François Gentils

On the numerical simulation of the diffuse arc in a vacuum interrupter

A 2D two-fluid numerical model is developed for the description of the diffuse arc in a vacuum interrupter under the possible influence of an axial magnetic field (AMF). This model incorporates the energy balance equations for both ions and electrons and takes into account the three components of the self-generated magnetic field. The possibility of both supersonic (at low current density) and subsonic (at high current density) plasma flow regimes is considered. Data from the literature are used to specify the values of the plasma parameters near the cathode, and the boundary conditions on the anode boundary rely on a simplified model of the anode sheath. Simulation results are presented for both ion flow regimes, with special attention to the current and plasma flow features. It is shown that there is always a contraction of the current flow, whereas the dynamics of the ions throughout the interelectrode gap is strongly influenced by their flow regime near the cathode. Finally, the influence of various o...

Numerical Investigations on the Pressure Wave Absorption and the Gas Cooling Interacting in a Porous Filter, During an Internal arc Fault in a Medium-Voltage Cell

A mathematical model and a numerical method have been developed to simulate the mechanical and the thermal physical phenomena in a energy absorber like a porous filter, during an internal arc fault in a medium-voltage apparatus. A 1D gas flow model in porous medium with variable porosity is described. The main point is the introduction of a new numerical scheme to take accurately into account discontinuous variations of the porosity and correctly simulate the creation of the transmitted and reflected waves. Numerical simulations are compared to experimental measurements performed on apparatus specially adapted for tests to provide a better understanding of the physical phenomena involved, for instance, the gas cooling and the shock wave absorption by the porous medium of the filter.

Current constriction of high-current vacuum arc in vacuum interrupters

Compared with previous paper [L. Wang et al., J. Appl. Phys. 100, 113304 (2006)], higher-current vacuum arc is simulated and analyzed based on magnetohydrodynamics model, and current constriction phenomenon in arc column is mainly paid attention to and analyzed in this paper. According to simulation results, it can be found that significant current constriction only appears near anode regions for lower-current vacuum arc. However, with the increase of arc current, current constriction also appears near the cathode side, and with the further increase of arc current, current constriction near the cathode side can become more significant than that near the anode side. The current constriction near the cathode side can be mainly caused by very high current level. The increase of axial magnetic field (AMF) strength will inhibit current constriction in the whole arc column. For influence of AMF distribution, saddle-shaped distributed AMF can more efficiently inhibit current constriction of arc column than bell-shaped AMF. The phenomenon of current constriction near the cathode side has also been found by many experiments, which also can verify the correctness of simulation results.

Two-dimensional modelling of internal arc effects in an enclosed MV cell provided with a protection porous filter

Medium voltage (MV) cells have to respect standards (for example IEC ones (IEC TC 17C 2003 IEC 62271-200 High Voltage Switchgear and Controlgear—Part 200 1st edn)) that define security levels against internal arc faults such as an accidental electrical arc occurring in the apparatus. New protection filters based on porous materials are developed to provide better energy absorption properties and a higher protection level for people. To study the filter behaviour during a major electrical accident, a two-dimensional model is proposed. The main point is the use of a dedicated numerical scheme for a non-conservative hyperbolic problem. We present a numerical simulation of the process during the first 0.2 s when the safety valve bursts and we compare the numerical results with tests carried out in a high power test laboratory on real electrical apparatus.

Experimental study of HBC fuses working at short and medium pre-arcing times

Pre-arcing stage is the first working step in high breaking capacity (HBC) fuse operation and affects the following step, namely, the arcing step. We have performed realistic HBC fuse tests for short ( 10 ms) pre-arcing times by varying the phase angle of the electrical fault (defined as the phase angle of the fault current once the supplied voltage is applied to the fuse) in the range from 0° to 160°, for two values of the power factor (cos φ ∼ 0.9 and cos φ ∼ 0.1). Experimental values of the pre-arcing time and the arcing time (t are ) are given for t prearc /t MC ? 1 to ∼4.2, and discussed from the energetic point of view by taking into account the inductive source term. The adiabatic assumption classically used in the modelling is also examined. The influence of the pre-arcing step on the arcing step is analysed by means of the Joule integral, the energy dissipated in the fuse and the mass and length of the fulgurite.

Plasma backflow phenomenon in high-current vacuum arc

Based on the two-temperature magnetohydrodynamic model, a high-current vacuum arc (HCVA) in vacuum interrupters is simulated and analysed. The phenomenon of plasma backflow in arc column is found, which is ultimately ascribed to the strong magnetic pinch effect of HCVA. Due to plasma backflow, the maximal value of ion density at the cathode side is not located at the centre of the cathode side, but at the paraxial region of the cathode side, that is to say, ion density appears to sag at the centre of the cathode side (arc column seems to be divided into two parts). The sag of light intensity is also found by experiments.

Theoretical and experimental analyses of the synergism in the dielectric strength for C3F8∕C2HF5 mixtures

Two theoretical approaches are applied in order to describe the behavior of the experimental breakdown voltage in C3F8∕C2HF5 mixtures. First, available cross section data of the electron-molecule processes are scaled to simulate the dielectric strength (DS) behavior in both C3F8 and C2HF5 gases at the level of the Boltzmann equation (BE) solution corresponding to a homogeneous electric field model. Then, the DS of C3F8∕C2HF5 mixtures is evaluated and compared to the experimental breakdown voltage measured using different electrode geometries as sphere/plane type via experimental techniques, i.e., power frequency and negative or positive polarity lightning impulse. Due to the limitation of the BE approach to describe the observed experimental data, the Hunter-Christophorou [J. Appl. Phys. 57, 4377 (1985)] model regarding the positive synergism phenomenon is finally applied. Both models allow us to compare relative values of the “chemical” and “physical” DS components in C3F8∕C2HF5 mixtures.