Thermal response of a three core belted PILC cable under varying load conditions

The most important indicator of the health of electrical plant items is the condition of their insulation. In the case of underground cables an important issue is the operating temperature and indeed the thermal history of the cable. There are several factors which will determine the thermal behaviour of a given cable installation. These include the assumed ampacity, the cable construction and circumstances of installation, the thermal properties of the surrounding soil and the ambient temperature. The work presented in this paper involves the use of COMSOL multiphysics finite element software to develop an integrated electrical, thermal and mechanical model of buried single or multiphase cables that simulates the behaviour occasioned by a varying load. The principal heat source in the problem is the Joule heat dissipated in the conductor(s). The transfer of this heat to the surroundings is governed by the geometry and material properties of the conductor, insulation, screening, sheathing and trench fill material as well as the ambient conditions. The thermal and electrical systems are coupled via the temperature dependence of the resistivities of the conductor and sheath materials. The input data to the model is provided in the form of time series describing the variations in load over a 24-hour cycle and the output takes the form of the thermal response of the cable for given installation and ambient conditions.