Modification of dependence of DC insulation resistivity of low density polyethylene (LDPE) on temperature and electric field using inorganic additives

One of the reasons which has hampered the use of polyethylene (PE) as an insulating material for HVDC cables is the inherent dependence of its dc insulation resistivity ρ_v on temperature T and electric field E. The objectives of the present work are: (i) to investigate the possibility of modifying the dependence of the insulation resistivity ρ_v of LDPE on temperature and electric field by doping LDPE with an inorganic additive, and (ii) to find a mathematical model representing that dependence. Measurements of dc resistivity were made using relatively thick samples (approximately 2 mm) of undoped and doped LDPE at different electric fields ranging from 17 kV/mm to 33 kV/mm for temperatures from 50 °C to 80 °C. Results indicate that the use of the additive has a significant effect on the rate of decay of the insulation resistivity with temperature in the doped material. Based on these measurements, the dependence ρ_v = f(E,T) was found to conform to the law ρ_v = ρ_oe^{-[α*(E)T+β*(E)]}. Nonlinear curve fitting was used to determine the coefficients α*(E) and β*(E). It is concluded that use of additives to LDPE can be a promising method for manufacturing insulating materials for HVDC cables.