2017, Khakpour, A., Methling, R., Franke, S., Gortschakow, S., Uhrlandt, D.

A vacuum interrupter reaches its interruption limit once high-current anode phenomena occur. High-current anode modes lead to an increase of the anode surface temperature and an increased generation of metal vapor, which may result in a weakening of the dielectric recovery strength after current zero. In this work, different discharge modes in a vacuum arc for AC 50 Hz including diffuse, footpoint, anode spot type 1 and type 2, and anode plume are investigated. Electrodes made of CuCr7525 with diameter of 10 mm are used. The final gap length is about 20 mm. Time and space resolved optical emission spectroscopy is used to examine the temporal and spatial distribution of atomic and ionic copper lines. The distribution of atomic and ionic lines parallel and perpendicular to the anode surface is investigated. Radiator density is also determined for CuI, CuII, and CuIII near the anode surface.

2017, Methling, R., Khakpour, A., Wetzeler, S., Uhrlandt, D.

A switching arc in a model chamber is investigated by means of optical emission spectroscopy. Ignition wire is applied to initiate an arc of several kiloampere between tungsten−copper electrodes. Radiation emitted by the arc plasma is absorbed by a surrounding PTFE nozzle, leading to an ablation–dominated discharge. Video spectroscopy is carried out using an imaging spectrometer combined with a high–speed video camera. Carbon ion and fluorine atom line emission from the heating channel as well as copper, oxygen and nitrogen from ignition wire and ambient air are analyzed with focus on the low–current phases at the beginning of discharge and near current zero. Additionally, electrical parameters and total pressure are recorded while the general behavior of the discharge is observed by another video camera. Considering rotational symmetry of the arc the corresponding radial emission coefficients are determined. Finally, radial temperature profiles are calculated.

2017-05-15, Baeva, M., Uhrlandt, D., Murphy, A.B.

A collisional-radiative model for the ground state and fifty effective excited levels of atomic iron, and one level for singly-ionized iron, is set up for technological plasmas. Attention is focused on the population of excited states of atomic iron as a result of excitation, de-excitation, ionization, recombination and spontaneous emission. Effective rate coefficients for ionization and recombination, required in non-equilibrium plasma transport models, are also obtained. The collisional-radiative model is applied to a thermal arc plasma. Input parameters for the collisional-radiative model are provided by a magnetohydrodynamic simulation of a gas-metal welding arc, in which local thermodynamic equilibrium is assumed and the treatment of the transport of metal vapour is based on combined diffusion coefficients. The results clearly identify the conditions in the arc, under which the atomic state distribution satisfies the Boltzmann distribution, with an excitation temperature equal to the plasma temperature. These conditions are met in the central part of the arc, even though a local temperature minimum occurs here. This provides assurance that diagnostic methods based on local thermodynamic equilibrium, in particular those of optical emission spectroscopy, are reliable here. In contrast, deviations from the equilibrium atomic-state distribution are obtained in the near-electrode and arc fringe regions. As a consequence, the temperatures determined from the ratio of line intensities and number densities obtained from the emission coefficient in these regions are questionable. In this situation, the collisional-radiative model can be used as a diagnostic tool to assist in the interpretation of spectroscopic measurements.

2017, Baeva, M., Uhrlandt, D.

The paper is concerned with the state-of-the-art nonequilibrium modelling of a DC tungsten-inert gas arc plasma. The advanced description involves the two-way interaction between the plasma and the electrodes. Results in atmospheric pressure argon demonstrating important features of the arc plasma are presented and discussed. First results in the presence of metal vapour released from the molten anode are presented. Outlook for further developments in nonequilibrium arc modelling are discussed.

2017, Gortschakow, S., Khakpour, A., Popov, S., Franke, S., Methling, R., Uhrlandt, D.

Melting and evaporation of the anode surface strongly influence the interruption capability of vacuum circuit breakers, because they lead to injection of atomic vapour into the inter-electrode gap. Determination of the vapour density and its dynamics with respect to different anode phenomena is therefore of great importance. Results of Cr density measurements in a high-current vacuum arc by using broadband absorption spectroscopy are presented. The vapour density of atomic Cr is determined after the formation of anode spots as well as close to the current zero. Cr I resonance lines at 425.43 nm have been used for the analysis. An AC current pulse with maximum value of 7 kA and a frequency of 100 Hz is applied to a vacuum arc between two cylindrical butt electrodes made of CuCr7525 with a diameter of 10 mm. The high-current anode modes are observed by means of high-speed camera imaging. The temporal evolution of the Cr ground state density is presented and discussed.

2017, Kalanov, D., Golubovskii, Y.B., Uhrlandt, D., Gortschakow, S.

The description of radiation transport phenomena in the frames of collisional-radiative models requires the solution of Holstein-Biberman equation. An advanced solutuion method for 3D plasma obejcts is proposed. The method is applicable for various line contours in a wide range of absorption coefficients. Developed approach is based on discretization of the arbitrary plasma volume on a Cartesian voxel grid. Transport of photons between the cells is computed using the ray traversal algorithm by Amanatides [1]. Solution of the particle balance equations with computed in advance radiative transfer matrix is demonstrated for various typical arc shapes, like e.g. free-burning arc and cylindric arc. Results are compared with corresponding calculations using previously developed approaches. As the method is suited for finite geometries and allows for a strict solution of the radiation transport equation, applicability ranges of previous approximations can be specified.

2017, Murashov, I.V., Frolov, V.Y., Uhrlandt, D., Gortschakow, S., Ivanov, D.V., Sivaev, A.D.

Nowadays multi-chamber arresters are widely distributed as devices of lightning protection of overhead power lines. A mathematical modelling of processes in the discharge chamber of multichamber arrester is necessary to carry out in order to improve its breaking capacity. A three-dimensional mathematical transient model of thermal, gas-dynamic and electromagnetic processes taking place in the discharge chamber of multi-chamber arrester is presented in the article. Basic assumptions, model equations, a computational domain and the boundary conditions are described. Plasma turbulence is taken into account. The results of the calculation i.e. distributions of plasma temperature and overpressure in the discharge chamber at different time points are shown. The analysis of the results was carried out. It is shown that the presence of cavities in the electrodes design promotes electric arc extinction in the discharge chamber of multi-chamber arrester.