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Reference Number NIA_NGET0054
Title Load cycling and radial flow in mass impregnated HVDC Submarine cables
Status Completed
Energy Categories Other Power and Storage Technologies(Electricity transmission and distribution) 100%;
Research Types Applied Research and Development 100%
Science and Technology Fields ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering) 100%
UKERC Cross Cutting Characterisation Not Cross-cutting 100%
Principal Investigator Project Contact
No email address given
National Grid Electricity Transmission
Award Type Network Innovation Allowance
Funding Source Ofgem
Start Date 01 October 2011
End Date 01 October 2015
Duration 48 months
Total Grant Value £2,200,000
Industrial Sectors Power
Region London
Programme Network Innovation Allowance
Investigators Principal Investigator Project Contact , National Grid Electricity Transmission (100.000%)
Web Site
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 cool-down period after a power reduction or turn-off. To establish an informal North Sea cable working group 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.
Abstract 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, enclosed by a lead sheath that prevents water ingress. Recent installations typically operate at 400 - 450 kV and have a continuous power rating per cable of up to 500 MW. Two such HVDC links are presently in operation between Norway and the European continent. 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. Ohmic loss in the conductor is the main source of heat generation in a loaded cable. Hence, the conductor will always be at a higher temperature than the surroundings, and there will always be a heat flow and an associated temperature gradient in the radial direction through the cable insulation. The thermal expansion coefficient of the mass impregnation is ten times that of paper During load increase, the associated thermal expansion causes the volume of the insulation to increase and the lead sheath is inelastically deformed. If the elastic properties of the armouring combined with the external water pressure do not compress this volume sufficiently during cooling, cavities will form in the insulation. Moreover, the greater temperature reduction and thus a larger thermal contraction of the inner parts of the cable than of the outer parts, is also expected to contribute to cavity formation. These cavities greatly reduce the dielectric strength and may cause long breakdown channels extending tens of centimetres and even meters, in the axial direction. Moreover, thermal cycling may, over time, lead to a lasting and irreversible displacement of the mass impregnation. The inner insulation layers become depleted, while mass accumulates between the outer insulation layers and the lead sheath. The existing knowledge about the importance and significance of the various factors expected to influence cavity formation and their interaction is limited, even though such relationships essentially determine the power rating and safe operational patterns for a subsea HVDC mass impregnated cable. In other words, subsea transmission systems are presently operated under constraints that potentially are unnecessarily strict. Research This project proposes the following method: The project will be delivered as part of a consortium with TenneT and Statnett who will provide cable samples from the Nor-Ned interconnector for analysis by academics from the University of Trondheim and the Norwegian Sintef Energy research facility. Research activity is focuses on: Load cycling and Partial Discharge measurements of full-scale cables sections in the laboratory Parameter estimation and small scale experiments Modelling of radial mass flow and Operational LimitsNote : Project Documents may be available via the ENA Smarter Networks Portal using the Website link above
Publications (none)
Final Report (none)
Added to Database 31/08/18