National Grid Electricity System Operator
National Grid TO Innovation Team
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High Voltage Direct Current (HVDC) Mineral Insulated (MI) Cables have complicated stress processes that are particularly vulnerable in the cooling stages immediately associated with power reductions or emergency shut downs, especially when occurring during the delivery of short
term overloads, however the behaviour of MI cables under different load
conditions is not clearly understood. This knowledge would be of great benefit to
Mass impregnated HVDC subsea cable has for long been and still remains the
state-of-the-art technology. The electrical insulation of such cables consists of
paper impregnated with a high viscosity oil (the ”mass”), enclosed by a lead
sheath that prevents water ingress.
Recent installations operate at typically 400 - 450 kV and have a continuous
power rating per cable of up to more than 500 MW. Two HVDC links are
presently in operation between Norway and the European continent, and more
are expected to come. In a future pan-European electrical power grid, subsea
cables in the North Sea are expected to play a crucial role, both for exchanging
power between the UK, Scandinavia and the European continent, and for
transferring power generated in large off-shore wind farms.
It is generally accepted that the cooling period after a power reduction or turn-off
is the most critical part of the operation of subsea mass impregnated HVDC
cable. Consequently, the power rating of such cables, both with regard to short term
overloads and on a continuous basis, is largely set by considering the risk
of having a dielectric breakdown during a power reduction or turn-off. However, as will be described in some detail below, the behaviour of the cable insulation
under different load conditions, and thereby the risk of having such breakdowns,
is far from fully understood. Hence, it is reasonable to assume that the true
capacity and operational flexibility this cable technology can offer, are not fully
To determine what load conditions (power ratings and load patterns) typical high
voltage direct current (HVDC) mass impregnated paper insulated cables can be
subjected to without risking cavity-induced dielectric breakdowns during a cooldown
period after a power reduction or turn-off.
To establish an informal North Sea cable working group towards collaboration on
HVDC link projects, potential sharing of spares holding and repair resources.
- Obtain a detailed physical understanding of the processes that lead to cavity
formation and the importance of various operational, environmental and cable
design parameters to these processes
- Develop a numerical model that quantitatively describes the radial mass flow and
cavity formation under load cycling
- Determine the operational constraints for one or more HVDC subsea cables
presently in service.