Impacts of moisture absorption on electrical properties of rigid polyurethane foam for composite post insulator of UHVDC transmission line

Abstract Polyurethane-based porous material is a promising candidate for internal insulating medium of composite post insulators of ultra-high voltage direct current transmission line because it avoids the gas leakage of SF6 or N2 post insulator and is superior to the epoxy resin post insulator in weight. In this paper, firstly, a series of rigid polyurethane foams with various diphenylmethane diisocyanate contents are synthesized and cell morphology is investigated. Thereafter, the effects of temperature and exposure time on hygroscopicity of polyurethane foams at various humidity levels (non-immersion in water) are explored. With the ultimate goal of polyurethane foams for use in internal insulation of composite insulator, the impacts of moisture absorption on electrical properties of polyurethane foams are studied systematically. The moisture absorption results of polyurethane foams of various ratios demonstrate that they increase with the relative humidity and temperature and they exhibited a saturation state. The volume resistivity and breakdown strength of polyurethane foam with various diphenylmethane diisocyanate contents all decrease significantly with the increase of relative humidity. The great differences of breakdown field are found at different Weibull slope of intrinsic breakdown. Simultaneously, the permittivity and loss of foam specimens all increase significantly after the moisture absorption, especially foams of 1:0.9 and 1:1.3. It is found that the appropriate ratio of raw material could result in a significant improvement of the dielectric losses.