Circuit rating methods for high temperature cables

For the safe and efficient operation of power transmission systems, each system component must have an accurate current rating. Since the advent of formal power networks a wide variety of methods have been employed to calculate the current carrying capacity of power cables, ranging from simple analytical equations to complex numerical simulations. In the present climate of increasing power demand, but where finance for large scale network reinforcement schemes is limited, providing an accurate rating becomes paramount to the safe operation of the transmission network. Although the majority of the transmission network in the United Kingdom comprises overhead lines, many vital links make use of high voltage cable circuits. Advances in our ability to manipulate the properties of dielectric materials has led to increased interest among the cable community as to whether new cables could be designed which could deliver improved power transfer performance in comparison to traditional technologies. One way in which this might be possible is if the existing conductor temperature limit of 90C common to XLPE based cable systems could be lifted. At the present time a number of polymer systems exhibit potential in this area - however prior to investing significant resources in their development, it would be valuable to scope out the magnitude of the benefits that such cable systems could deliver to a network operator. In order to determine the scale of the operational benefit available, a comprehensive rating study would need to be undertaken. However most existing cable rating methodologies were not designed for situations with conductor temperatures in excess of 100C and may not be suitable for the task. To allow a quantitative analysis of the benefits available from permitting higher cable conductor temperatures, cable rating techniques for all major installation types have been reviewed and improved. In buried cable systems high temperature operation can lead to significant problems with moisture migration which is not easily modelled by traditional calculations. To overcome this a full dynamic backfill model has been created which explicitly models moisture movement and allows its impact on the thermal profile around a high temperature cable circuit to be established. Comparison is also made to existing forced cooling techniques to benchmark the scale of the benefits attainable from high temperature operation. Cable joints become critical in such forced cooled systems - to ensure that the joint temperatures do not exceed acceptable levels a full finite element based modelling process has been developed, allowing detailed rating studies to be undertaken. It is not always possible to bury cable circuits, for instance where they are installed in surface troughs or tunnels in urban areas. By applying modern computational fluid dynamics methods it is possible to develop more comprehensive rating methodologies for these air cooled cable systems, allowing the benefits of high temperature operation in such circumstances to be demonstrated. By utilizing these techniques for an example cable design it has been possible to provide an in depth discussion of the advantages available from high conductor temperature operation, while simultaneously noting the potential problems which would need to be mitigated should such a cable design be deployed in an operational setting.