According to the characteristics of large thermal capacity of the soil and less influence of cable load on the transient temperature rise of the soil, a simplified transient thermal circuit model of single-core cable based on the monitoring ambient temperature of the trench is proposed using lumped parameter methods. The thermal resistance and thermal capacity of the model are calculated by using the empirical formula. For multi-loop cables, both the change of actual running current and the strong thermal interaction between the cables will result in the change in external thermal resistance. Therefore, the external thermal resistance is modified by the load current and the monitored air temperature. Different step current is applied to the cable and to the real-time calculation of cable conductor temperature. By comparing the result of the calculation of the model with that of the finite element method, the accuracy of the thermal circuit model is verified. Based on the transient thermal circuit model, the maximum permissible current-carrying can be predicted.

ampacity, thermal circuit, thermal parameter, trench laying cables, temperature measurement