Pressboard roughness and ageing influence on mineral oil electrification

Electrification of mineral oil and pressboard during power transformers operation may lead to high energy electrical discharges and finally, to the transformers failures. This paper presents the results of a study concerning the influence of pressboard roughness and ageing condition on the mineral oil electrification. The experiments were performed on unaged and thermally aged (1000 h at T = 135°C) pressboard samples. The samples roughness, electrical properties (dc electrical conductivity, permittivity and loss factor) and electrification current were measured at different ageing times. The volume charge density values for the compact layer at the pressboard-oil interface as well as inside the oil were also computed. Finally, it is shown that the thermal ageing process leads to increase of pressboard samples roughness and a proportional relation between oil volume charge density and the roughness exists.