Search

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).