This presentation was held at the 2020 NAFEMS UK Conference "Inspiring Innovation through Engineering Simulation". The conference covered topics ranging from traditional FEA and CFD, to new and emerging areas including artificial intelligence, machine learning and EDA.

Resource Abstract

The transmission of electrical power at high voltage over any significant distance can be achieved either by using pylons supporting overhead lines or by using underground power cables. Although generally cheaper to install, the use of overhead lines has a number of issues, not least of which is the environmental impact of such installations and the potential health and safety concerns close to residential developments. In consequence, much of the power distributed in the UK is distributed by cables that are buried underground in troughs, ducts or tunnels.

Inevitably, with the continually developing UK power network infrastructure, new cable circuits may, in some cases, have to traverse existing underground circuits that have been operating successfully for many years. This can cause significant problems as underground power cables are carefully designed to be able to transmit the required amount of power without exceeding the limiting conductor temperature of 90 degrees centigrade. In isolation, the temperature of the conductor depends on the power losses generated by the cable and the thermal conditions of the local environment. For a direct buried circuit this is heavily influenced by the depth of burial below the ground surface.

For the scenario where a new cable circuit needs to traverse an existing cable circuit, the new circuit can either pass above or below the existing circuit. Notwithstanding the restrictions on either of these options, the mutual heating effect between the circuits in the vicinity of the crossing will almost certainly mean maximum conductor temperatures are exceeded on both circuits, and hence the cables will need to be de-rated i.e. not carry the amount of load for which they were designed. From a distribution utility point of view this is a highly undesirable option. The only practical alternative in this situation is to somehow provide additional cooling to the cables in the region of the crossing, such that both circuits can operate at the desired rating.

This paper describes a study of one such scenario where a new 400kV circuit is required to traverse two existing 132kV circuits. The aim of the study was to estimate the maximum conductor temperatures on both circuits for seasonal rated currents in the presence of an external cooling mechanism, consisting of air flowing in ducts that pass between the cable circuits. Experimentation is almost always not possible for installations of this type, so the only practical alternative is to use numerical simulation in the form of conjugate heat transfer analysis in a 3-D numerical model involving both heat conduction and fluid convection. As a conceptual study, the possibility of using induced natural convection as well as forced-flow convection was considered on a full 3-D model of the complete installation in the vicinity of the cable crossing.

The heat transfer model was constructed using the CFD functionality available in Dassault Systemes 3DEXPERIENCE platform including CAD generation, meshing, analysis and results post-processing.