This is the conclusion in the final report from the OffshoreGrid project, a scientifically based techno-economic study of an offshore grid in Northern Europe within the Intelligent Energy Europe program. The report states that more than 1/3 of all offshore wind power projects (321 in total) should be clustered in hubs, with only a single transmission line to shore (being in most cases a High Voltage Direct Current (HVDC) link). The clustering would lower investment costs from €83 billion to €69 billion up to 2030, compared to connecting each of the wind farms individually to shore. In addition, by interconnecting several hubs to form a multi-terminal HVDC grid, system benefits of €21 billion over a 25 year period could be generated for an additional investment of app €8 billion. In total this adds up to savings of €27 billion (see figure below).

Figure: OffshoreGrid project - infrastructure cost comparison

HVDC transmission for wind power could soon reach 1 GW

The clustering of wind parks into larger hubs is well underway in the German North Sea. ABB recently won an order worth around $1 billion from the Dutch-German transmission system operator TenneT to supply the Dolwin2 HVDC transmission link connecting offshore North Sea wind farms to the German mainland grid. The 900 MW HVDC converter and cable system, operated at ±320 kV, is to be installed by 2015. This project follows the 800 MW Dolwin1 link scheduled for 2013 and the 400 MW Borwin1 which was installed in 2009. Siemens is also installing multiple transmission links off the German coast, notably the 864 MW SylWin link scheduled for 2014, the 800 MW BorWin2 and the 576 MW HelWin1 links both scheduled for 2013. The offshore platforms SylWin alpha and DolWin alpha are both being certified by Det Norske Veritas (DNV).

Breaking the current in a HVDC grid is no longer a showstopper!

The lack of a HVDC circuit breaker is by many seen as the most pressing technical showstopper to a multi-terminal HVDC grid. However, in a paper published at a Cigré conference in September this year, ABB claims to have successfully tested a hybrid HVDC breaker concept that provides ultrafast breaking capability – within 2 milliseconds – and maximum breaking current up to 16 kA (16 000 Ampere). ABB states that such a breaker could soon be commercially available for HVDC grids rated at ±320 kV.

HVDC VSC reaches voltage level of 500 kV

All the HVDC links mentioned earlier are based on Voltage Source Converter (VSC) technology. The Skagerak 4 interconnector between Norway and Denmark – scheduled for 2014 – will break a new record for VSC voltage. At 500 kV, the link will provide 700 MW of transmission capacity in a single cable (a ±500 kV link would be able to support twice the power capacity, or 1400 MW). In fact, it is not the VSC converter that is limiting the maximum voltage, but rather the HVDC cables. The Skagerak 4 project will use Mass Impregnated (MI) cables, but in the future it is expected that polymeric extruded HVDC cables will take over also on the highest VSC voltage levels.

All in all, the outlook for an offshore grid is much more positive than it was just a year ago. Now coordination challenges are in the hands of EU – and national decisions makers – to sort out the many regulatory issues that stand in the way, such as the incompatibility of national support schemes for offshore wind power!

Figure: An outlook on offshore grids in Northern Europe in 2030. Source: OffshoreGrid project

Figure: Atlantic Wind Connection

The Atlantic Wind Connection (AWC) project off the US east coast could be the first ever offshore Multi-Terminal HVDC (MTDC) network. When completed, AWC could transmit 6,000-7,000 MW of offshore wind power to shore, enough to supply more than 2 million US homes. According to plans all permits are expected to be in place by 2013, the first of five construction phases would be completed as early as 2016 and the complete network could be in place by 2021. There seems to be consensus that a MTDC network in Europe will not materialize before after 2020.
The AWC project will include the following facilities:

• Nine offshore and seven onshore nodes with voltage source converter (VSC) stations. Each offshore node is rated 500-1,000 MW.

• Two independent HVDC circuits totalling more than 1,200 km of circuit length, mostly located between 16 and 29 km offshore. Each circuit contains two power cables operated at 320 kV DC voltage, and a fibre optic communication cable. The transmission backbone is split in two to avoid faults in one circuit affecting the other.

AWC is a cheaper and more flexible Solution

The Brattle Group calculates the project cost to approximately $5 billion and says it would be far cheaper than connecting each wind farm to shore with independent cables. The flexibility in routing wind power to different market areas and relieving onshore transmission congestion are both strong arguments in favour of the project; A 2009 US DOE Congestion study deemed the Mid-Atlantic region a “Critical Congestion Area”.

Partly funded by Google

The project is led by independent transmission company Trans-Elect with Atlantic Grid Development as the project developer. Financial backing is provided by Google (37.5%), Good Energies (37.5%) and Marubeni Corporation (15%). The US Mid-Atlantic Bight holds potential for up to 60,000 MW of offshore wind power in relatively shallow waters, and the U.S. government this year announced four zones for offshore wind energy development in the area. The 450 MW NRG Bluewater Wind project off the coast of Delaware and scheduled for 2016 could be the first to connect to the AWC transmission backbone.