Internally force-cooled power cables; optimum working temperature in the range 70¿358 K

Current ratings are compared for both copper and aluminium conductors, internally cooled with a variety of coolants, working in the temperature range 70 K to above ambient. Capital and running costs are then estimated for optimised designs of cable cooled with some of the more promising of these coolants. It is shown that there is no economic advantage in the deep refrigeration of the conductors, and that the optimum temperature of of operation for a force-cooled cable is near to ambient. With 400 kV cables, the preferred coolant is a low-viscosity mineral oil. Water may also merit consideration as a coolant, but further study is needed of the problems of segregation from the dielectric, and design of feed joints and terminations. 1000 kV cables are also considered, but because of the high cost of reactive compensation, unit capital costs are higher than for 400 kV cables. The preferred coolant for 1000 kV cables is a low-viscosity mineral oil. With oil-cooled cables, some economic, advantage can be gained by refrigerating to coolant-inlet temperatures of down to about ¿40°C.