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Monday, 18 January 2016

Evaluation and selection of AC transmission lay-outs for large offshore wind farms

This paper studies different energy transmission solutions for AC offshore wind farms. This transmission of energy is based on AC submarine cables that present a strong capacitive behavior. Therefore, an analysis is necessary to determine transmission characteristics such as, the number of submarine cables, voltage or rated power. For that purpose, three different transmission configurations will be considered: unique HVAC, various HVAC and MVAC, combined with three submarine cables of different characteristics. By using a design procedure, it is shown that based on the electric characteristics provided by the manufacturer of the submarine cable, it is possible to determine the most efficient energy transmission solution, from the perspective of the submarine cable. Different variables will be taken into account, including transmission current, active power losses, the cost of the transmitted energy and the reactive power compensation required. In addition, the consequences of the selected transmission solution to other more general aspects of the wind farm such as, necessity of the offshore platform or local inter turbine network are also discussed.


1.      Wind energy
2.       Transmission of electrical energy
3.       AC-cable




Fig. 1: General layout of HVAC offshore wind farm.
Fig. 2: General layout of MVAC offshore wind farm.
Fig. 3: General layout of offshore wind farm with multiple HVAC connections.


Fig. 4: Module of the current through the submarine cable vs cable length. With compensation at both ends (red) and onshore compensation (blue). a) 5x30MW-36kV configuration. b) 150MW 150Kv
Fig. 5: Active power losses for 50km cable length, with compensation at both ends (red) and onshore compensation (blue) a) 150MW-150kV, 2x75MW-87kV, 3x50MW-66kV y 5x30MW-36kV configurations b) 150MW-220kV
Fig. 6: a) Rayleigh distribution for different average wind speeds b) Generated power on wind farm on function of the wind speed.
Fig. 7: Energy transmission cost for different layouts.

In agreement with built wind farms, a MVAC transmission system is the best option near to shore. This is because submarine cables are very expensive. With big cable lengths the cables costs do not compensate the money saved in the offshore platform. With short cable lengths (<20Km) MVAC connections are better than other layouts. Moreover, at 150 MW rated power MVAC configuration can be the best option to 60Km cable length. However in this case the clusters are of (40-50MW) and the submarine cables operates at 70-80% (or more depending the cable length) of their load capability. This can cause an inadmissible voltage drop in the transmission system or other harmful effects. In this paper only conduction losses in the submarine cables have been considered, armor losses or dielectric losses have also not been taken into account. But this simplification affect to cable parameterization and not to layout selection procedure. 220kV HVAC system is not the best option for any cable length. But the cable used in this evaluation has 3 times higher resistive component than other cables.
[ 1 ] S. Lundberg, "Wind farm configuration and energy efficiency studies series DC versus AC layouts," Thesis, Chalmers University of Technology 2006.
[ 2 ] S. Lundberg, "Evaluation of wind farm layouts," EPE Journal (European Power Electronics and Drives Journal), vol. 16, pp. 14-20, 2006.
[ 3 ] Å. Larsson, A. Petersson, N. Ullah, O. Carlson, “Krieger’s Flak Wind Farm”, Nordic wind power conference, May 2006
[ 4 ] S.D. Wright, A.L. Rogers, J.F. Manwell, A. Ellis, “Transmision options for offshore wind farms in the united states,” AWEA 2002
[ 5 ] S. Chondrogiannis, M. Barnes, “Technologies for integrating wind farms to the grid (Intering report)”, DTI 2006.