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The partial discharge plays an important role in the ageing and the rupture process of solid or mixed insulation systems. It has been recognized that the failure of this insulation can be joined to the presence of partial discharge often in inclusions sparkling. Liquid filled cavities can be considered as the most likely defects that can exist in capacitors. In this paper we describe the partial discharge evolution at low temperatures in all-PP film capacitors according to the time and the applied voltage. We distinguish two regimes of discharges for all the range of temperature and the low temperatures encourage the breakdown of capacitors at weak voltage, we assign this phenomenon to the increase of the viscosity of filling liquid.

The equivalent series resistance of polypropylene capacitors has been determined under rated voltage, in the range 1 kHz–1 MHz, between 220 K and 370 K by a calorimetric technique. The original feature of this determination of capacitor losses lies in the use of the isothermal calorimetry and in the measurement of an electrical power and not of a temperature increase. The frequency dependence of the equivalent series resistance, at various temperatures, enables to separate the losses in the conducting material from those in the dielectric and to get their respective variations as a function of frequency and temperature. These variations of the equivalent series resistance with frequency at a given temperature have been reproduced by using an equivalent circuit composed of resistors, inductors and capacitors. This model has been verified for non-sinusoidal waveforms such as those met with in a filtering circuit and is used to evaluate by simulation the losses of the capacitor.

The time dependence of the surface potential of an insulating material is theoretically investigated in the case of simultaneous surface and volume ohmic conduction. When the material presents a constant surface conductivity, the first part of the decrease with time of the surface potential is strongly influenced by the initial distribution of charges and cannot be used to determine the surface conductivity. On the other hand, at rather long times after the surface has been charged, the potential of the surface follows an exponential variation with time, the expression of the corresponding time constant is given in different cases. This model is found in good agreement with experiments performed with photo-oxidized epoxy resins placed in a humid atmosphere.

The electrical conduction of purified aromatic liquids in the tip-plane electrode geometry was studied for high electric fields. The current voltage curves, the current pulses were measured for dc applied voltage in toluene, benzyltoluene, and in the saturated hydrocarbon ndecane. In n-decane, current pulses are similar to corona discharges in electronegative gases and to pulses previously studied in liquid cyclohexane. In aromatic liquids, a discontinuity in the current-voltage curve was also observed for an electric field of 4.5±0.5 MV.cm -1 whereas current pulses were detected for higher fields. This behavior is attributed to the ionization and electronic attachment of aromatic molecules

The great influence of O 2 on the electrical aging of polypropylene films. The lifetime of the capacitor model with 2x13.6 μm-thick PP films is about 25 h with O 2 saturated liquid and more than 1500 h without O 2 . Two different degradation are obtained on a given film and good agreement is shown between electrical (breakdown voltage) and physicochemical measurements (FTIR spectroscopy)

The authors have investigated the high field conduction of aromatic liquids (toluene and benzyltoluene) in the tip-plane geometry and compared it to the conduction of n-decane under the same experimental conditions. For various tip radii they have determined the threshold field E/sub s/ for which current becomes space-charge controlled, the threshold field in the case of which current pulses are detected, the charge of each pulse and their repetition rate. Results on DC and AC voltage have been compared. It is shown that the basic prebreakdown phenomenon in aromatic liquids, i.e., electron avalanche in the liquid phase, is similar to that in aliphatic liquids. The main difference in the high field behavior of aliphatic and aromatic liquids comes from a deposit on the tip formed by polymerization of the liquid in the ionization region. Thus, after a very short period of voltage application during which the tip surface changes, current pulses are observed for about a two-fold higher electric field.

Partial discharges in a capacitor-type structure have been registered with a digital discharge analyzer for increasing voltages up to breakdown. The capacitor contained two 13.6 /spl mu/m thick PP films pressed between two plane electrodes. The discharges were located by FTIR mapping at the edge of the electrodes. At high applied voltages, electrochemiluminescence and discharges give light emission, the first phenomenon being predominant. The variations with time of the light emitted by the discharges have been correlated with the phase-distributions of their number and charge.

The electrical conduction of purified toluene, in liquid and gaseous states for negative point-plane electrode geometry, has been studied for high electric field. In toluene liquid, the results depend greatly on the previous use of the point. After a long use of the point, electrical conduction is influenced by an organic deposit at the used tip. This deposit is due to the polymerization of toluene. In toluene vapor, the polymerization by electric field becomes significant. The visualization of the phenomenon at the extremity of the cathode point shows the formation and the growth of a solid filament which consists of aliphatic and aromatic groups.

The charging of SiO/sub 2//glass samples under pulsed electron irradiation have been experimentally studied. More specifically, results about the secondary electron emission yield of the uncharged surface have been obtained and successfully compared to a semiempirical model. Moreover, we have pointed out a strong deviation of the evolution of the secondary emission yield (during a series of pulsed irradiation) compared to the theoretical expected one; these contradictory behaviors are explained by the influence of the trapped charges on the secondary electron emission from the charged sample surface.

Electrochemical properties of biphenylnitriles and phenylbenzoates are given and are compared with those of other liquid crystal (LC) families. The experimental results prove that a small gap and the reversibility of the reactions occurring at the electrodes favourably influence the LC life-time during d.c. dynamic scattering.

On etudie le comportement du carbonate de propylene pur en haute tension apres avoir recherche par voie electrochimique les reactions d'oxydation ou de reduction de ce solvant ou de ses impuretes.

Non-linear losses in heterogeneous iusulation (e.g. impregnated paper) are often attributed to ions oscillating between insulating paper walls without penetrating or even adhering on them. To investigate this problem, we studied the physico-chemical properties of capacitor paper and of other insulating solids having the same blocking behavior. We have investigated the transport of electrolytes across paper impregnated with organic solvents due to an applied electric field or to a pressure gradient, the adsorption of electrolyte, and the net charge carried by paper by means of electrophoresis. It turns out that in the organic liquids tested, paper behaves like a semi-permeable barrier. Synthetic polymers also exhibit the blocking effect when they are made selective by grafted weak acid groups. We demonstrate that the ion blocking effect is of a physical and chemical nature and not of a mechanical one. The chemical nature of paper and impregnant modifies the amount of absorbed electrolyte and above all changes the sign of the net charge on the paper; on the other hand, the pore diameter modifies the concentration value for which selectivity no longer operates.

La conduction de solutions d'électrolytes dans le cyclohexane a été étudiée entre électrodes recouvertes de films d'alumine ou de polyphénylène-oxyde lors de l'application d'un échelon de tension continue. L'injection faible observée aux temps courts dans les solutions de TlAPi a les mêmes caractéristiques qu'entre électrodes métalliques nues (même variation avec le champ, avec la conductivité de la solution). Les ions injectés sont les ions de l'électrolyte, ils sont créés dans la zone de force-image. Aux temps longs le film qui bloque les ions du liquide, devient un injecteur très fort d'ions dans le liquide. La nature de ces ions n'a pas été déterminée.

The transient space‐charge‐limited current technique developed in crystals is made to fit polar liquids: First, from a theoretical point of view the injection is no longer only limited by space charge, the electrode–liquid contact being non‐Ohmic; second, from an experimental point of view, the electric field distribution is at every moment determined by means of the electro‐optic Kerr effect, and known ions are injected by permselective membranes. Applying this technique makes it possible to estimate the mobility of different carriers injected into different polar liquids and brings out high mobility values of about 2 × 10−3 cm2 V−1·sec−1. Account is taken of the influence of bulk liquid motion to explain the high values of ionic velocities obtained.

Conduction in dielectric liquids has been reviewed many times (Lewis, 1959; Sharbaugh and Watson, 1962; Felici, 1971; Gallagher, 1975). Since then, few major improvements of the understanding of conduction mechanisms have been achieved: all the possible mechanisms had been previously put forward. The main contribution of the latest works has been the experimental verification of some of the above models and, more especially, the definition of their domains of validity. There are many reasons for such a slow progress. It is impossible to control the nature and concentration of impurities acting on the electric conduction of very resistive liquids, conductivity remaining more sensitive to impurities than physicochemical techniques. Industry uses liquid insulants only for particular applications; they were essentially mineral oil and chlorobiphenyl liquids which gave full satisfaction, and it needed the prohibition of the PCBs to bring about a new interest in conduction studies.

During unipolar (or ambipolar) injection of charge carriers into an insulating liquid, a liquid motion is generally observed. This motion, generated by the charge carriers, is thought to be responsible for the high carrier mobility which is about 2×10−3 cm3 V−1 sec−1 in nitrobenzene, i.e., an order of magnitude higher than the true mobility value (1.9×10−4 cm2 V−1 sec−1). Recent schlieren photographs strongly support the assumption that when the applied voltage is high enough, for nitrobenzene of the order 103‐104 V, the “self‐generated” motion is turbulent shortly after the voltage is applied. This turbulence invades the undisturbed fluid at practically constant velocity. Neglecting the true carrier mobility and making certain assumptions concerning the turbulent structure, the charge transport and consequently the entrainment can be described to a good approximation by a Lagrangian diffusion process and the charge distribution is governed by a diffusion equation. The latter remains valid during the stea...