V. M. Atrazhev

Mechanisms of Impulse Breakdown in Liquid: The Role of Joule Heating and Formation of Gas Cavities

The impulse dielectric behavior of insulating liquids is of significant interest for researchers and engineers working in the field of design, construction, and operation of pulsed power systems. Analysis of the literature data on transformer oils shows that potentially there are several different physical processes that could be responsible for dielectric breakdown by submicrosecond and microsecond impulses. While for short submicrosecond impulses ionization (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, the thermal effects associated with Joule heating start to play an important role. This paper provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high-voltage (HV) impulses with duration sufficient to cause local heating, evaporation, and formation of prebreakdown gas bubbles. The proposed model is based on the assumption that dielectric breakdown is developed through percolation channels of gas bubbles, and the criterion of formation of these percolation chains is obtained. To test the developed model, the breakdown field-time characteristics have been calculated for the liquid with chemical composition close to that of transformer oils but with known thermodynamic characteristics (n-hexane). Its dielectric strength has been obtained as a function of externally applied pressure and temperature. The analytical results show good agreement when compared with the experimental data available in the literature.

Transport properties of electrons in gaseous xenon

Calculation of the electron transport coefficients such as the mobility, transverse and longitudinal diffusion coefficients and corresponding characteristic energies was carried out in Xe gas at density 0.2 g/cm3 (9×1020 cm-3). Comparison between the calculation results and the experimental and theoretical data of other authors allows to choose the momentum transfer cross section of electron scattering by Xe atom. This cross section coincides with ones obtained by other authors in various energy ranges. The calculations of the electron transport coefficients based on this cross section are in good agreement with experimental ones in a wide range of the parameter E/N. An estimation of the validity boundary of gas-kinetic theory is carried out for the calculation of electron transport coefficients in dense Xe gas.

Work functions for a HV cathode in nonpolar liquids

When negative HV is applied to a needle of small radius of curvature (r/sub p/ /spl times/10/sup -12/ A) is observed above a voltage U/sub FN/, the Fowler-Nordheim law. For larger point radii, only a regular current pulse regime is detected above a threshold voltage U/sub s/ (Trichel regime). Analysis of these voltages U/sub s/ and U/sub FN/ as a function of conduction band energy V/sub 0/ of different liquids, shows that the apparent work function for high electric field strengths in liquids is lower than the one without applied field (2.1 compared to 4.5 eV for a tungsten cathode). This difference can be explained if one assumes the existence of a plasma-like layer around the HV cathode. >

Onset voltage for corona pulses in gaseous Ar under high pressure and in liquid Ar

HV, applied to a point cathode (radius of curvature /spl sim/1 to /spl sim/8 /spl mu/m) in a nonpolar liquid, brings about a regular current pulse regime above a threshold value V/sub s/. This regime is similar to the Trichel pulse regime for corona in air. We present results of an investigation of this threshold voltage V/sub s/ in argon over a wide range of densities. In gaseous argon, the V/sub s/(P) values increase with pressure. In the liquid, V/sub s/ is independent of external pressure. The results show that V/sub s/ is mainly a function of fluid density N and cathode tip radius /spl tau//sub p/. The theoretical analysis of the phenomenon is carried out according to the criterion /spl int//spl alpha/(x)dx constant, where a(x) is the Townsend ionization coefficient. It is shown that if the dependence of V/sub s/ on N for low density gas is extrapolated into the region of liquid densities, it greatly exceeds the experimental values. The dependence of V/sub s/ on the fluid number density /spl sim/ is derived. Its variation is found to be non-monotonic at very high N values. >

Breakdown processes in gas micro-bubbles in liquids under electric stress

The present work is concerned with a theoretical analysis of the breakdown characteristics of gas-filled micro-bubbles formed in insulating liquids stressed with electric field. It is assumed that the gas inside these bubbles is air which allows the use of experimental Paschen curve data for air in this analysis. Two main discharge mechanisms have been considered, the Townsend discharge and impulse breakdown. The combination of bubble diameter, D, gas pressure, p, and duration, τ, of the field stress determines the type of breakdown. Parameters which are required for the Townsend mechanism of breakdown and impulse breakdown to occur inside gas bubbles have been obtained and these conditions have been represented as boundary lines in the (Dp, τp) coordinate system. It is shown that there are such combinations of these parameters which satisfy neither Townsend nor impulse breakdown conditions. Experimental data on breakdown in air for these intermediate values of (Dp, tp) between the Townsend and the impulse discharges are not available in the literature and the breakdown behavior under such conditions is not well defined.

Nonmonotonic distribution of population of the a 3 Σ u + triplet state rotational levels in corona discharge in cryogenic helium gas

We observed the spectra within the wavelength range of 910–990 nm of emission of the corona discharge in supercritical helium gas at 6–11 K. This spectral range contains the molecular bands of the He*2 excimer radiative transitions (c3Σg+ ⇒ a3Σu+) between the electron-vibrational-rotational levels of the c3Σg+ and the a3Σu+ triplet states. We have determined the populations of the rotational levels (quantity of molecules in the given excited state among the excited molecules of the discharge) of the c3Σg+ excited state from experimental values of the intensity of the respective electron-vibrational-rotational lines. The calculated population distribution is nonmonotonous. The population of the level with the rotation quantum number K′ = 18 (the level number) is higher than those of the levels with the other K′ values.

Molecular and atomic spectra emitted by normal liquid and supercritical 4 He excited by corona discharge

The properties of corona discharge were study by electro-physical and spectral methods in the supercritical phase at 6 K and 11 K as well as in normal liquid helium at 4.2K within the pressure range of 0.1-10 MPa. The electro-physical investigations (measurement of corona current as a function of the applied voltage) gave information about the mobility of charged particles in the medium. The observed corona current is very small, less than 10-7 A. The measured current-voltage characteristics allow for the calculation of the charged particle mobilities (i.e., electrons for negative corona and positive ions for positive corona). The observed charge mobilities decrease as a function of external pressure in both supercritical and liquid phases of helium. The light emitted from the ionization zone was analyzed and assigned to atomic and molecular lines. Distortion of the spectral lines due to the dense helium environment was analyzed. The observed P and R branch rotational line shifts of He2* are markedly different from the previous theoretical predictions. The observed atomic line shifts and widths depend strongly on the applied pressure in both the supercritical and the liquid phases, which can be understood in terms of a bubble formation around the emitters.

Spectra emitted by helium excited by corona discharge

Spectra obtained in corona discharge in liquid and in gaseous Helium are under contrastive analysis. The spectra observed in LHe are emitted by excited atoms surrounded empty cavities (bubbles). The spectra emitted in gaseous gas under high pressure have asymmetric profile due to strong perturbation of radiating atom surrounded by perturbers.

Atomic and molecular spectra of normal liquid 4He excited by corona discharges

Liquid 4He at 4.2 K and different pressures up to 8 MPa is excited by positive and negative corona discharges. Emission of He I atomic lines and He2 molecular bands is observed. In negative corona, the line spectrum has distinct blue-shift and broadening, which becomes stronger with increasing pressure. The rotational structure of the molecular bands is resolved at pressures of 0.1–0.2 MPa. A blue shift of the Q-branch maximum with pressure is observed. A rotational temperature of 900 K is estimated from the d3Σu + − b3Πg molecular band emission. Positive corona was produced on a point anode with smaller electrode radius and higher voltages than for the negative corona. The electric currents for the negative and positive corona are similar. The radiation from the positive corona discharges has spectral features which differ qualitatively from the negative corona discharges. The spectra with a positive corona have marked asymmetries with greater intensities in the longer wavelength (red) wings.

Cavity initiation through an evaporating mechanism for the pulse breakdown in liquid

This paper presents an analytical model which describes the dielectric strength of insulating liquids stressed with the impulse electric fields. The Joule heating by the space charge saturated current may cause over-heating of pure liquids or may result in generation of nucleation centers associated with impurities in the case of practical liquids. Evaporation of the liquid from these impurities has been analysed in the paper and formation criterion for percolation chains of gas bubbles in impure dielectric liquids has been established. Based on this percolation condition, the dielectric behavior of n-hexane has been studied. Breakdown volt-time characteristics of liquid n-hexane have been calculated for different temperatures, and its dielectric strength has been obtained as a function of externally applied pressure.