FAEEF7E3d01 - IEEE Transactions on Electrical Insulation...

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IEEE Transactions on Electrical Insulation Vol. 28 No. 3, June 1993 Discharges in All-PP Film Model Capacitors 315 M. Nemamcha, J. P. Gosse and B. Gosse C. N. R. S, Laboratoire d’Electrostatique et de MatCriaux Dielectriques, Laboratoire associC 1 1’UniversitC J. Fourier, Grenoble France ABSTRACT Discharges in model capacitors have been studied with a digital analyzer. The dielectric was polypropylene impregnated with an organic liquid. We have investigated the effect of the nature of the liquid (aliphatic or aromatic hydrocarbon) and the influ- ence of parameters such as voltage, pressure and temperature on the main characteristics of the discharges. Discharges were also monitored during aging tests of the models. We conclude that aging of the models we tested at high electric fields (- 135 V/pm) is not due to discharges but to the chemical degradation of polypropylene submitted to electrical and thermal stresses. 1. INTRODUCTION UCH work has been done on discharges within air or M gaseous inclusions in a solid insulation [l-31. Mech- anisms by which discharges cause progressive insulation degradation and breakdown have been considered [4,5]. On the other hand, liquid-impregnated insulation has been studied less extensively. Most of the background work deals with paper-oil insulation systems [6], whereas the major lifetime limiting mechanism in capacitors made of polypropylene impregnated with an organic liquid is the result of discharges at the foil edges [7]. Nevertheless, in previous work [8] on all-film model capacitors, we have observed that under severe conditions of field and temper- ature, impregnated polypropylene was greatly damaged though our discharge detection system gave no signal for the voltage applied during the aging test. To determine if another mechanism might cause the degradation of our model capacitors, we have performed a thorough study of discharges in our models and of their voltage dependence. We have used new computer-aided equipment to register the discharge characteristics for dif- ferent applied voltages and so to obtain the inception volt- age of the discharges. We will discuss this voltage which depends on the sensitivity of the experimental setup. We have also localized the discharges by recording the light emitted by the model capacitors, in order to correlate the phase distributions of the number of discharges n(4) and of the mean discharge magnitude q(4) with their location, as has been done for several electrode systems containing cavities filled with gas [9, lo]. Lastly, the electrical behav- ior of an impregnated insulation is strongly dependent on the nature of the impregnant. It is well known that the liquid has to be gas absorbing [Ill to provide the capaci- tors with good performance at low temperature. This is the case for aromatic liquids, but not for aliphatic hydro- carbons which are gas evolving. But this property is not related to discharges in the liquid phase. In this work, we compare the characteristics of discharges in our mod-
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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FAEEF7E3d01 - IEEE Transactions on Electrical Insulation...

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