Calculation of cable thermal rating considering non-isothermal earth surface

The study presents an algorithm to compute the heat transfer coefficient for thermal cable rating when modelling the non-isothermal earth surface with the additional wall method. The position of the fictitious images is computed analytically by recognising that the complex geometrical arrangement can be converted into a much simpler one using the Fourier transform. This allows for the estimation of the heat transfer coefficient with simple formulas. The resulting equations are very easy to implement and are entirely compatible with the standardised methods (IEC and IEEE) for rating power cables. Hundreds of finite elements simulations have been performed to validate the proposed method which yielded results with typical differences of <5% for standard installations.