Pre-breakdown Discharges Research Articles

Simulation of pre-breakdown discharges in high-pressure air: II. Effect of surface protrusions

Analysis of deviations from the similarity law, observed at high and very high pressures in experiments on discharge ignition and breakdown in corona-like configurations, can serve as a useful, albeit inevitably indirect, source of information about microprotrusions on the surface of the electrodes. In this work, such analysis was performed by means of 2D numerical modelling. Conical or cylindrical protrusions on the surface of the inner electrode were studied and the kinetic scheme includes the electrons, one species of positive ions, and negative ions , , and . It is shown that the deviations from the similarity law, observed in the experiment, may indeed be attributed to enhanced ionization of air molecules in regions of amplified electric field near the microprotrusions. A qualitative agreement with the experiment in all the cases is achieved for protrusion heights of the order of 50 µm. Such values may appear rather high, however there is no other explanation in sight at present. The enhancement of the field electron emission from the surface of the negative electrode due to the amplification of the electric field on the microprotrusion was estimated and found insignificant in the range of values of the protrusion aspect ratio where the enhanced ionization in the gas phase is already appreciable.

Simulation of pre-breakdown discharges in high-pressure air. I: The model and its application to corona inception

A ‘minimal’ kinetic model of plasmachemical processes in low-current discharges in high-pressure air is formulated, which takes into account electrons, an effective species of positive ions, and three species of negative ions. The model is implemented as a part of numerical model of low-current quasi-stationary discharges in high-pressure air based on the use of stationary solvers, which offer important advantages in simulations of steady-state discharges compared to standard approaches that rely on time-dependent solvers. The model is validated by comparison of the computed inception voltage of corona discharges with several sets of experimental data on glow coronas. A good agreement with the experiment has been obtained for positive coronas between concentric cylinders in a wide range of pressures and diameters of the cylinders. The sensitivity of the computation results with respect to different factors is illustrated. Inception voltages of negative coronas, computed using the values of the secondary electron emission coefficient of 10−4–10−3, agree well with the experimental data. A simplified kinetic model for corona discharges in air, which does not include conservation equations for negative ion species, has been proposed and validated. Modelling of positive coronas in rod-to-plane electrode configuration has been performed and the computed inception voltage was compared with experimental data.

Degradation of organic molecules by streamer discharges in water: coupled electrical and chemical measurements

This paper presents an experimental study of pre-breakdown discharges in water (usually called ‘corona’ or ‘streamer’) used to degrade organic molecules for depollution purposes. Streamers in water are generated by high voltage pulses of short duration in order to minimize energy losses by Joule heating. Measurements of the electrical energy used to develop streamers can be taken only when losses due to the electrical conduction in the test cell becomes negligible, i.e. at high voltage. Coupled to electrical measurements, the formation of hydrogen peroxide and the degradation of organic compounds such as 4-chlorophenol and 4-nitrophenol is studied. Chemical effects are correlated with the amount of electrical energy released, which depends mainly on conductivity and applied voltage. On the other hand, the chemical yield of these processes, expressed in terms of mole/joule is independent of voltage and conductivity.

Deterioration of LPOF Cable Insulation Due to Ingress of SF6 Gas

Contamination of oil in low-pressure, oil-filled, paper-insulated, high-voltage cables connected to the high-pressure chambers of gas-insulated switchgear, due to the leakage of SF6 gas, have often been reported by electric power utilities. As a consequence this contamination can impair not only the dielectric behavior of oil and oil-paper composite but also the dielectric integrity of adjoining termination. This experimental investigation explores the impact of trace amounts of SF6 gas in oil on its dielectric strength under the power frequency and lightning impulses. The results show that minor leakages of SF6 gas improve the power frequency dielectric strength of oil. However, longer contact of gas with oil can significantly reduce it both under the power frequency as well as the lightning impulses of both polarities. Vaporization of liquid is found to cause pre-breakdown discharges which lead to its final breakdown.

Self-controlled pre-breakdown discharges in planar symmetry

In measurements on Teflon FEP films charged in /spl les/50 /spl mu/m air gaps by microsecond impulse voltages, a uniform charge deposition on the films was observed. A regular increase of the film surface potential from a threshold value of the peak impulse voltage was found. However, for 300 /spl mu/m air gap it was observed that abrupt charging occurs at a lower threshold voltage, indicating breakdown, and the charge deposition on the film becomes nonuniform. The behavior in air gaps <50 /spl mu/m is explained here using Townsend's theory of pre-discharges. It is shown how and why the interposed insulating film acts to make the system self-controlled, thus avoiding breakdown in the air gap, despite the large values of the applied peak impulse voltage.

Pre-breakdown discharges through isotopically enriched nitrogen: possible contributions from highly vibrationally excited molecules

The spatial and temporal ionization growth of pre-breakdown discharges in is compared with that in . The spatial growth of the ionization current shows differences which are attributable to differences in secondary-ionization processes caused by metastable-particle activity. The secondary processes are less efficient in than they are in . This is consistent with a previous suggestion (Fewell et al 1996 Chem. Phys. 206 257-67) that diffusion not only of the first electronically excited state but also of highly vibrationally excited electronic ground states of the nitrogen molecule plays a significant role in pre-breakdown discharges through nitrogen.

Diffusion coefficients of two metastable states of the nitrogen molecule

Abstract Previous experiments of Ernest et al. (1986) showed the existence of a metastable state of the nitrogen molecule with a coefficient for diffusion through ground-state nitrogen of about one quarter of that of the well-known A 3 Σ u + state. Both states were populated by pre-breakdown discharges through pure nitrogen. This paper presents further results on the diffusion of metastable excited molecules through nitrogen. The effects of varying background gas number density N and the reduced electric field E N were explored, and data were taken in two ionization chambers with two different cathode materials. Consistent results are obtained with the different chambers and cathodes. The characteristic times for the decay of metastable population density are independent of E N but scale with 1/N, consistent with the existence of two slowly diffusing metastable states of nitrogen. The values at 295 ± 4 K of the product DmN, where Dm is the metastable diffusion coefficient, are 551 ± 25 am−1s−1 ((5.51 ± 0.25) × 1016cm−1s−1) for the first slowly diffusing state and 158 ± 17 am−1s−1 for the second. Both states show very small quenching in collisions with ground-state nitrogen molecules. The first state is identified as the A 3Σu+ state. Far from having a small effect, the action of molecules in the second slowly diffusing state can dominate the ionization growth near breakdown.

Identification of slowly diffusing metastable states of the nitrogen molecule

Abstract Experiments [Haydon et al., Chem. Phys. 206 (1996) 245.] have confirmed the existence of a slowly diffusing metastable state of the nitrogen molecule in addition to the well-known A 3 Σ u + state. The second state is populated in prebreakdown discharges and is characterized by a coefficient for diffusion through ground-state nitrogen of about one quarter of that of the A 3 Σ u + state. Recent experimental work on the structure of the nitrogen molecule provides much evidence bearing on the question of the nature of the second slowly diffusing state. Examination of a wide variety of possible candidates leads to the conclusion that, on presently available evidence, this state is most likely to be a highly vibrationally excited X 1 Σ g + state.

Mode-structure influences on the laser-induced optogalvanic effect in pre-breakdown discharges

The technique of quantitative laser-induced optogalvanic perturbation of pre-breakdown discharges, established in previous work, is developed through an investigation of the effect of the mode structure of the laser radiation. Results are presented using three different mode structures: single mode and two qualitatively different multimode structures. These show that one of the multimode structures, readily generated by a ring dye laser, is unsuitable for quantitative optogalvanic investigations. The other controlled multimode structure gives results broadly similar to those from single-mode perturbation. However, optogalvanic signals are harder to locate with single-mode perturbation and metastable-particle-density decay rates are higher, for a given laser intensity, thus making analysis more difficult. Also, effects possibly due to saturation become apparent at relatively low laser intensities with single-mode perturbation. Both multimode and single-mode perturbation show effects with variation of laser intensity that are not in accordance with existing theoretical expectations. These effects might be due to a subset of the 1s5 metastable population being unable to absorb the laser radiation, because of its polarization state. This implies that the magnetic substates of the 1s5 state are surprisingly resilient to collisional redistribution of population among them.

The effects of multi-mode CW laser-induced perturbations of neon metastable states on optogalvanic phenomena in pre-breakdown discharges

Optogalvanic effects have been induced in the pre-breakdown regime in neon by the use of a CW ring dye laser operating in the wavelength range 550-650 nm. Those effects that arise when the dye laser output is allowed to operate in the multi-mode configuration at a wavelength tuned to a particular neon transition from one of the four 1s to one of the ten 2p energy levels have been investigated. The strong influence of the energetic metastable 1s5 particle has been examined in detail by inducing its transition to the 2p4 level with laser radiation tuned to 594.5 nm. The effect of collisional coupling between the closely spaced 1s levels complicates the interpretation of the observed temporal and spatial changes in the nanoampere pre-breakdown ionization currents following the laser-induced perturbation. The relevant coupled differential equations, relating the particle densities appropriate to the levels of the 1s manifold and the 2p4 state, have been solved. These solutions, in conjunction with an extension of the Molnar treatment for the temporal growth of ionization in the presence of two or more metastable particles, have led to determinations of the diffusion coefficients, the fundamental time constants associated with the diffusion processes, the corresponding quenching parameters linked to the effects of trace impurities, as well as the specific action of the laser radiation. Observations of the variation of the fundamental time constant with laser intensity agree well with theoretical predictions but lead to some unexpected consequences concerning the identity of the unperturbed residue.

Ionization and current growth in N2 at very high electric field to gas density ratios

Measurements and analyses have been made of electron impact ionization and of current growth in pulsed, low-current, prebreakdown discharges in parallel-plane geometry in N2 at very high electric field to gas density ratios E/n and low products of the gas density n and electrode separation d. The E/n range and nd ranges were 1&amp;lt;E/n&amp;lt;52 kTd, where 1 Td=10−21 V m2 , and 6×1018 &amp;lt;nd&amp;lt;3×1020 m−2 (or 0.02&amp;lt;pd&amp;lt;1 Torr cm, where p is pressure) and were below breakdown values. Measurements were made of the transported charge on the time scales of electron transit, ion transit, and metastable decay. Measurements were also made of the growth of steady-state discharge currents as a function of discharge voltage. The contributions of avalanches resulting from ion- and metastable-induced secondary electrons were determined from the ratio of electron-excited N+2 391.4-nm emission integrated over all avalanches to the integrated emission during the laser-initiated electron pulse. Calculations based on ionization by electrons only show good agreement with the measured charge transported during the electron avalanche and with the current multiplication. Analysis of 391.4-nm emission data and of charge transported at E/n≥10 kTd and voltages near breakdown using the assumption of electron impact ionization leads to large apparent secondary electron yields at the cathode which increase significantly with nd. At E/n≥10 kTd, and for the voltages of our experiments, fast N2 produced in charge transfer collisions of N+2 with N2 appears to provide most of the secondary electrons. The apparently small contribution of ionization of N2 by N2, N+2 , N, and N+ is consistent with our assumption that each of these ionization cross sections is equal to that for N2-N2 collisions and so are much smaller than for Ar+ -Ar or Ar-Ar collisions.

Relative time parameters of optogalvanic responses

New optogalvanic response relative time features, such as signal growth and decay speeds, are suggested. These criteria can characterize signal time response parameters, as well as determine which particular type of gas internal quantum process is dominant during a given stage of response development. The general theoretical concepts reported here are compared with experimental results which are based on resonant optogalvanic effects in neon discharges and on non-resonant optogalvanic signals in neon and argon prebreakdown discharges. These new time characteristics are conceptually pleasing and permit study of the influence of individual factors such as bias and incident laser irradiation on overall gas response.

Dynamic nonoptogalvanic signal polarity and magnitude in prebreakdown gas discharges

Nitrogen laser pulse irradiation of prebreakdown discharges in Ne and Ar result in pulse responses strikingly similar to those reported for dynamic optogalvanic signals. For the latter, response polarity depends primarily upon atomic transition. Here, it depends primarily upon bias. Nevertheless, analysis of the results points to similar internal processes within the gas concerning metastable generation and destruction. Photoionization-assisted electron heating is an additional photon process relevant here which changes relative populations of excited states and plays a key role in reversing prebreakdown signal response polarity. It can also explain the mechanism of reported electron temperature increase in optogalvanic experiments which cannot be explained on the basis of atomic transitions.

Electric Strength of Polymers Under Vacuum at Cryogenic Temperatures

Abstract The electric strength of solid polymers under high vacuum is investigated at a temperature ranging from 293°K down to that of liquid helium (5°K) under power frequency alternating- and direct-voltages. The electric strength of polymers was found to be very dependent on the pre-breakdown partial discharges when measured in cryogenic medium whether gas or liquid. When helium was used as coolant, discharges may extend very far from the electrodes and thus exerted high electric stresses on large areas of the sample under test. These discharges may produce flash-over and serious troubles. It may also lead to localised breakdown of polymers which may give apparent reduction in dielectric strength. A large increase in the electric strength of polymers was observed when used at cryogenic temperatures under vacuum; the increase was very significant for polyethylene and polyimide specially under direct voltages. The results showed promising properties and it is concluded that the high vacuum will offer the...

Pre-Breakdown Discharges in Highly Non-uniform Fields in Relation to Gas-insulated Systems

In clean SF6 gas-insulated power transmission systems significant pre-breakdown discharges are rarely observed. This is so because the prevailing electric field is only moderately non-uniform. In practical systems, however, conducting particle contamination creates regions of highly non-uniform fields. Initial ionization, under contaminated conditions, takes the form of pre-breakdown discharges. An understanding of pre-breakdown discharges is essential for evaluating the conducting particle effects and the insulation capability of SF6 gas-insulated systems. Rod-to-plane gaps provide a convenient way of studying pre-breakdown discharges in SF6 under highly non-uniform fields under controlled laboratory conditions. The relatively few published reports of investigations of prebreakdown discharges in SF6 are reviewed. Based on the authors' and other published experimental results, several pre-breakdown regimes (for example, single pulse and multiple pulse pre-discharges and incomplete breakdowns etc.) are identified. The randomness of the time delay of the first pre-discharge current pulse is attributed to the rate of production of initiatory electrons near the anode and the growth of electron avalanches thus generated. The importance of the spatial electric field distribution around the rod electrode (anode) is emphasized. Since in practice transient overvoltages may be superimposed on the system 60 Hz ac voltage, the effect of continuous corona and the space charge due to the 60 Hz ac voltage on the overall insulation performance of SF6 gas-insulated systems under combined ac and impulse voltages also is discussed.

Voltage Breakdown Testing of Relays in the 1000-V Range

The ac voltage breakdown between adjacent conductors separated by an insulator on relays has been examined using two types of breakdown test sets and a sensitive corona (partial discharge) detector. A large difference in the breakdown voltages was observed on relays from the same population using two breakdown test sets. The characteristics of the test sets were examined to identify the cause of the breakdown difference. The breakdown voltage is independent of the voltage ramp speed from 55 to 8300 V/s. It is also independent of the series resistance up to 300 k\Omega. The different detector systems do not contribute to the breakdown difference. The output capacitance of the breakdown test sets appears to be quite important to the voltage breakdown of the samples. Modification of the output of the test sets leads to similar breakdown values. The prebreakdown discharges observed using the corona detector suggests that breakdown is a gas breakdown type. Effects on the insulator surfaces caused by the testing are considered in light of the prebreakdown discharge measurements and are expected to be minimal. These results should be applicable to the moderately high voltage testing of other products, although the numerical values will probably differ.

Development of discharges in liquid nitrogen in non-uniform electrical field

This paper reports the results of measurements of the dielectric strength of liquid nitrogen and of investigations of prebreakdown phenomena and the development of discharges at large point-plane and point-sphere electrode gaps. The tests were carried out under alternating and direct voltage conditions as well as with voltage pulses of 250/1250 μs of both polarities. It has been found that there are marked differences in the development of discharges and the formation of a gaseous phase, depending on whether the polarity of the point electrode is negative or positive. With the point negative, a very strong space charge effect on the development of the main discharge becomes manifest, due to space charge generated during the stage of prebreakdown discharges.

Stark broadening of H α and H β in ionized gases with space-charge field

In ionized gases created by prebreakdown discharges, the heat transfer from electrons to heavy particles is weak since it is often governed by electron-neutral collisions. Thermal ionization does not take place and the conductivity can be small enough to allow space-charge fields to appear. Under these conditions, the low frequency component of the field, which acts on the radiating hydrogen atom, is due to the vector sum of the ionic microfield and the space-charge field. Thus, a spatial anisotropy of the low frequency distribution of the field appears, which is derived in this paper and introduced in the general expressions of hydrogen-line broadening. It is shown that the emitted profile varies spatially and that the value of the space-charge field can be deduced from the difference of broadening between the light emitted perpendicular and parallel to this field. Calculations for H α and H β are given within the range of validity of the expressions used.

On the production and the decay of delayed molecular ultraviolet radiation in rare gas Townsend discharges

We have studied the radiation produced in pre-breakdown discharges in Ar, Kr, and Xe, in a coaxial geometry, in the pressure range 50-600 torr. In these discharges, consisting of a single Townsend avalanche, radiation continues to be emitted for an appreciable time (up to some tens of microseconds) after the end of the current pulse. The intensity of the uv component of the radiation pulse is well expressed by a formula of the type I(t)similar, equals exp (−λ1t) −exp (−λ2t), where λ1 and λ2 are specific decay times. λ1 is a function of the pressure; λ2 is almost pressure independent.The results are discussed on the assumption that excited dimers are formed in the avalanche by three-body conversion of metastable atoms. The three-body conversion frequency for the dimer formation is 6·6×10−33 cm6s−1 for Arm, 44×10−33 cm6s−1 for Krm and 25×10−33 cm6s−1 for Xem. The mean lifetime for the excited dimers is 3·7 μs for Ar2*, 1·7 μs for Kr2* and 0·5 μs for Xe2*. At the lower pressures, two-body collisions become a relatively important destruction mechanism for the metastable Xe levels.

Ionization, attachment and ion—Molecule reactions in oxygen

Values of the reaction rates for the processes O −+O 2 → O 2 −+ O and O −+2O 2 → O 3 −+O 2 have been determined from a mass spectrometric study of high pressure, pre-breakdown discharges in oxygen. Measurements are reported for values of the parameter E/N of between 60 × 10 −17 and 120 × 10 −17 V cm 2. For the second process the possibility of the reverse reaction occurring at the higher values of E/N is considered and values obtained for the reaction rate. A comparison of the values obtained simultaneously for the ionization and attachment coefficients with available data is included.