Manfred Lindmayer

Simulation of the gasdynamic and electromagnetic processes in low voltage switching arcs

The switching arc in a low voltage circuit breaker is a very complex phenomenon, which is determined by the interaction between heat conduction, gas flow, current flow, and magnetic forces. A realistic simulation of the switching arc must consider all these effects. In this paper a three-dimensional simulation model is described, which is based on the differential equations for the mass, momentum, and heat balance. Furthermore, it contains a differential equation for the calculation of the current flow and the resulting magnetic field. This set of equations is solved by the finite volume method. First results are presented, which show the shape of the arc and the gas flow while the arc is moving between rail electrodes in a simple arc chamber.

Mathematical modeling of the high frequency behavior of vacuum interrupters and comparison with measured transients in power systems

When the gap length of a vacuum switch at current zero is too short (i.e. contacts open shortly before current zero), high frequency transients, known as multiple reignitions or virtual current chopping, may occur under certain network conditions. In order to estimate the probability of such transients a mathematical vacuum breaker model, based on numerous measured data, has been developed. It is able to simulate the HF-behaviour in single or three phase networks. To verify the abilities of the breaker model, complex measurements of the interruption of a modern 11 kV dry type transformer with a vacuum breaker under no-load conditions have been carried out. A comparison between the measured data and the results of the simulations, which require an equivalent HF-network for all involved devices (especially the transformer), is presented.

Vacuum interrupters with axial field contacts. 3-d finite element simulation and switching experiments

A magnetic field parallel to the current flow in the arc column can improve the breaking capacity of vacuum switches by affecting the arc mode. To calculate the field distribution in the interelectrode space of axial field contacts, a model has been generated by using a finite element program. A segmented coil behind the contact plates creates the magnetic self field. The three-dimensional analysis takes into account all parts of the current path within the contact structure. The generation of eddy currents by the 50 Hz excitation is also considered, as well as means to reduce them. Examples of calculated results with respect to current density distribution, axial magnetic field and phase shift between current and field are given. The simulations are completed by switching experiments with axial field contacts at currents to 45 kA.

Plasma density decay of vacuum discharges after current zero

The knowledge of the physical processes around current zero and their modeling is important for the development of vacuum interrupters towards higher switching capacity and higher voltages. One essential feature is the recovery of the arc gap after current zero from the well-conducting state of the high-current plasma to the isolating state of the cold gap. Like in other switching media, there is still plasma present in the switching gap at current zero, which needs time to recombine and disappear. The presence of post-arc charge is associated with post-arc current under the influence of the transient recovery voltage, which in turn interacts with the surrounding network. As the charges carried by the post-arc current leave the gap, the post-arc current also influences the plasma decay. A simulation of this combined behavior needs an adequate mathematical post-arc model as well as the knowledge of data of the charge-carrier densities and their decay. It is the aim of this work to determine the necessary data, based on a simplified one-dimensional post-arc model, and to compare model calculations with experiments.

Effect of the barrier material in a Townsend barrier discharge in nitrogen at atmospheric pressure

A Townsend-like barrier discharge in nitrogen at 7 kHz frequency is studied experimentally and theoretically. The discharge is homogeneous under a certain range of parameters, which depends on the material of the barriers. The higher the dielectric permittivity of barriers is, the narrower is this range. It is shown that the discharge properties do not only depend on the total capacitance of barriers but they also explicitly depend on the permittivity of a dielectric sheath near the surface. Measured ranges of existence of a Townsend discharge agree with the calculations based on a self-consistent model. Also, the two-dimensional simulations of the barrier discharge show that the stability of the discharge relative to radial fluctuations may depend on the permittivity of barriers. The effect of barrier material is interpreted as the influence of dielectric permittivity on the electric field induced by surface charges.

Arc motion and pressure formation in low voltage switchgear

To understand the complex nature of arc motion in current limiting low-voltage switchgear detailed information about the interaction between pressure distribution, arc movement and plasma flow are necessary. One of the major factors concerning these interactions is the geometry of the arc chamber. Systematic experimental investigations have been carried out to study the influence of the gap between the arc runners and the insulating side walls of the arc chamber in combination with different outlets at the top of the chamber. A high time resolution optoelectronic measuring device for the observation of arc motion, and pressure transducers at different positions of the chamber give a good impression of each stage of arc movement and the corresponding pressure development. Significant differences depending on the geometry of arc chambers can be shown. The findings are explained quantitatively and qualitatively.

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.

On the ion energy distribution of high current arcs in vacuum

With the experiments presented in this paper, applications of a retarding field analyzer (RFA) for the measurement of the ion energy E/sub i/ in a vacuum arc plasma are discussed. The examined plasma was produced by a sinusoidal half-wave vacuum arc current. The experiments were concentrated on evaluating the plasma parameters at the last three milliseconds before current zero. In a current range from 300 A/sub rms/ to 10 kA/sub rms/, the ion energy distributions and their peak values were evaluated. With the increase of the arc current, a decrease of the ion energy was found. By additional investigations of the angular distribution of the ion energies, a transition from a collision dominated interelectrode plasma to a freely expanding plasma was observed, depending on the arc current.

Contact Resistance and Arc Erosion of W/Ag and WC/Ag

A multitude of tungsten/silver (W/Ag) and tungsten carbide/silver (WC/Ag) contact materials of different compositions, origins, manufacturing methods, and contact sizes are investigated over a wide current range I = 60-1800 A. Resistance is measured after each switching-off operation and evaluated statistically. The results are a strong dependence of contact resistance on switching current and contact size, with high values caused by semiconducting layers at low arc currents and low metallic values at high currents. Investigations of the contact surfaces after switching help to explain this phenomenon. The resistance decrease nearly coincides with a considerable increase in erosion. While at low currents there is no significant difference in the erosion rates between W/Ag and WC/Ag, the latter tends to have lower rates at high currents.

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.