Introduction: 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 Utilities.

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 exploited.

Objectives: 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.

Project Deliverables:

• 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.