The EU climate and energy package — known as the 20-20-20 target — places Europe as a world leader in the field of sustainable and renewable energy. By 2020, the continent could add around 600 TWh in new renewable electricity generation.

However, predictions for offshore wind energy suggest that by 2020, offshore wind will be responsible for 28 percent of the total wind energy generation, itself a third of all renewable electricity generation, according to EU forecasts. This estimate equals a total of 44 GW of installed offshore capacity throughout Europe by the end of this decade, an average of 4.1 GW annually.

Since most of the offshore energy in Europe is to be installed in the Baltic or North Sea, the integration aspects of this offshore power to the different national electricity grids constitute a very important challenge to what is already a very challenging engineering task. Several European studies, for example Tradewind and OffshoreGrid, have shown that electricity networks in Europe will require major reinforcements. Nevertheless, along with important onshore grid reinforcements Europe will also need to develop an offshore grid infrastructure to efficiently integrate large amounts of offshore wind.

The first step towards a European offshore grid network was taken on 7 December 2009, at the EU Energy Council in Brussels where nine European countries — Belgium, Denmark, France, Germany, Ireland, Luxembourg, the Netherlands, Sweden and the UK — signed a political declaration for joint cooperation on the development of a transnational electricity infrastructure in the North Sea. A year later, the countries taking part agreed to make available, by 2012, a series of deliverables on grid configuration and integration, market and regulatory issues, planning, and authorisation procedures for the construction of the mooted transnational offshore grid.

Inside this vision of Europe's future grid, the North Sea Transnational Grid project (NSTG) aims to identify and study the technical and economic aspects of connection of offshore wind power and trade between countries. The project is jointly executed by the Energy Research Centre of the Netherlands (ECN) and the Delft University of Technology; it was started in October 2009 and will continue for four years.

Transmission technology for the NSTG

There are two transmission technologies available for the connection of offshore wind farms to onshore networks: high-voltage AC (HVAC) and high-voltage DC (HVDC). The choice of which transmission technology to use will be based on efficiency and economic viability.

In comparison with HVDC systems, HVAC transmission systems have a wider dissemination, are more straightforward to install and present a lower offshore footprint. To date operational offshore wind farms in Europe have been connected through HVAC systems. The main reasons include the fact that only a few offshore wind farms currently have power ratings above 200 MW, and almost all are less than 30 kilometres from shore.

However, it is not always economically viable (or technically feasible) to use AC. For instance, the BARD Offshore 1 (or BorWin1) wind farm, scheduled to be operational in 2012, will be connected using HVDC. The 400-MW BorWin1 will be located 130 km from the German coast, justifying the choice of DC as to cross long distances by means of submarine cables (~60-100 km) the HVDC solution starts to be preferable. Traditional HVAC lines have higher losses (due to skin effect and capacitive leakage current) and demand additional equipment to provide reactive power compensation. On the other hand, DC cables do not suffer from leakage current and thus, in steady state, their electricity transmission is limited only by cable resistance.

The Role of Modularity

The North Sea Transnational Grid project, with its intention of interconnecting around 60 GW of offshore wind power between several countries in the North Sea up to 2030, is a very ambitious initiative. For projects of such dimension and complexity, choosing the most appropriate construction architecture is extremely important right from the start. The NSTG will have to organically grow with time from its initial, inherently simple phase to its desired final form, expected to exhibit a much more complex topology.

System architecture may be defined in several ways. One possible definition involves verification of how the operative elements of a system are arranged into blocks, and how these blocks interact. Two distinctive types of system architecture — integral (or closed) and modular (or open) — are generally recognised. In analysing the complexity involved in the development of a system such as the NSTG, it is immediately apparent that an integrated architecture is not the most convenient choice for construction and expansion of the system. Modifications to features and/or components are likely to occur regularly during the initial and development phases. Nevertheless, there should be little redesign of the whole system given technical difficulties and the high costs involved.

For complex systems such as the NSTG, one potential solution is to adopt, from the early stages of development, a modular architecture approach. In a modular-architecture system, each module may be designed practically independently from each other, which allows changes to be made to one module without affecting the others. Therefore, it becomes important to be able to clearly distinguish the objectives and primary functions of each system's modules and the possible interactions between them. The task of establishing the modules' functionalities inside the system can be accomplished through design hierarchy and standardisation.

Design hierarchy & standardisation

Design hierarchy and standardisation are two important concepts for complex systems such as the NSTG, since more than one stakeholder will be involved and, indeed, necessary for funding and development of the entire system.

In the modularisation process of a complex system, the first task is to establish which are the parts that can be considered modules or subsystems. For instance, in an offshore transmission grid, wind farms, HVDC converter stations, DC transmission cables and potential protective systems naturally constitute the basic building blocks or modules involved in the installation.