Ampacities of Underground Electric Power Cables in the Presence of External Forced Cooling

This paper presents a method of computing the ampacity of underground cables that are cooled by external pipes carrying a coolant. The method utilizes conduction shape factors and an effective thermal resistivity which represents the presence of both the surrounding soil and the cooling pipes. When used in place of the native soil resistivity, the effective resistivity value will produce the same cable temperatures as would exist when a cable is buried near external cooling pipes. The effective resistivity value can therefore be used in ampacity tables or existing ampacity programs, thereby extending their application to cases that involve forced cooling. An experimental horizontal three-cable/two-pipe system was used to verify the results obtained from the mathematical analysis. The predicted effective thermal resistivities for this system were used in an ampacity program and the resulting conductor temperatures were within a maximum of 3.4?? C of the experimentally measured temperatures for conductor temperatures up to 105?? C.