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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]

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 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]