The Controlling Thermal Resistance in Forced Cooled Pipe Transmission Lines

The heat dissipated in the conductor of a forced cooled pipe type cable system must pass through two thermal resistances in series: the conduction resistance of the cable insulation and the combined forced and natural convection resistance from the cable surface to the oil. The upper limit to the convection resistance was determined by natural con- vection heat transfer tests on a full scale model. It was found that the conduction resistance is more than four times larger than the convection resistance for cables designed for 138 kV and higher voltages. Therefore, to accurately predict the temperature inside the cable, the conduction within the insulation must be accurately modelled whereas a precise prediction of the convection heat transfer is unnecessary.