Computer simulation of the thermal environment inside a 275-kV underground electric cable tunnel

The thermal environment inside a proposed 275-kV electric cable tunnel in Hong Kong is discussed in this paper. The tunnel is 5 km long and composed of four sets of circuits buried in cable trenches. Each set of circuits dissipates 103.5 W of thermal power per unit meter length of tunnel. The problem is simulated by considering conduction in the solid materials surrounding the tunnel and convection in the tunnel air. Equations for thermal conduction inside the solid structures are solved numerically first. The induced natural convective air current is then predicted using the field-modeling technique to compute the air velocity and temperature. The predicted results of the tunnel air temperature are used for deciding whether a forced ventilation system is necessary to keep the air temperature below 40 °C. Further, the design of the mechanical ventilation system is illustrated by roughly estimating the required incoming air speed to maintain the tunnel below 40 °C.