HV Cables Thermal Rating under Steady-State Conditions in Unfilled Ducts

The increasing use of underground high-voltage cable systems in complex urban and infrastructure environments involves the adoption of trenchless laying methods, both for decreasing the occupation time of construction sites space and for crossing obstacles (i.e. roads, rivers, industrial areas), leading to an ever-increasing spreading of unfilled ducts laying sections; thus accurate thermal design methods for unfilled ducts are becoming of major concern. The most conventional thermal rating analytical method (IEC 60287) assumes a simplified heat transfer mode: empirical data are used to estimate the “dummy” thermal resistance. While this approach can be suitable for standard installations, it may introduce significant inaccuracies in complex installations showing casing ducts, both metallic and nonmetallic. This study compares the steady-state ampacity of HVAC cables laid in unfilled ducts as calculated using the IEC standard method and Finite Element Method (FEM) simulations based on the principles outlined in CIGRE Technical Brochure 963.