Improvement of the Standard Ampacity Calculations for Power Cables Installed in Trefoil Formations in Ventilated Tunnels

Forced ventilated underground installations are challenging to design due to the complex mathematics of turbulent fluid flow. Previous methods have tackled this problem, but only recently, the effect of confinement on a single cable has been considered. In this paper, the effect of cable confinement on the practical case of ventilated trefoil installations is investigated. The complete Computational Fluid Dynamics (CFD) problem is solved using Finite Element Method (FEM) simulations and then validated by laboratory experiments using pipes. FEM simulations are then used to vary the current, the wind velocity, and the distance of the trefoil from the tunnel wall to get a comprehensive picture of the phenomena. This data is used to create a new empirical equation for the Nusselt number for a wide range of operating conditions. The principle of similitude is validated for a scaled-down model of a large tunnel and then used to analyze the thermal rating of tunnels. The new equation improves the IEC standard equations by accounting for the effect of cable confinement on trefoil installations. An important finding of this paper is that a lower maximum permissible current rating is obtained, which if ignored can present a serious risk for thermal damage. It was also found that the cables must be installed three cable diameters away from the wall to prevent the derating effect of confinement.