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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. Index Terms--Cathode connection, heat transfer, magnetohydrodynamic (MHD), moving arc, numerical simulations, transverse magnetic field (TMF) vacuum interrupter, vacuum arc.

According to well-known standards, e.g., IEC 62271-100, vacuum circuit breakers and particularly vacuum interrupters (VIs) have to fulfill miscellaneous forms of dielectric strength requirements depending on the rated voltage. Besides conventional conditioning methods, e.g., with alternating current, further techniques have to be investigated in the case of higher voltage levels. This paper describes the high-frequency (HF) current conditioning method with the objective to increase the dielectric strength of modern and very compact VIs for medium voltage applications. A test setup for conditioning of VIs in kilohertz range is presented. The conditioning current has peak values up to 10 kA, with durations of microseconds and voltages up to 135 kV (peak). The dielectric properties of the investigated VIs are described with emphasis on the obtainable lightning impulse withstand voltage level after one or more HF sequences. In addition, the surface microtopography and roughness of contact surfaces are analyzed by means of scanning electron and atomic force microscopy before and after applying HF conditioning. Index Terms--Conditioning, dielectric breakdown, lightning impulse withstand voltage, vacuum arcs, vacuum interrupters (VIs).

According to the relevant IEC standards vacuum circuit-breakers have to meet various needs, e.g. the interruption capability, making operations, and dielectric strength. Besides the interruption of short-circuit currents, switching of capacitive currents causes high stress of the circuit-breaker. Switching of capacitor banks, overhead lines, or cables leads to very small currents in comparison with short circuit currents. After current interruption the circuit-breaker must withstand twice the peak value of the system voltage. Furthermore, restrikes can lead to voltage multiplication. This conjunction of relatively small breaking currents with high voltage stress must be considered in detail. This work introduces a test arrangement for combined tests of making operation, current interruption, and dielectric stress of a vacuum gap under capacitive switching condition. A test vessel permits investigations of various contact materials and designs. It is connected to a synthetic test circuit which provides the appropriate test currents and capacitive voltage. During the test sequence the contacts are stressed by inrush-currents up to 4.5 kA peak, followed by a breaking operation at 500 A peak and a subsequent capacitive voltage up to 50 kV peak. Both the appearance of pre-ignitions at contact closing and the behavior under capacitive voltage stress after breaking are indications of the contact surface conditions. Index Terms--Capacitive switching, inrush-current, pre-arcing, restrike, welding force, vacuum circuit-breaker.

The new IEC 62271-100 requires an extensive proof of the capability of capacitive switching for a breaker under test. For vacuum circuit breakers, dielectric properties are mainly determined by the condition and topology of the contact surfaces, which are modified by in-rush currents as well as load-breaking currents and other effects. A synthetic single-phase test device has been erected in order to simulate three-phase network conditions and to collect more data on the statistical properties of the relevant processes. The distribution of pre-ignition field strengths is evaluated for different contact strokes and surface conditions, when discharging a capacitor through the closing interrupter. On the other side, the probability of restrikes for a given switching condition defined by full contact gap do and peak recovery voltage [U.sub.re] is measured and compared with the cumulative probability of pre-ignition just at the field strength E= [U.sub.re]/[d.sub.0]. A correlation between pre-ignition and restrike probabilities suggests a breakdown mechanism being field-emission dominated. In addition a rather strong conditioning effect has been observed at smaller contact gaps smoothening the contact surfaces. Index Terms--Capacitor switching, vacuum interrupter, electric breakdown, contacts, electron emission, welding.

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. Index Termsi--Arcing time, axial magnetic field (AMF), contact erosion, contact material, contact melting depth, contact systems, copper and chromium, high-speed observation of vacuum arcs, radial magnetic field (RMF), vacuum arc, vacuum interrupter (VI).

Contact material based on copper and chromium (CuCr) is widely used for vacuum interrupters (VIs) and has found worldwide acceptance in medium-voltage applications, especially for high-current interruption. Contact material with a weight content of chromium between 25 and 60 wt.% is almost exclusively used. A new contact material was established based on a multilayer system to improve the interruption ability and mechanical properties and reduce the contact resistance. After a combined sintering and melting process in a high-vacuum furnace, a material of high density and low gas content is produced. The finished blank consists of the following layers: CuCr-sheathing, copper bulk material, and a stainless-steel support resulting from the lost mold. It turned out that the higher thermal and electrical conductivity as well as mechanical properties of the multilayer contact material improved the interruption ability of the VI. Investigations of switching behaviors were carried out in standard VIs. Additionally, the standard chromium content of 25 wt.% in CuCr and the influence of higher chromium content was investigated with respect to interruption ability. Afterwards, the microstructure on the contact surface was analyzed with scanning electron microscopy and energy dispersive X-ray. Index Terms--Contact material, copper chromium, interruption performance, short-circuit interruption ability, vacuum interrupter (VI).

Investigations have been carried out on the dielectric performance of the ceramic (high-purity alumina, [Al.sub.2] [O.sub.3]) surface in vacuum interrupters after switching. In order to examine the influence of the shielding on the protection of the ceramic surface against metal vapor condensation different types of vacuum interrupters (VIs) have been tested: VIs with and without shielding. Additionally, two contact materials CuCr:. 75:25 wt% and WCAg: 56:4:40 wt% have been investigated to compare the adhesion of different metal vapors to alumina ceramic surfaces. After having performed a HV conditioning of the VIs, dc arcs with arbitrary arcing times were triggered between the contacts simulating the generation of metal vapor during high current interruption and load break switching. Between the arcing tests the insulation levels of all VIs have been tested by means of HV ac source. Afterwards the VIs were opened and the microstructure of the metallic condensate on the inner ceramic surface was analyzed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The integral chemical composition of the metallic film was investigated by inductive coupled plasma (ICP).

For interruption operations at higher voltage levels sulfur hexafluoride (SF6) is the dominating insulation and interruption medium. In medium to high voltage levels vacuum has advantages compared to SF6. In order to expand the operation voltage levels of vacuum circuit breakers (VCB), two approaches are possible. The first is the enlargement of the single contact gap and second is the series arrangement. In both cases the Time–Distance-Curve of the contact travel is one factor that influences the interruption performance of VCBs. The usage of a single gap for higher voltage levels requires an increased contact distance. In dependence of different interruption operations, like short circuit or capacitive interruption, a controlled Time–Distance-Curve could be useful. For experimental investigations a vacuum single break test setup using a brushless servo motor with a frequency converter is built. The electrical control of the drive enables parameter variations of the Time–Distance-Curve. The velocity, acceleration, deceleration, and final contact gap can be adjusted as required for testing and later in field application. For fine control a mechanical simulation considering the regulation loop between frequency converter and drive as well as the mechanical components is employed. Finally, tests of the setup for interruption operations are presented. [ABSTRACT FROM AUTHOR]

Due to the impact of arcing on high-purity surfaces and bound residual gases, current interruption operations may influence the internal pressure in vacuum interrupters (VIs). In this paper, we investigate the temporal development of internal pressure in VIs immediately after a current interruption operation, both intrusively using a vacuum gauge and nonintrusively using the magnetron method. By means of an experimental test setup that consists of a vacuum test system to vary the internal pressure in punctured, nonvented VIs with active getter elements, we performed experimental test series under variation of internal pressure and arcing time. The experimental results indicate that arcing during the interruption operation predominantly causes a significant decrease in the internal pressure of up to several orders of magnitude, depending on the arcing time and the initial pressure before the current interruption operation. Based on a test series with passivated getter elements, we found that active getter elements have only a minor influence on the observed pressure decrease. With regard to a condition assessment of vacuum circuit breakers, we discuss the impact of the observed pressure reduction effect on the possibility of predicting the remaining service life of installed VIs. [ABSTRACT FROM PUBLISHER]

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]

The resistance of vacuum interrupters can be calculated from the geometry and resistivity of current carrying parts and the additional resistance of the contact points between movable and fixed contacts. Since vacuum interrupter contacts are designed as flat contacts facing each other, the resistance is mainly determined by contact force, hardness and resistivity of the contact material. It is known that the contact material changes consistency and structure during short-circuit interruptions within melting depth. Indeed, the overall resistance of a vacuum interrupter has been observed to increase by up to 60% after short-circuit making and breaking tests. Since the resistance increase across the switching device is considered by IEC and IEEE standards as one of the acceptance criteria for the integrity of the interrupter after tests, it is essential to understand the origin of this increase. Different causes are discussed, among them the change of grain structure, resistivity and hardness of the molten top layer of the contact, and flatness of the contact surface. An increase of hardness and a different position of contact points seem to be the main factors responsible for the increased resistance. [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 interrupters (VIs) are designed for a long operating life. Even after a large number of switching operations, they should still provide good arc extinction and be able to withstand high voltages. Switching operations produce substantial masses of metal vapor that condense on the inner component parts of the VI. The VIs are equipped with shields to protect the ceramic surfaces from being coated by metal vapor. Since the first VI was put into service, a large number of shield designs have been developed. If the shield design consists of a floating shield with two end shields, there are two gaps between these shields. These gaps are necessary for insulation when the VI is in the open position, but during the switching operation metal vapor can flow through these gaps and coat the ceramics. If the deposited area on the alumina ceramics reaches a critical value, the dielectric performance of the VI can become reduced. Standard VIs have been tested with a floating shield and two end shields that did not overlap the floating shield. After performing switching operations, the coated ceramics were examined. Electron probe microanalysis (EPMA) was used here to obtain a very detailed view of the thickness of the deposited layers. With EPMA, it is possible to detect very thin layers in the range of nanometers. To provide a better understanding of how strongly different deposited layers can affect the dielectric performance of the VI, surface resistance measurements were carried out. These were performed with a high-resistance meter, which is able to measure up to 10^16~\Omega using a 1-kV voltage source. Electric field simulation was carried out to know how the coatings do affect the electric field inside the VI and why they can cause breakdowns during lightning impulse voltage tests. [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 compares the switching properties of WCAg and WCCu for vacuum contactors and studies the influence of material composition and grain size of the WC powder. Different materials are compared with respect to their breaking capability, the state of their contact surfaces after arcing, their erosion losses, and their chopping current. The experiments were carried out with a demountable vacuum chamber in a synthetic test circuit. For the switching capability tests, the contacts were stressed by arc currents up to 7 kA root-mean square (rms), and a transient recovery voltage of 23 kV peak. The chopping measurements were carried out at a test current of 45-A rms. For all materials investigated there was an adverse ranking between interruption capability on the one hand and chopping current on the other. Materials with a higher WC content showed lower chopping currents, but also lower interruption limits. A general explanation is that all factors keeping the arc stable down to lower currents in the case of chopping conditions, also favor re-establishment of the arc at current zero on high current stress. Furthermore, materials with lower interruption limits exhibit stronger cracks and spikes. [ABSTRACT FROM AUTHOR]

In vacuum interrupters, metal bellows permit the motion of the moving contact with up to long contact strokes while maintaining the ultra‐high vacuum seal. In contrast to most other bellows applications, the opening and closing operation of the vacuum interrupter lead to high‐impulsive motions which additionally excite bellows oscillations. Future requirements of higher switching speeds and operations demand for a simulation‐driven optimization of the metal bellows to increase the mechanical lifetime of the vacuum interrupter. The damaging effect of the switching operations is investigated on the bellows. A static force is axially applied on the bellows to validate the mechanical model compared with experimental results. Furthermore, a modal analysis is performed to consider the bellows as a flexible body in a multibody simulation. To account for the multiple‐frequency oscillations during the switching operation, a transient simulation is carried out where the excitation of the bellows follows from a travel curve. The global hot spot of mechanical stress is identified for further lifetime evaluations by low‐cycle fatigue methods. [ABSTRACT FROM AUTHOR]