The Effect of Insulation Characteristics on Thermal Instability in HVDC Extruded Cables

This paper aims at studying the effect of cable characteristics on the thermal instability of 320 kV and 500 kV Cross-Linked Polyethylene XLPE-insulated high voltage direct-current (HVDC) cables buried in soil for different values of the applied voltages, by the means of sensitivity analysis of the insulation losses to the electrical conductivity coefficients of temperature and electric field, a and b. It also finds the value of dielectric loss coefficient bd for DC cables, which allows an analytical calculation of the temperature rise as a function of insulation losses and thermal resistances. A Matlab code is used to iteratively solve Maxwell’s equations and find the electric field distribution, the insulation losses and the temperature rise inside the insulation due to insulation losses of the cable subjected to load cycles according to CIGRÉ Technical Brochure 496. Thermal stability diagrams are found to study the thermal instability and its relationship with the cable ampacity. The results show high dependency of the thermal stability on the electrical conductivity of cable insulating material, as expressed via the conductivity coefficients of temperature and electric field. The effect of insulation thickness on both the insulation losses and the thermal stability is also investigated.