“Offshore wind farms of this size place particular demands in terms of grid access. We not only have the requisite technology and know-how but also a wealth of experience in connecting offshore wind farms to the grid,” says 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 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 to the coast.

On each of the platforms there are two 180-MVA transformers and medium-voltage switchgear. 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, Siemens says.

The transformers on the substation platforms at the London Array offshore wind farm step up the 33 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 fulfil the British grid’s requirements (Grid Code) on the quality of the electrical energy fed into the grid.

For that purpose Siemens says it will deploy its SVC Plus system, which operates with voltage-sourced converter (VSC) technology 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, Siemens says.

Siemens will also prepare the requisite design studies for grid access for all of the offshore wind farm’s electrical components and prepare the grid studies to demonstrate fulfilment of grid access requirements, Siemens adds.

The order volume is €128 million and the London Array offshore 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.