The effect of soil thermal conductivity and cable ampacity on the thermal performance and material costs of underground transmission line

Abstract

This paper presents an optimisation of material costs of a 400 kV cable line arranged in a flat formation for different values of soil thermal conductivity and cable ampacity. The Finite Element Method is adopted to calculate the temperature field within the cable system. The Particle Swarm Optimisation method is used to minimise the material costs of the cable system. To ensure safe and reliable operation of the cable line, the temperature of the cable core should be lower than 90 °C. Under this assumption, the design optimisation is performed to calculate the optimum size of cable bedding cross-sectional area and select the nominal cable conductor area correctly. An application of two commonly used cable backfill materials is studied: Fluidized Thermal Backfill and Sand-Cement Mix. The calculations are performed for current rating varied from 1,000 A to 1,400 A, and thermal conductivity of soil varied from 0.6 W/(m × K) to 1 W/(m × K). The results showed that for a low soil thermal conductivity, i.e. 0.7 W/(m K), it is possible to reduce the overall material cost of the system (cables cost and thermal backfill material cost) when applying Fluidized Thermal Backfill (FTB) instead of commonly used Sand-Cement Mix. The reason is that FTB application allows reducing the cross-sectional area of power cables, which cost has a major effect on the overall material cost of the underground transmission line.