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.

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.

In high-voltage applications, insulators may be exposed to corona discharges during long periods. In this experimental work, corona discharge tests of different durations are carried out in air at atmospheric pressure on polymethyl methacrylate (PMMA) samples. The resulting surface degradation is studied with several techniques. The surface damages are observed with environmental scanning electron microscope and atomic force microscopy. The results show that electrical trees occur on the surface of material and their distribution depends on the corona discharge duration. The chemical changes on PMMA surface are analyzed by Fourier transform infrared spectroscopy and a chemical degradation mechanism is proposed. Evolution of surface resistivity with corona aging is also implemented by using a classical I(V) method. In addition, to study the dielectric behavior of PMMA, the monitoring of kinetics of the trapped charge under electron irradiation in a scanning electron microscope is performed. The charging ability of PMMA under electron irradiation and its time constant of charging decrease with electrical aging.

This paper is devoted to recovery voltage as a diagnostic non destructive method for an assessment of an insulating state of power transformers. The insulating pressboard used in power transformers is tested for different moisture levels. Experiments were carried out with samples from the same material intended to power transformers building. A charge and discharge of a capacitor having pressboard paper as dielectric is a suitable tool for an appreciation of the polarization and relaxation processes. The polar character of the water's molecules has an influence on the permittivity. Then the polarization of the paper will be affected by a moisture presence. Time evolution of the recovery voltage for different durations of the charges was examined. An increase of the recovery voltage with the moisture content was observed. The maximum of recovery voltage occurs much later starting from the moment of the opening of the circuit for higher moisture for all examined levels. Thus, the time separating the short circuit and the maximum's occurring can be considered like an important parameter allowing an appreciation of the moisture state of oil impregnated paper.

The electrical discharge and breakdown characteristics of a triple junction (TJ) in high voltage (HV) electrical insulation are becoming an important issue in recent industrial applications. In this paper, a comparative study is conducted between three insulation materials used in HV: silicone, porcelain, and heat tempered glass, with electrode shape parameters such as inter-electrode distance (d), applied voltage (V), and geometry investigated for positive polarity, using an experimental approach. For optimum test organization to support our research and highlight the parameters and their interactions, we have established a mathematical regression model of the discharge current. It is polynomial, with good performance and a high level of precision. Besides, this yields one monovariable and another two-variable quadratic model. The latter can simulate the electric discharge simultaneously with two variables: V and d, and an adjusted determination coefficient () of nearly 0.99. Consequently, the two-variable quadratic model has been adapted to V and d ranges of 10 to 50 kV and 1.9 to 8 cm, respectively. Finally, this study provides us with new applicable knowledge about the TJ effect on electrical insulation with recommendations to optimize the design for higher performance.