Cooling of underground transmission lines : heat transfer measurements

Abstract

The heat dissipated in the conductor of a forced cooled pipe type cable must pass through two thermal resistances in series: the conduction resistance of the cable insulation and the convection resistance due to forced and natural convection from the cable surface to the oil. The upper limit to the convection resistance was determined by natural convection heat transfer tests on a full scale model of a pipe type cable system. It was found that conduction resistance is more than four times larger than convection resistance for cables designed for 138 kv and higher voltages. Therefore, to accurately predict the temperature inside the cable for a given oil temperature and current, a precise prediction of convection heat transfer is necessary. The solution for conduction within the cable must include effects due to cable splices and the proximity of one cable to another.