Parameters of extruded insulation for H.V.D.C. cables

The work described attempts to define the steady state d.c. resistivity and governing parameters of materials for H.V.D.C. extruded cables. With high density polyethylene steady state was never achieved in tests of several days´ duration. With EPR, polypropylene and cross-linked polyethylene steady state was achieved within twenty hours at low stresses (<; 10 MV/m) and at temperatures above 50°C. Attempts are made to use the relationship, ρ = ρ₀e^-(αθ+βE), which is used for paper-oil insulation. Although consistent values for α are obtained, it is felt that further work is required to define β; also its variation with temperature may require modification of the resistivity equation. Additions of inorganic materials to polyethylene are seen to bring about steady-state conduction in a few hours.