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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 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)

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.