National Grid is currently evaluating DC cable schemes to increase the transmission capacity of the UK network, particularly for increasing the import of electrical energy from renewable sources in Scotland.
The calculation of current ratings for DC cable is significantly more complex than that for AC cable. The rating is often determined by electric stress constraints rather than considerations of thermal ageing. Ratings are also strongly influenced by thermally induced pressure transients within the cable. In some cases the rating of the cable can be restricted by the cable being too cold.
As the normal operating voltage of the cable increases the cable can experience high levels of electric stress while the cable is hot. As the cable cools it is susceptible to electrical failure. Some manufacturers require the converter station to reduce its operating voltage if the current on the link is reduced. The implementation of these current dependent voltage control systems may help protect the cable, but this approach does not align well with the concise cable rating sheet used as part of the CUP package. This introduces an additional level of complexity for Network Operations.
Modelling the complex interactions of thermal and electrical parameters is essential if National Grid is to make a thorough assessment of tenders for HVDC cable schemes. The modelling of transient thermal conditions and the behaviour of the cable insulation under reversals of power flow will provide guidance for the development of dynamic rating algorithms and operational regimes suitable for high power HVDC cable circuits. The thermal and electrical models will be constructed in such a way that the outcomes of planned R&D work on pressure transients and partial discharge ageing can readily be incorporated at a later date.
The models will also be suitable for assessing the effect of fast polarity reversals on the cable system. This will provide guidance on more flexible operation of existing and future HVDC links. In some circumstances the opposite scenario will apply (a cable link where the direction of power flow is rarely if ever reversed). In this case the outcomes of this project could allow restrictions on cable voltage or overload capability to be lifted; again increasing the flexibility of the link.
Objectives
This project aims to:
- Develop tools for the rating and technical assessment of high power HVDC cable options. The work will initially concentrate on cables with mass impregnated insulation
- To provide National Grid with techniques to evaluate continuous, transient and dynamic (real time) ratings for DC cable circuits and to evaluate the options and limits for features such as current dependent voltage control.
