Effects of modelling assumptions on the rating calculation for externally forced cooled high-voltage cables

To increase the rating of a high-voltage cable circuit the cable group can be externally forced cooled, using additional coolant pipes in proximity to the buried cable group. This complicates modelling of the heat transfer problem to obtain ratings as coolant temperature and therefore heat transfer coefficient varies along the cable route. The most common approach for obtaining the circuit rating is the finite difference (FD) method outlined in Electra 66. This method is computationally efficient and quick to solve. To investigate the assumptions underlying this approach and provide confidence over a range of model parameters, this paper presents the development of an extended 2-D heat-transfer finite element method (FEM) model. The ratings of two cable circuits have been modelled using this approach and are compared with results from Electra 66. Cable ratings from the two methods are consistent in trend but offset favourably by 2.6% using the FEM model for all burial depths tested. With the FEM model verified for standard assumptions the model provides a useful tool for rapid investigation of sensitivity to model assumptions. A sensitivity analysis to changes in AC resistance, burial depth, dielectric loss, soil thermal resistivity and surface boundary condition is presented.