The present work concerns the moisture effect on electrical features of insulating paper. Despite the progressive replacement of paper by different materials, several blocks and other insulating screens in transformers contain cellulose which is characterized by a very high hygroscopicity. These cellulosic products appear as a source of moisture. This moisture can lead to a serious damage in the electrical equipments. The aim of this work was also to focus our attention on examination of dielectric losses evolution with the moisture level. For a better understanding of phenomena, we have separately examined the moisture influence on both conductivity and permittivity of the paper.

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The use of paper in electrical insulation, especially in power transformers, remains a topical subject. The interest for the insulating paper, despite the emergence of many synthetic materials is ascribed to its low cost price and its ability to an oil impregnation. However its strong hydrophilic character remains the main difficulty for its implementation in electrical insulation. This affinity to water absorption and/or adsorption shows that a good understanding of the moisture effect becomes imperative. Several studies were carried out on the subject but a systematic study of the moisture effect on overall behavior of paper has not been achieved. This work is meant to be a contribution for this systematic investigation. The first step of this study, which is the object of the present article, concerns an examination of time variation of the electrical currents, under dc stress, for different moisture levels. A particular attention is given to the polarization and depolarization currents. These currents are examined during alternate charges and discharges sequences. An increase of the current, during the application of continuous voltage was noticed. This unexpected phenomenon would be related to an electro-dissociation and/or polarization processes.Index Terms -Paper insulation, dielectric materials, moisture effect, dielectric polarization, electrolytic dissociation, electric field effects.

An experimental investigation of the charging and discharging currents in polypropylene and polystyrene under 1000 V DC stress were presented. The results showed a distinct difference between the charging and discharging currents. As known, the charging current after the DC voltage application decreases rapidly during the first hundred seconds then approached a stabilisation. The discharging current flowing in the opposite direction of the charging current was observed. It was concluded that space charge have formed in the sample. The charging and discharging currents for both the polypropylene and the polystyrene increased with increasing thermal aging time at 80 °C. It was established that the rise in the current depends on the thermal aging time which induces anomalous discharging current flowing in the same direction as the applied field.

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