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Transnational Offshore Grids

THE CASE FOR AN OFFSHORE GRID

The exploitation of the offshore wind potential in Europe brings new challenges and opportunities for European power transmission. The long-term European offshore potential amounts to up to 150 GW in 2030, according to EWEA estimates in 2008. The majority of the currently projected offshore wind plants will be situated close to the European coastlines, not further than 100 km from the shore, due to the high costs of grid connection, limited grid availability and the lack of regulatory frameworks. Looking at the North Sea alone, with a potential of hundreds of GW of wind power, an offshore grid connecting different Member States would enable the transfer of this wind power to load centres and, at the same time, facilitate the competition and the trade of electricity between countries. A multi-terminal offshore grid would reach offshore wind plants far from shore, as foreseen for German and UK waters.

The project developer Airtricity introduced the offshore Supergrid® concept in 2005. Supergrid® combines the following:

• the connection of offshore wind plants;
• the balancing of wind power variations; and
• the provision of transmission of electricity between different markets for cross-border trade.

A commercial proposal has been worked out for a first phase of 10 GW of wind power – the construction of an offshore ‘super grid’ which would be on a modular basis. The fact that wind farms will be able to operate collectively at variable speed and frequency, independent of the land-based grid, is expected to optimise turbine generating efficiency and offset losses incurred as a result of the increased transmission distances.

Figure 4.2: Vision of High Voltage ‘Super Grid’ to Transmit Wind Power Through Europe

Source: Dowling and Hurley (2004)

At first, large, multi-GW offshore arrays would connect to nearby networks, before being modularly extended and ultimately interconnected. A further advantage of this system will be the full controllability of power flows, eventually allowing an ‘all-European’ market for electricity, including ‘firm’ wind power.

Presently, the idea of a transnational offshore grid is being addressed by several other parties. The Norwegian TSO Statnett proposes the progressive development of a grid linking Scandinavia with UK, Germany and The Netherlands (Figure 4.3). On its way, it would connect offshore wind farms, as well as existing offshore oil installations that need to reduce their CO₂ emissions. The technology to be used is a HVDC VSC (see ‘Ensuring adequate transmission capacity and access for wind power’ above).

Figure 4.3: Offshore Grid Proposal by Statnett

Source: Statnett (2008)

Greenpeace (Woyte et al., 2008) has studied the concept of an offshore grid serving electricity trade between European countries around the North Sea and at the same time providing transmission of up to 70 GW of offshore wind power capacity – a target that could be achieved between 2020 and 2030 (Figure 4.4). The study has also evaluated the smoothing effect of aggregating the offshore wind power using such a grid. The offshore grid topology proposed seeks the maximum synergy between existing plans and reinforcements, aiming to improve the cross-border exchange between countries. For example, it includes the East Connector in the UK, which alleviates the heavy north–south congestions, as well as an offshore connection along the French, Belgian and Dutch coasts.

Figure 4.4: Offshore Grid Examined in the Greenpeace Study

Source: Woyte et al. (2008)

The effects of grid configurations described above on the power flows in the European transmission system are being analysed in the TradeWind project. A common element to all these proposals is the fact that the offshore grid would provide multiple functions and serve the functioning of the European electricity market. Therefore, it should be considered as an extension of the existing onshore grid, falling under the responsibility of the various governments, TSOs and regulators involved.

TECHNICAL SOLUTIONS FOR THE OFFSHORE GRID

Compared to onshore sites, offshore wind farms will have large power capacities and be comparable in size to conventional power plants, typically in excess of 400 MW. Modern transmission technologies operating at high and extra high voltage levels will be required to transmit high levels of power over longer distances. Two main types of offshore transmission systems exist, based on either alternating or direct currents (HV-AC or HV-DC).

For wind farms close to shore, the HVAC system offers the best solution, as it provides the simplest, least expensive and proven technology for grid connection, and is similar to the transmission network used on land. However, as transmission distances increase, the losses from the HVAC system increase significantly. To avoid ineffective operation, AC cable length should be limited to a length of approximately 120 km. HVDC technology offers a number of advantages, but has a distinct investment cost structure, as it involves the installation of expensive converter stations. The break even distance (now around 90km) depends on the cost developments, and will move closer to the shore as HVDC system costs decrease.

Conventional thyristor-based HVDC technology has generally been used for point-to-point power transmission. Offshore wind farm arrays would benefit from a multi-terminal transnational offshore grid system. Recent advances in HVDC technology, using insulated gate bipolar transistor (IGBT)-based converters, seem to offer a solution and facilitate the cost-effective construction of multi-terminal HVDC networks. These modern HVDC-IGBT systems offer clear technological advantages, especially in the area of controllability and efficiency. A specific advantage of HVDC systems is reactive power control capability, favouring grid integration and system stability. The technical and economic aspects of offshore transmission systems are being actively investigated by the supply industry and by electric power companies in order to be ready with the most cost-effective solutions when large-scale offshore wind power takes off.