Dietmar Gentsch

Switching of capacitive currents and the correlation of restrike and pre-ignition behavior

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 d/sub 0/ and peak recovery voltage U/sub re//spl circ/ is measured and compared with the cumulative probability of pre-ignition just at the field strength E=U/sub re//spl circ//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.

High-speed observations of arc modes and material erosion on RMF- and AMF-contact electrodes

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.

Numerical Simulation of a Moving High-Current Vacuum Arc Driven by a Transverse Magnetic Field (TMF)

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.

Switching behavior of different contact materials under capacitive switching conditions

For vacuum circuit-breakers capacitive current switching means a specific operating condition. Though it differs notably from the short-circuit current interruption situation, the circuit-breaker design needs to be reconsidered and adapted. Capacitive switching combines high inrush-currents at the connection of a capacitive load and considerable low breaking currents at its disconnection. A reliable dielectric performance of the breaker is required since the occurrence of a restrike under recovery voltage stress can cause voltage escalation. In terms of a reliable performance of the circuit-breaker the choice of contact design and material takes centre stage. The behavior of various contact materials and designs are studied during a series of tests, representing complete switching cycles for a 24 kilovolt (kV) system voltage. The test series is comprised of making operations and current interruptions followed by a subsequent capacitive recovery voltage. Under these conditions diverse variants of copper-chrome composite materials are analyzed and contrasted. Thereby the effect of additive materials is observed. The pre-arcing behavior and the occurrence of restrikes are observed as an indication of the alteration of the contact systems dielectric condition during the test series. In this regard the tested CuCr contact types show a distinctly differing behavior in their pre-arcing and restrike performance. Furthermore a serious erosion of the contact surfaces can be observed occasionally. This can affect the dielectric condition of the contact system significantly. In addition optical investigation of arcing periods by high-speed movies show the localization of an arc being established on the contacts and particles escaping the contact gap can be noticed.

Study of vacuum circuit breaker performance and weld formation for different drive closing speeds for switching capacitive current

Experiments have been conducted to study the influence of the closing speed of the drive mechanism on the performance of vacuum interrupters used for capacitive switching. Given the experimental conditions (no load-opening), it was found that the probability of breakdown is about 30% less with a fast closing speed (contacts closing on average before a half-cycle of the inrush current) compared to a slow speed (contacts closing on average after a half-cycle of the inrush current). However, the probability of breakdown depends strongly on the contact history. In particular, it was shown that the weld formed during the inrush current phase exhibits a strong macroscopic deformation of the contact surface that is more pronounced for slow closing speed when the arc duration is the longest and the heat transfer to the electrodes is the highest. This deformation produced, at the end of the set of experiments, a local electric field that was enhanced by up to 3.5 times compare to that of the original surface. Contact cross-sections show that the weld is formed by mass transferred from one contact to the other and by further melting of this additional material after its integration.

Arcing Behavior on Different TMF Contacts at High-Current Interrupting Operations

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.

Theoretical and Experimental Investigation of New Innovative TMF–AMF Contacts for High-Current Vacuum Arc Interruption

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 contacts 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.

Simulation of a High Current Vacuum Arc in a Transverse Magnetic Field

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 (ges5 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.

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.