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
What is critically needed is an underground and/or subsea linkage capable of carrying >10 GW through a single conductor. Such supergrid backbone links could tie multiple offshore wind farm hubs together with points on shore, for much less money than 10 cable connections. I have been working on one solution to this problem (elpipes) for several years now. My solution to this problem uses large, rigid conventional conductors and polymeric insulation. The pipe-like segment modules are tied together through flexible splice modules; the entire elpipe is mounted on powered carriages and rolls into the conduit (essentially identical to a gas pipeline) in a controlled manner. This method of installation allows all splices to be made under highly controlled conditions at one end of the line (good for splice reliability). The wheeled, train-like nature of the elpipe also enables faster repairs and even preventive maintenance.
Other high capacity underground options include superconducting lines (which are problematic in terms of reliability) and gas insulated lines (GIL), which are no closer to being a proven technology for HVDC than elpipes.