Flexible Rating Options for DC Operation (FRODO)
National Grid Electricity System Operator
National Grid TO Innovation Team
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Innovation Funding Incentive
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
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.
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
- 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.