William Bussiere

Influence of sand granulometry on electrical characteristics, temperature and electron density during high-voltage fuse arc extinction

Emission spectroscopy and the recording of electrical characteristics are used to study the influence of the granulometric composition of arc-quenching material. Six adjacent intervals of grain size are studied: each of them is 50 µm wide, and the upper limit of the global interval is lower than 1000 µm. The final dimensions of the fulgurite are found to be proportional to the granulometry. Concerning the electrical parameters, two groups are considered: the extinction time constant, the dissipated energy and the I2t value decrease with the granulometry, whereas the maximum value of the arc voltage increases with the granulometry. The rate of decrease of the temperature depends on the time interval observed: at the beginning of the arcing period, the decrease is fastest for the smallest granulometric parameters; and during the last part of the arcing period the trend is reversed. At the beginning of the arcing period, the electron number density increase varies between 1019 and 1020 cm-3 ms-1. When the electric current decreases, the electron number density decreases exponentially with a time constant value ranging from 0.40 ms to 0.25 ms.

Numerical study of the short pre-arcing time in high breaking capacity fuses via an enthalpy formulation

In order to study the short pre-arcing time in high breaking capacity (HBC) fuses, we use a mathematical model including the phase change of the fuse-element heating coupled with the Laplace equation for the potential and Ohms law. The thermal model is based on the enthalpy formulation of the heat equation with a source term representing the Joule heating. For the time range considered (up to 10 ms), we assume no heat transfer between the fuse-element and the surrounding sand. To solve numerically the governing equations, we employ a semi-implicit scheme for time integration and a finite element method for space discretization. Using electrical and thermal properties of the silver fuse-element, we present pre-arcing characteristics (temperature, current density, potential) for a fuse-element used in industrial protection circuits.

Evaluation of the composition, the pressure, the thermodynamic properties and the monatomic spectral lines at fixed volume for a SiO2-Ag plasma in the temperature range 5000-25 000 K

The pressure, the composition, the internal energy, the heat capacity and several monatomic spectral line intensities are calculated at constant volume for a plasma composed of SiO2 and Ag for several initial densities and in the temperature range 5000-25 000 K at thermodynamic equilibrium. We show that with a small quantity of material in the plasma we obtain a high pressure. From the heat capacity and composition calculation, we deduce that the main reactions are the ionization of Ag, the dissociation of SiO2 to SiO with further dissociation and ionization of Si and O in the considered temperature range. Furthermore, with the monatomic spectral line calculation, we deduce that the oxygen spectral line has a behaviour rather different from those emitted by Ag and Si.

Comparative study of an argon plasma and an argon copper plasma produced by an ICP torch at atmospheric pressure based on spectroscopic methods

The aim of the paper is to test the accuracy of classical spectroscopic methods in the visible domain dedicated to measurements of temperature and electron density in order to conclude about the validity of thermal disequilibrium. The influence of various factors is studied: accuracy of the intensity calibration, Abel inversion of the experimental spectra, excitation temperature deduced from the relative method, absolute excitation temperature, influence of the transition probability accuracy, influence of the Biberman factor value, electron temperature from the line-to-continuum intensity ratio, electron density deduced from Stark broadening, and electron density deduced from the continuum intensity. This spectroscopic investigation is carried out for argon plasma and argon copper plasma both produced by means of an ICP torch operating at atmospheric pressure. Results are given with uncertainties for each evaluated parameter. We show that, first, the electron temperature deduced from the line-to-continuum intensity ratio has to be considered with great care; second, for argon plasma no evidence of thermal disequilibrium can be discerned, whereas for argon copper plasma a small disequilibrium of 1.2 to 1.4 at most is experimentally observed.

Effects of dielectric material on arc plasma pressure and ablation measurement in high-power apparatus

We measure some characteristic parameters to investigate the electrical discharge in different types of insulators in the energy range 150-500 J. By varying the type of insulator and the confining geometry, we quantify their specific influence on the pressure, the dissipated energy, and the ablated mass. These considerations are useful to increase the knowledge of the physical mechanisms which play a role in the high-current plasma-wall interaction. In agreement with previous theoretical works, we show two opposite behaviors between polyethylene and polyoxymethylene in terms of ablated mass and graphite formation.

Measurements of time-resolved spectra of fuse arcs using a new experimental arrangement

A new experimental set-up has been designed to observe spectral studies in fuse arcs. The fuses studied here are high breaking capacity fuses. They consist of a fuse element (in silver or copper) surrounded by silica sand. In this study, the arc dissipates 1200 J, the peak of electric current is about 2000 A and the duration of the arc is 3 ms. The maximum cross section of the squeezed arc is about 0.5 mm * 5 mm. The whole of the visible spectrum has been registered, from 360 nm to 800 nm, in the exploding part of the arc. The spectrum consists of continuous light together with spectral lines. The continuous light intensity is closely correlated with current intensity. The spectral lines appear mostly when the current decreases. The behaviour of the spectra shows that there is a big gradient of species concentrations in tha arc, together with a high grandient of temperature. The temperature is measured from Si II lines in the arc core. The calculation gives about 30 000 K at 1.3 ms. The measured values of electronic densities vary between 10E17 and 10E18 per cm3

Porous Filter Optimization to Improve the Safety of the Medium-Voltage Electrical Installations During an Internal Arc Fault

Electrical power distribution equipment, such as medium-voltage (MV) switchgear, must be designed to withstand the pressures and temperatures of gases resulting from an internal arcing fault. An original way to limit the external effects of the arc consists in channeling downward the gas flow across a filter composed of a granular porous medium in order to absorb the abrupt pressure wave and to cool the hot gas flow. In this paper, we propose an optimization of the MV switchgear configuration to enhance the porous filter efficiency where we manage to strongly reduce the external manifestations of the arc fault. On one hand, we employ the numerical simulation tool lying on a physical model where the major events are taken into account. On the other hand, real experimental tests have been performed according to the IEC standards and pressure and temperature histories obtained by numerical simulation are compared with the experimental measurements.

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.

Estimation of the burn-back rate in high breaking capacity fuses using fast imagery

The viewing of the erosion rate of the fuse element in high breaking capacity fuses is carried out using fast imagery. The rotating drum camera we have used provides up to 160 frames to observe the arc extinction throughout the phenomenon which lasts 4 ms. From these frames, we show that three stages follow each other with different values of burn-back rates: the maximum values are obtained at the beginning of the phenomenon and are equal to 6.65 m s-1 and 5.81 m s-1 for silver and copper fuse elements, respectively. The direct observation of the burn-back mechanism shows a reproducible disequilibrium depending on the nature of the electrode: the cathode erosion rate is 1.7 times that of the anode rate in the case of silver, and 1.2 times that of the anode in the case of copper.

Numerical method for pre-arcing times: Application in HBC fuses with heavy fault-currents

This works deals with calculations of pre-arcing time prediction for fuse links used in industrial protection circuits in case of heavy faults-currents. An enthalpy method to solve heat-transfer equation included two phase-changes is presented. The mathematical model couples thermal and electrical equations based on the principle of energy conservation and the Ohms law respectively. In order to determine current density and temperature evolution in the fuses, three typical fuse links have been chosen for the calculations with circular, rectangular and trapezoidal reduced sections at their centre. Silver physical properties, mathematical equations and the numerical method are reported. Calculations results show that for the fuse link with rectangular reduced section a major heat-transfer mechanism took place compared to the other ones.