Thermal transients in a high-voltage cable system with natural and artificial cooling

As the overload capacity of cables is governed by their temperaturerise/time characteristics a convenient and relatively rapid means of predicting these characteristics is of great utility. A resistance-capacitance type of electrical analogue simulating the thermal problem of three single-core high-voltage oil-filled cables laid direct in soil in horizontal formation is described. The temperature-rise/time characteristics resulting from step functions of cable copper loss are obtained for points in the cables and the surrounding soil, and those for the cables are shown to comprise three distinct exponential terms. The effects of dielectric and sheath losses are also investigated. A modified form of the analogue is used to simulate cooling by means of the flow of water through two tubes laid between the cables. The thermal time-constants in these conditions are seen to be much smaller than without water cooling, and a large uprating is possible. Of the total heat evolved 73% is dissipated into the water. A simplified analogue of one single cable is also described, and the transients obtained are compared with those resulting from published analytical methods.