A. Denat

Study of streamer inception in cyclohexane with a sensitive charge measurement technique under impulse voltage

Abstract This paper presents an experimental study of streamer initiation in liquid cyclohexane. The aim is to compare specific features of streamer initiation in positive and negative polarity. Experiments are carried out in point-plane geometry under impulse voltage. A new differential charge measurement system is presented. It allows to obtain a high detection sensitivity under impulse voltage (below 0.1xa0pC). The investigations presented consider the influence of applied voltage, impulse rise time, hydrostatic pressure. As previously observed under DC voltage, negative streamer initiation is due to an avalanche in the liquid phase, and is not affected by pressure, up to 10xa0MPa. In contrast, the mechanisms leading to positive streamer initiation are not clearly established. In this polarity, an increase of streamer initiation voltage is observed when the pressure is raised.

Self-healing of capacitors with metallized film technology: - experimental observations and theoretical model

Abstract Capacitors made of metallized polypropylene films suffer partial discharges, called self-healing, due to weak electrical defects. Those defects are destroyed by an electrical arc that extinguishes when enough metal of the electrodes is vapourized around this point. From experimental results, we have elaborated a model of the self-healing phenomenon. The present work shows that the volatilized area of self-healing is mainly due to the Joule heating of the metal by the current pulse induced by the local breakdown, and the local discharge stops (=arc extinction), although the voltage applied to the film remains constant, when the power density becomes lower than some critical power density.

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.

DYNAMICS OF MICROSCOPIC BUBBLES GENERATED BY A CORONA DISCHARGE IN INSULATING LIQUIDS : INFLUENCE OF PRESSURE

Abstract In this paper, we present an experimental study of the successive rebounds of a bubble as a function of several parameters, hydrostatic pressure P ∞ (in the range 0.1–10 MPa), electrical energy W i deposited by the current pulse (1–20 nJ) and nature of the liquid. Two loss mechanisms have been clearly identified, viscous losses and losses correlated with the acoustically radiated energy. However, the very important damping of the successive rebounds of the bubble cannot be quantitatively explained by only these two mechanisms. Our experiments show that other important dissipative processes are to be considered during the bubble life time.

Luminescence from Liquid Helium Excited by Corona Discharges

Liquid helium (LHe) at 4.2 K was electronically excited using a corona discharge for both negative and positive high voltages. The experiments were carried out for different pressures in the range from 0.1 to 10 MPa at constant temperature. The light emitted from the zone close to the tip was spectroscopically analyzed showing features from atoms and excimer helium. The shifts and widths of the observed lines and bands were found to depend on the applied hydrostatic pressure and on the tip polarity. Our analysis showed that classic pressure broadening theory cannot account for the observed widths and shifts rather than the presence of bubbles which surround single excited atoms and molecules. For positive tip polarities red shifted features distinct from pure He and He2* were observed and tentatively assigned to red satellites.

Insulating properties of some liquids after an electrical arc

The insulating properties of a liquid are commonly studied before a breakdown occurs. In HV industrial apparatus such as high energy plastic metallized and liquid impregnated films capacitors, some local breakdowns, named self-healing, occur and the ability of the liquid to remain insulating after electrical degradation is of interest. To understand the influence of the liquid on the behavior of such capacitors, the post-arc behavior of the liquid alone has to be studied in a range of energy (/spl sim/mJ) close to the self-healing process. The present work deals with the study of the ability of different liquids to restore in the millisecond range their insulating property after an electrical arc. Using an experimental apparatus based on the repetitive charging with a dc HV generator and discharging through an arc of a resistor-capacity system, different kind of liquids were tested. The liquids we used were alcane, esters, alkyl benzene and polyaromatic. We showed as expected that liquids with the smallest ratio of carbon versus hydrogen in their molecular formula present the best restoration property after an electrical arc, property that is crucial for liquids impregnating metallized capacitors.

Influence of a metallic particle at a metal/insulator/gas triple junction in air and SF6

In gas insulated switchgears (GIS), high voltage electrodes are supported by solid insulators. A triple junction (TJ) exists at the interface between metal, insulating solid, and gas. This area constitutes a preferential zone were partial discharges (PDs) can appear. A further degradation of insulation performance can occur if solid particles accumulate at that place. This paper presents experiments carried out to obtain a quantitative account of the influence of a metallic particle located close to a triple junction area, both in dry air and SF6. PDs are detected and measured by conventional electrical measurements, and observed optically with a high-sensitivity intensified camera. Measurements under AC voltage show the influence of the particle on PD inception and flash-over voltage, in comparison with the same geometry without particle. In SF6, the particle induces a large decrease of PD inception and breakdown voltages, which become nearly independent of the gas pressure.

Pressure waves and bubble generation in non-polar liquids initiated by a high divergent electric field

This paper describes experimental studies on: (i) acoustic transients emitted after current pulse and cavitation bubble collapse with special emphasis on the chronology of events and, (ii) the influence of fluid viscosity on bubble dynamics. Several interesting results are obtained: the relative amplitude of the acoustic transient is proportional to W/sub i//sup 1/2/ where W/sub i/ is the injected energy and, the relative amplitude of the shock wave is independent from the applied pressure. Our experimental results are in good agreement with theoretical models. For small bubbles in viscous fluids, bubble lifetime is much higher than the Rayleigh time and rebounds of the bubble are practically undetectable.

Work functions for a high-voltage cathode in non-polar liquids

When a negative high voltage is applied to a needle of small radius of curvature (r/sub p/ < 0.2 /spl mu/m), a field emission current I(U/sub FN/) is observed. For larger point radii, a regular current pulse regime is detected above a threshold voltage U/sub s/. Analysis of the U/sub s/(V/sub o/) and U/sub FN/(V/sub o/) dependences shows that the apparent work function for high electric field strengths is lower than the one without voltage (1.5 eV compared to 4.5 eV). This difference can be explained if one assumes the existence of a plasma-like layer around the high-voltage cathode.

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

Results of an investigation of the threshold voltage U/sub s/ for corona discharges in fluid argon for a wide range of densities are presented. The dependence of U/sub s/ on the fluid number density N is derived. It is found to be nonmonotonic at very high N values.