Simulation of Electrical Field Distribution of HVDC and EHVDC Cable during Load Cycles in Type Test

Extruded HVDC and EHVDC cable systems are taking a leading role in long distance and off-shore power transmission. Cable systems with higher rated voltages and elevated working temperature are being developed with the rising demand for large transmission capacity. When DC cables are qualified during type test, electrical field distribution inside cable insulation could be critical under certain thermal conditions. The operational thermal limits could be obtained by determining electrical field distribution in the cable. Temperature profiles were taken from load cycle tests of type test of 400 kV and 535 kV DC XLPE cables, respectively. XLPE electrical conductivity parameters were obtained using cable slices. With XLPE conductivity analytically fitted with field and temperature coefficients, the electrical field distribution in the cable insulation under different thermal conditions are investigated by simulation. The simulation results show that a large temperature gradient over the cable insulation can lead to breakdown at insulation screen of the cable during type test, especially for EHVDC cables. The permissible working conditions of DC cable should be determined considering both conductor temperature and temperature gradient. With understanding of electrical field distribution in the cable insulation, thermal conditions during type test can be controlled properly to ensure a successful cable qualification.