Comparison different electrical connection topologies for offshore wind farms.
Carries out a cost comparison between HVDC connection topologies.
Analysis of the costs associated with reliability of electrical connection equipment.
The AC auxiliary option is best due to lower protection costs and higher revenue but its use is limited by distance.
This paper outlines possibilities for connecting 2.4 GW of power from two separate wind farms at Dogger Bank in the North Sea to the GB transmission system in Great Britain. Three options based on HVDC with Voltage Source Converters (VSC HVDC) are investigated: two separate point-to-point connections, a four-terminal multi-terminal network and a four-terminal network with the addition of an AC auxiliary cable between the two wind farms. Each option is investigated in terms of investment cost, controllability and reliability against expected fault scenarios. The paper concludes that a VSC-HVDC point-to-point connection is the cheapest option in terms of capital cost and has the additional advantage that it uses technology that is commercially available. However, while multi-terminal connections are more expensive to build it is found that they can offer significant advantages over point to point systems in terms of security of supply and so could offer better value for money overall. A multi-terminal option with an auxiliary AC connection between wind farms is found to be lower cost than a full multi-terminal DC grid option although the latter network would offer ability to operate at greater connection distances between substations.
- Wind turbines;
- Offshore wind farms
The increasing demand for wind power production and reduced visual impact is driving the development of offshore wind farms. The GB Government has issued plans to install more than 40 GW of renewable power generation by 2020, with most of the energy being delivered from new offshore wind farms around the coast of Great Britain ,  and . The Dogger Bank Round 3 offshore site in the North Sea is expected to be the largest, with an initial planned capacity of 7.2 GW .
Due to higher wind speeds and abundant open areas offshore wind farms are seen as a promising option for large-scale power generation. However, the harsh offshore environment and large distance from the mainland grids represent a significant challenge to be overcome. Achieving this may require the use of high-specification wind turbines that can be operated remotely, with more reliable control systems since these are at present the biggest single source of failures in wind turbines . Efficient and reliable transmission systems which permit power transfer with reduced losses and minimum operational issues for mainland grids will also be required.
Offshore wind farms can be connected to onshore grids using AC or DC transmission. The maximum economic distance for the AC transmission option is limited by the need for appropriately sized and located reactive compensation as well as the need for measures to deal with transient over-voltages and harmonic resonance  and . A DC transmission system is an option which minimises the impact of onshore grid disturbances on offshore power production due to a decoupled connection between the wind farms and the onshore grid  and . Another advantage of a DC system is that onshore converter stations can be used to provide additional services such as reactive power provision to the onshore grid at no additional cost; in some cases, independent of wind power production offshore .
Many offshore wind farms will be located a significant distance from the shore, including most of the Crown Estate Round 3 sites . Due to the high potential capacity of Dogger Bank and the long distance to shore (a minimum of 144 km), HVDC transmission is seen as the only viable option for transferring the power back to the onshore transmission system. Two different HVDC technologies are available: voltage source converters using IGBTs (VSC-HVDC) and line-commutated converter (LCC-HVDC). VSC-HVDC has several technical advantages over LCC-HVDC. For example the use of self-commutated semiconductors removes the need for communication systems for power transfer, VSC has black start capability unlike LCC making it preferable for connection to ‘weak’ AC grids like offshore wind farms. Furthermore there is no requirement for harmonic filters and other compensation equipment such as STATCOMs meaning there is less space required on the offshore platform. VSC-HVDC also offers a high level of controllability which allows for the use of multi-terminal topologies  and .
In Refs.  and  it is shown that a point-to-point VSC-HVDC connection improves voltage quality in the grid compared with an AC connection where wind variation may cause propagation of voltage fluctuations. This work also highlights the advantages of decoupled operation of the transmission system and investigates different potential control strategies. In this paper the AC voltage is controlled at the wind farm level taking account the wind variation. Multi-terminal VSC-HVDC is studied in Refs.  and  in terms of flexible control capabilities and as a future option for connecting a large amount of power from offshore wind farms. These studies suggest that this is a very attractive option for interconnection between countries and also for connection of offshore oil and gas platforms. A VSC-HVDC transmission system with additional AC auxiliary cables providing a connection between wind farms is a promising solution if the distance between the wind farm substations isn’t too great and a variety of options have been shown in studies conducted by National Grid . It is already a well-known technology and that may improve system reliability and security in a more cost effective way.
VSC-HVDC is a relatively young technology but the scale of delivered and planned projects is advancing rapidly to the point that it can compete with long established and high power LCC-HVDC technology. The ABB NordLink connection proposes the largest point to point connection between two onshore locations and will consist of a 1400 MW, ±525 kV bipole connection between Norway and Germany . In 2013 the 400 MW, ±150 kV Borwin1 connection to the Bard1 German offshore wind farm was the first VSC-HVDC scheme to connect an offshore wind farm to shore. In addition to this even larger projects are under development in the German offshore sector such as the 900 MW, ±320 kV Dolwin2 project . Early VSC-HVDC projects were based on two or three level converter technology using pulse width modulation however it is likely that newer modular multilevel technology will be preferred in most future developments due to reduced losses and station footprint  and .
This paper seeks to investigate the merits of different connection options for far offshore wind farm installations including the possibility of introducing interconnection between two wind farms in relatively close proximity. It does this by exploring three VSC-HVDC connection schemes designed to transfer 2.4 GW of power from two separate Dogger Bank wind farms to the GB transmission system in Great Britain (GB). The study is based on option 1 from the National Grid “Round 3 Offshore Wind Farm Connection Study” shown in Fig. 1. The studies focus on connecting wind farm 1 and wind farm 2 to the onshore grid with each farm sized at 1.2 GW. The magnitude of power flows into the GB network suggests the use of two onshore connection points , and the scenarios presented in this study are based on this assumption.