Fundamental Studies on Impulse Breakdown of Oil-Impregnated PPLP Systems

Abstract

PPLP (Polypropylene Laminated Paper) is a prominent insulating material with high dielectric breakdown strength and low dielectric loss, having been actually used for OF cables in EHV/UHV transmission systems. Nevertheless, the dielectric breakdown mechanism of PPLP has not been cleared sufficiently. The work reported here concerns the breakdown studies in oil-impregnated paper systems with a standard 1.2/50μs impulse voltage wave simulating lightning. The effects of PP (Polypropylene) films on the impulse breakdown strength of oil-impregnated PPLP systems were studied, compared with the impulse breakdown mechanism of oil-impregnated kraft paper systems. As a result, this paper emphasizes that concerning the impulse break-down mechanism of laminated oil-impregnated papers, there is a great difference between the cases of including PP films and of being made of kraft papers only. It has been found from the detailed investigation of the experimental results that the positive charges, especially trapped on the surface of the PP films, may be most important factor affecting the impulse breakdown strength. A fundamental and practical suggestion is made from the viewpoints of raising the impulse breakdown strength of the OF cables insulated with PPLP, based on the experimental results.