Introduction to Offshore Wind Power - Getting the Power to the People
Following the latest Crown Estates leasing allocations of offshore areas available for development of wind turbine fields, a number of wind turbine manufacturers are developing large capacity wind power generators. These range between 3.5 and 5MW rated power output.
These large turbines will require to be supported farther out to sea and in deeper water than at present.
This is an article on renewable energy and in particular offshore wind turbines. We begin by having a look at the current status of large output wind turbines, going on to examine a typical offshore field using HVDC technology.
Current Status of Large Output Offshore Wind Turbines
There are a number of wind turbine manufacturers in Europe developing 4 or 5MW output wind turbines in response to the UK Crown Estate offering large areas for lease and development of offshore windfarms. (The Crown Estate is the landowner of the seabed and foreshore within British territorial waters and leases areas to windfarm operators for development among other things to offshore windfarms).
These areas are the farthest from the British coast and in the deepest waters so far allocated by the Crown Estate for windfarm development.
This means that larger capacity wind turbines can be used because of the increased wind velocities available. However because of the depth of water, the turbines will have to be mounted on typical tried and tested oil and gas structures that will either be fixed to the seabed using a conventional steel jacket of a floating structure such as a semi-submersible.
Below are sketches of a fixed steel jacket and a floating structure from the author’s experience in the offshore oil and gas industry. These are suitable supports for large capacity wind turbines as they move out into deeper waters.
The large output of power will be transmitted by subsea cables, and to prevent loses the power will be converted to High Voltage Direct Current (HVDC) using the latest state of the art Voltage Source Converter (VSC) system, such as the Siemens 800MW capacity VSC.
This will be installed on a moored floating platform along with the rest of the electrical equipment required by the VSC system. The author’s interpretation of this is shown below.
High Voltage Direct Current-Installation and Operation of The Siemens Wind Power Offshore Substation (WIPOS)
Siemens have the contract to connect the Veja Mate and Global Tech1; two offshore windfarms in the North Sea that between them have an output of 800MW. This current will be converted to HVDC before being transmitted 125km by undersea cable to shore. Here it will be transmitted about 75km to Diele near Papenburg where it will be converted to 400kV AC suitable for transmission to the German national grid.
The HVDC units and associated transformers, AC cable compensating reactors and High Voltage Gas Insulated Switchgear (GIS) will be installed in buildings fitted to the deck of the semisubmersible floating platform, which will be moored by anchors to the seabed. The floating semisubmersible design appears to be the preferred method, probably because all the electrical equipment can be installed, connected and tested ashore, before the structure is towed to its location. Once there it does not require the use of a heavy lift floating crane, nor pile driving barge for installation.
Once the correct location has been confirmed, the vessel is ballasted and the anchors are lowered onto barges and towed out to a predetermined location where they are dropped to the seabed. Divers or a ROV (Remote Operated Vehicle) will be on hand to ensure anchors are securely fixed to the seabed when their hawsers/ anchor chains are being tensioned by the semisub’s winches. A satellite positioning system (SPS) can be then used to maintain the vessels position and avoid drifting and subsequent straining of the incoming and outgoing power cables.
Once connected to the windfarm power output, the Voltage is stepped up to 300kV AC then converted to 300kV DC before the cable is running down to the seabed connecting to the subsea cable.