Harald Fink

Optical investigations of dynamic vacuum arc mode changes with different axial magnetic field contacts

By using a high-speed charge-coupled device (CCD) video technique, three different axial magnetic field contact systems (i.e., unipolar, bipolar, and quadrupolar systems) are investigated at an arc current of 10 kA. Video recordings were compared to computer simulations of light emission emitted at the side-on of diffuse and diffuse columnar arcs. The computer images reproduced typical trends, such as stronger light intensities in front of the cathode caused by higher-plasma densities in this region. A low-current dc vacuum arc was initiated by contact separation before the high current was injected at a fixed contact distance of 10 mm. Videos were taken from two directions perpendicular to each other to localize the vacuum arc properly. From these investigations, the transient development of vacuum arc under different axial magnetic field profiles can be visualized. The results were interpreted with respect to the behavior of the vacuum arc in the second half cycle after an eventual reignition.

Vacuum interrupters with axial magnetic field contacts based on bipolar and quadrupolar design

Axial magnetic field (AMF) contacts are applied within vacuum interrupters especially for high short-circuit currents. In this paper, two AMF contact designs based on a bipolar and a quadrupolar magnetic field arrangement and their characteristics are presented. In the first section, both principles are discussed. This is followed by three-dimensional field simulations performed by means of a finite-element-analysis program. The magnitude of the axial magnetic flux density, the phase shift between current and magnetic flux density, and the residual AMF after current zero are investigated during arcing with a constant contact gap. Furthermore, the influence of motion during the contact opening on the previously-mentioned parameters is investigated. An important parameter during the design phase of AMF electrodes is the electromagnetic attractive force, which is provided by the AMF contacts during current flow. This force can reduce the required contact force provided by the drive of the vacuum circuit breaker. Finally, it is reported about the short-circuit performance of both contact systems. Contact plates after high-current interruption are presented revealing the arcing behavior during the high-current phase.

Multilayer contact material based on copper and chromium material and its interruption ability

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.

New developments of vacuum interrupters based on RMF and AMF technologies

New developments of vacuum interrupters based on radial magnetic field (RMF) contacts and axial magnetic field (AMF) contacts are presented. Important basis of the design of optimised contact constructions is the electromagnetic field calculation. In the first part of the paper the optimisations of AMF and RMF contact systems by means of the field simulation are presented. In the second part test results are shown which indicate the high current switching performance of both systems. The interrupting capability of both systems has been verified by the experiments up to 12 kV/63 kA and 36 kV/40 kA.

Condensed metal vapor on alumina ceramic in vacuum interrupters

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).

Bipolar axial magnetic field contact

In this paper, the bipolar axial magnetic field contact is introduced. The magnetic field has been calculated and evaluated. The vacuum interrupters were built and tested. From the results, it shows that this principle is suitable for very high current and high voltage applications.

Investigations on condensed metal vapour on the surface of alumina ceramics within vacuum interrupters after switching

Basic 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 vapour condensation different types of vacuum interrupters (VIs) have been tested: VIs with and without shielding. Additionally, two contact materials CuCr 25 wt.-% and WCAg 40 wt.-% have been investigated to compare the adhension of different metal vapours to alumina ceramic surfaces. After having performed a high-voltage conditioning of the VIs, DC arcs with arbitrary arcing times were triggered between the contacts simulating the generation of metal vapour during high current interruption and load break switching. Between the arcing tests the insulation levels of all VIs have been tested by means of an AC high voltage source. Afterwards the VIs were opened and the microstructure of the metallic condendate on the inner ceramic surface was analysed by means of scanning electron microscopy (SEM). The integral chemical composition of the metallic film was investigated by inductive coupled plasma (ICP). The conductivity of the surface condensate resulting from the condensation of the metal vapour was measured in order to compare the material investigations with the electrical behaviour of the surface.

Conditioning of Series Vacuum Interrupters (VIs) for Medium Voltage by Applying High-Frequency (HF) Current to Increase the Dielectric Strength of VIs

According to well known standards e.g. IEC 62271-100, vacuum circuit breakers and especially vacuum interrupters (VIs) have to fulfil miscellaneous forms of dielectric strength requirements depending on the rated voltage. Besides conventional conditioning methods e.g. with alternating current (AC), further techniques have to be investigated in the case of higher voltage levels. This work 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 set-up for conditioning of VIs in the kHz 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 investigated VIs are described with emphasis on the obtainable lightning impulse withstand voltage level after one or more HF-sequences. Additionally the surface microtopography and roughness of contact surfaces are analysed by means of atomic force microscopy before and after applying HF-conditioning.