while keeping the total nominal resistance as low as possible is demonstrated. [ABSTRACT FROM AUTHOR]

Vacuum circuit breakers are widely used in the medium-voltage area. The majority of the installed vacuum tubes are equipped with electrodes using the transverse-magnetic-field design forcing the electric arc on a circular motion to avoid severe local overheating. A vacuum test switch was used to investigate the arc movement behavior between spiral- and cup-shaped electrodes at high-current interrupting operations. The switch was equipped with viewing ports allowing an observation from two rectangular views. Mounted sample contacts were made of CuCr 75/25 in different diameters. A digital 8-bit high-speed camera was used to record the arcing process with frame rates of 33 000 frames per second. Behavior patterns were investigated and compared with the arc voltage and the instantaneous current. Parameters such as the arc velocity and the current density on the contacts could be determined by means of the recordings. A static simulation model delivered Lorentz forces for a comparison between both designs. The experiments were conducted with short circuit currents from 20 to 60 kA (root mean square) with a frequency of 50 Hz. [ABSTRACT FROM AUTHOR]

This paper presents the results of simulations using a model that describes constricted high current (> 15 kA) vacuum arcs driven by a transverse magnetic field in a 2-D configuration (parallel rail electrodes). The simulations investigate a number of cases of practical interest for the use of vacuum interrupters. The influence of the electrode gap distance on the arc motion is discussed. It is found that faster arc velocities are obtained for larger gaps. For large gaps (⩾ 5 mm), the Lorentz forces and pressure gradients acting on the plasma jets originating from the hot electrodes strongly affect the arc structure: The arc tends to expand on a longer distance and can efficiently preheat the next area of current attachment. The model also describes the jump of the arc over an electrically nonconductive part of an electrode (slit). This is possible due to the ability of the arc column to expand in the direction of motion and to prepare current attachment at a point beyond the slit. The characteristics of the jump depend on a function of the slit width, electrode gap, and current. Finally, the thermal effect on the electrode surface and the electrode bulk for an arc returning several times to the same position is investigated for a fixed dc current. The results show that the minimum surface temperature increases the first few times the arc returns, before stabilizing at a temperature given by the balance between the arc heat flow and the cooling by metal evaporation and conduction into the electrode. [ABSTRACT FROM AUTHOR]

This paper deals with the numerical simulation of the constricted high-current vacuum arc (> 15 kA), driven by a transverse magnetic field (TMF), as found in vacuum circuit breakers applying the TMF arc control. The magnetohydrodynamic approach, together with the detailed heat transfer and evaporation equations for the electrodes, is used to describe the arc behavior self-consistently, restricted to 2-D. A newly developed model describes the cathode attachment of the constricted arc, as a large laterally extended foot points, instead of regular cathode spots. The arc maintains itself when the electrode temperatures are higher than 3400 K on the cathode and 2900 K on the anode. This model leads to the characterization of the physical quantities of the arc plasma and describes the arc motion. A stepwise movement of the arc results due to different instantaneous velocities of the current attachment areas at the cathode and anode. [ABSTRACT FROM AUTHOR]

Vacuum interrupters, particularly with high short-circuit interruption ability, are mostly equipped with contact systems based on two different principles: the widely used radial magnetic field (RMF) contact and the axial magnetic field (AMF) contact system. In this investigation, contact electrodes performance of an improved RMF system was compared with both an unipolar and a quadrupolar AMF contact system. By using a high-speed complementary metal oxide semiconductor digital video camera, the different systems were observed during arcing under short-circuit conditions at different current levels, concentrating on arc modes development with different arcing times. Contact erosion and thermal stress of the high-current vacuum arc on the contacts was basically evaluated on the basis of contact melting depth, with the result of comparable melting depths at insignificantly higher thermal stress of the RMF versus AMF systems. The microstructure of the copper and chromium compound contact material cross section was analyzed by means of a scanning electron microscope. [ABSTRACT FROM AUTHOR]