Calculation of thermal distribution and ampacity for underground power cable system by using electromagnetic-thermal coupled model

Joule loss in conductor and eddy current loss in sheath are critical for calculating thermal distribution and ampacity of underground power cable. These two types of losses are usually well estimated in most cases by IEC-60287. However, the problem becomes complicated for the underground power cable system as analyzing these losses is not easy due to the electromagnetic coupling between a large number of cables in such system. Nevertheless, IEC-60287 has recommended a simplified method for calculating the ampacity of multi-circuit cable system by ignoring the proximate effect between circuits. In spite of inaccuracy, this simplified method is commonly used in practical applications for its ease of use. In this paper, an electromagnetic-thermal coupled model is proposed and numerically calculated by finite element method (FEM). A case study of 110 kV underground power cable system is given. Results show that the ampacity obtained from our proposed model agrees very well with that from IEC-60287 for one-circuit cable system, whereas it apparently deviates from what IEC-60287 recommends for multi-circuit cable system. Such deviation is primarily caused by the capability of our model to precisely calculate the losses.