الفهرس 
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Abstract
The present work introduces a comprehensive investigation on the effect of gas cavities on the behavior of the widely used oil-paper insulating materials. Experiments on the breakdown voltage, partial discharge, inception voltage, and deformation level have been done in order to explain the differences between the impact of gas cavity on the oil-paper insulation system behavior in cases of either AC or DC applied electric stress. More investigations have been implemented in order to understand the role that the charge carriers play affecting the oil-paper insulation behavior especially when HVDC is applied. Different factors that may explain the charge carriers role in case of DC electric stress such as the oil gap distances, the cavity volume, the polarity of the applied voltage, and different insulation system arrangement have been studied.
The electric field distribution in oil-paper insulation has been calculated when either AC or DC electric stress is applied with the using of finite element method technique (FEM). The aim of this part is to understand the main differences in the behavior of the insulating materials under the effect of AC and DC electric stress in presence of small gas cavities. charge carriers role can be illustrated by comparing the experimentally obtained results and the expected insulation system behavior that depends on the electric field calculations.