Dependence of DC insulation resistivity of polyethylene on temperature and electric field

Recently, the development of HVDC cables with polymeric insulation has received a renewed attention. The present work aims at determining the dependence of the dc insulation resistivity on temperature and electric field for low-density polyethylene (LDPE) which is used for manufacturing modern high voltage ac cables. Using a highly precise technique, resistivity measurements were made using thick (~2 mm) disc samples at different dc electric fields ranging from 17 kV/mm to 33 kV/mm for a temperature range from 50°C to 80°C. Results indicate that the dc insulation resistivity of the investigated LDPE (ρ) is a rapidly decreasing function of both temperature (T) and electric field (E). This relationship conforms generally to the law ρ=ρ₀ e^-(αT+βE). The values of ρ₀, α and β were determined using computer curve fitting techniques. Comparison between the present results and similar previously reported measurements made on less clean grades of LDPE indicates that the inherent dependence of the dc insulation resistivity of LDPE on temperature and electric field has not been appreciably improved by using a ultra-clean polymer. It is concluded that the successful utilization of LDPE for HVDC cables will require the modification of such dependence