The installed length of cable circuits on the 400kV network has increased substantially in the last decade, particularly in London but also in rural areas where the use of overhead lines has become politically unacceptable. Cable circuits typically have a lower continuous current rating than overhead lines due to the increased thermal resistance between the cable and the ambient environment and as such can form the overall limiting factor on the amount of power which can be transferred through a given network link. Historically, National Grid has planned and purchased cable circuits based on the required continuous rating. Emergency ratings would then be calculated based on a set preload for a given time and would typically be applied in the event of a circuit outage elsewhere on the network.
This approach provides a concise cable rating sheet as part of the CUP package which can be readily used by Network Operations. While this approach works well where the level of load to be transferred is known in advance, it provides for only a limited number of rating combinations based on a series of assumptions about the cable system thermal environment. Given the increasing variability of the UK climate, coupled with the trend towards higher generation of electrical energy from renewable sources, this may not always lead to the best utilisation of a cable asset as its true power transfer capability over periods of 24hrs or less may be under-estimated through this traditional approach.
This study will investigate the development of dynamic rating algorithms applicable to a variety of common cable circuits through both numerical modelling, simulation and laboratory based experiments. The proposed work will be carried out within the Tony Davies High Voltage Laboratory at the University of Southampton. Principle targets for this study include: 1. The development and experimental verification of an algorithm for dynamic ratings applicable to buried cable circuits 2. Further development of (1) for application to cables in air, for example troughs and tunnels. 3. The examination of possible integration of tunnel dynamic ratings with ventilation control options under investigation in the CCTV (Control of Cable Tunnel Ventilation) project. 4. Development of a framework detailing the data collection requirements and other pre-requisites for any future deployment of dynamic cable ratings within National Grid. 5. An analysis of the potential benefits of using dynamic ratings, particularly in terms of constraint cost reduction.