Methods for rating directly buried high voltage cable circuits

To manage a cable network effectively it is desirable to rate cable assets precisely. To investigate the conservatism of cable ratings provided by accepted modelling approaches several models have been employed to provide comparative continuous ratings for a directly buried cable group arrangement. Operational ratings are performed using expedient cable rating calculations, typically employing analytical or one dimensional (ID) finite difference (FD) methods. The case for a directly buried cable group is considered; ratings are obtained using three methods including the international standard analytical approach (IEC 60287), ID FD and a 2D finite element analysis (FEA) approach. A continuous equation for resultant heat-flux has been derived to more precisely model the dielectric loss in 2D and 3D FEA models. Comparing results shows that all the models have good agreement when modelling an isothermal ground surface condition and validate the 2D FEA model against established approaches. The FEA model developed has been used to investigate more realistic ground surface boundary conditions. Results for different cable burial depths and ground surface boundary conditions show that using the established methods can lead to overestimation of cable ratings in the case of shallow buried cables.