Modelling of insulated cables has received considerable attention in recent years. Since installation and maintenance of insulated cables facilities are expensive, design and operation should be optimized to maximize current rating. The current rating of a insulated cable depends on the maximum temperatures that core and sheath can support. As a consequence, the dynamic behaviour of insulated cables becomes an important issue for planning and operation of insulated cables facilities. The usual methodology used to compute either steady state or dynamic current rating in underground installations is based on electrical and thermal analogy based models. The thermal-electrical analogy consists of considering each layer of each cable as a thermal resistance and a thermal capacitance, and heat sources as current sources. This paper presents a method to model the dynamic behaviour of insulated cables from a thermal point of view. The proposed model is based on a radial partition of the cable. Each annular layer obtained is represented by thermal resistances, which connects it with their adjacent layers, a capacitance, which represents the thermal inertia, and a current source, representing the different heat sources of the installation. The model also considers the heating mutual influence between different insulated cables. The non conductive (convection and radiation) heat transfer models, such as ducts or galleries, are computed using empirical approximations and they are adapted to conductive heat transfer equivalent models by using equivalent thermal conductivities. In addition, a full electric model of the installation is considered to obtain accurate sheath currents depending on the type of sheath grounding system has been selected. The resulting model is a linear electric circuit which is solved using eigen analysis. In addition, superposition can be applied to deal with dynamic load profiles since the resulting electric circuits are linear. The performance of the method is illustrated in 20 kV and 45 kV installations used by Gas Natural Fenosa (GNF). Results presented compare the different critical times, i.e., the time to reach maximum allowed temperature, depending on the number of lines in the installation, the grounding system and the type of emplacement.
Keywords: Insulated cable, dynamic current rating - dynamic conductive heat transfer
Cigré 2012 Session 44, Paris, France, 27-31 August 2012
Published: August 2012.