Offshore wind farm now has grid access

London Array grid connection by Siemens Energy links the offshore wind farm via substations, high-voltage subsea cables, VSC-based SVC Plus, and reactive power compensation at Cleve Hill, enabling up to 1,000 MW for 750,000 homes.

With its 175 Siemens wind turbines and a total capacity of 630 megawatts (MW) the London Array offshore wind farm will — following completion — be the largest of its kind in the world.

After Siemens Energy was already appointed to supply the turbines for the wind farm, the company also received the order to connect London Array to the power supply network, following major contracts awarded by the consortium earlier.

Purchasers are Dong Energy, E.ON and Masdar, the wind farm's owners. The order volume is 128 million euros. The wind farm is scheduled to be completed by 2012 and will be hooked up to London's power supply network via the Siemens grid connection, as the UK National Grid 2 GW substation illustrates in related infrastructure developments.

The wind farm is being erected in the Thames estuary approximately 20 kilometers off the Kent and Essex coasts. An option is also provided for upgrading London Array to as much as 1000 MW. The wind farm will thus become the first in the 1-gigawatt class. That will be sufficient to supply 750,000 British households with eco-friendly electricity, which is equivalent to approximately a quarter of the population of Greater London.

"Offshore wind farms of this size place particular demands in terms of grid access, as supergrid initiatives gather momentum across Europe. We not only have the requisite technology and know-how but also a wealth of experience in connecting offshore wind farms to the grid," said Udo Niehage, CEO of the Power Transmission Division of Siemens Energy.

Siemens will supply the electrical equipment for two offshore substation platforms, which will be installed right at the wind farm. The substations bundle the power generated by the 175 Siemens SWT-3.6 wind turbines, each rated at 3,6 MW, before it is transported via high-voltage subsea cable, similar to the Scotland-England link now underway, to the coast. On each of the platforms there are two 180-MVA transformers and medium-voltage switchgear, comparable to equipment in ABB's Irish grid contract for network expansion. The requisite protection and instrumentation and control equipment is also installed on the platforms. Distribution over two platforms has the advantage that the cable routes within the wind farm are short, and power transmission losses are kept as low as possible to enhance the wind farm's energy efficiency.

The transformers on the substation platforms step up the 33 kilovolts (kV) generated by the wind turbines to a transmission voltage of 150 kV. High-voltage subsea cables transport the electricity to the grid access point, which is located in Cleve Hill. In addition to a substation with four 180-MVA power transformers (400/150 kV) Siemens will also install four 50-MVAr reactive-power compensators at this access point to fulfill the British grid's requirements (Grid Code), in line with new grid standards adopted in Spain, on the quality of the electrical energy fed into the grid.

For that purpose Siemens will deploy its new SVC Plus system. It operates with innovative voltage-sourced converter (VSC) technology, proven in Siemens HVDC projects abroad, and is continuously controllable with the aid of insulated-gate bipolar transistors (IGBTs). The central feature of SVC Plus, a further refined statcom (static synchronous compensator), is its modular multilevel converter technology. By contrast with other self-commutated converter topologies the voltage waveform generated is practically sinusoidal because of the multilevel technology. This means that the low-frequency harmonic filters required in solutions used to date are no longer needed and significantly less space is required for the overall system.

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