Analytical and FEM-Based Modeling of Thermal Resistances for Power Cables With Corrugated Sheaths

An analytical expression for the thermal resistance of the corrugated sheath layer of power cables is developed in this paper. (The corrugated sheath layer means a layer composed of both metallic sheath and air gaps.) The model is based on multiple finite element (FEM) studies. The proposed analytical model defines the resulting thermal resistance of the corrugated sheath layer composed of a metallic sheath and air gaps. All three heat transfer mechanisms are considered, namely, conduction, convection and radiation. The geometric parameters of the corrugated sheath presented in this study cover most constructions encountered in practice, thus it might be used for cable ampacity calculations. The presented analytical expression is illustrated with the current rating calculations that consider heat losses in the corrugated sheath. The 400 kV low pressure oil filled (LPOF) cable found in CIGRE TB 880 was studied. Based on the results of the FEM analysis, a conclusion was reached that the thermal network for this cable presented in CIGRE TB 880, based on the approach described in the IEC Standard 60287, overestimates its ampacity values as presented in the technical brochure.