Increased cable ampacity using multiple coaxial conductors and inductive compensation

Temperature buildup due to resistive losses in the conductor may limit the current capacity (ampacity) of a cable. One method for increasing cable capacity is to reduce joulean heating by using two or more coaxial conductors in a single cable to carry the desired current. To distribute the current among several concentric layers, inductive compensation in the form of internal transpositions or series connected reactors is required. Due to the geometrical arrangement of the conductors, additional eddy losses are induced by an alternating magnetic field acting on the outer layers of the conductor. A general result is obtained for the ac resistance of a multiple conductor cable, incorporating the proximity effect of the adjacent conductors. It is shown that resistive losses in the cable can be minimized by correctly choosing the thickness (radial build) of each layer. To illustrate the mathematical results, a simple example comprising two coaxial layers of conductor is presented. The example shows that lower losses (in comparison with a single layer cable of the same radius) can be obtained by using two coaxial layers in conjunction with inductive compensation. The results of this analysis should be applicable to gas-insulated (SF6) conductors which must carry large continuous currents.