Accelerated Ageing Method of Mineral Oil Under High Electric Field and Partial Discharges

During the operation of electric power transformers, their insulation systems (most often made of cellulose paper and mineral oil) are subjected to thermal, electrical, and mechanical stresses. Consequently, a continuous degradation process occurs, impacting the dielectric properties of both oil and paper. Even though the thermal ageing process of oil-paper insulation is well known, ambiguities still exist regarding the electrical stress influences upon the insulation materials. The paper presents an experimental study on the degradation of mineral oil when exposed to high strength electric field and partial discharges. In order to examine the influence of high strength electric field on oil's dielectric properties, an accelerated experimental method is proposed. The laboratory tests are conducted by causing local partial discharges in a special designed ageing cell contained a small amount of mineral oil. Dielectric spectroscopy is employed to evaluate the influence of ageing process on the real and imaginary parts of relative permittivity and dielectric loss factor. The results obtained show the ability of the proposed method to enable an accelerated oil degradation, proven by important changes of its dielectric properties.