Siemens, the only provider of direct-current (DC) offshore wind connection projects, in October unveiled a potentially game-changing technology that it says enables cheaper and simpler grid connection of wind turbines miles off the coast.
The DC technology presented at the National Maritime Conference in Bremerhaven, Germany, consists of a platform housing the transmission equipment that is 80% more compact than the conventional large central converter platforms used to convert DC into alternating current (AC) for transmission and distribution. The new compact design neatly holds diode rectifier units (DRUs)—replacing conventionally used air-insulated transistor modules—the transformer, the smoothing reactor, and the rectifier in one tank.
If a number of smaller platforms can be built, a DC cable can connect several of them sequentially in a wind farm and then route them to an onshore transformer substation, the company explained. Typically, two DRUs (each with a transmission capacity of 200 MW) are installed on one platform. Up to three of these new platforms can be connected to each other to create an offshore grid node that replaces collection platforms in the wind farms—essentially creating a link that can connect up to 1.2 GW of offshore wind capacity to the mainland (Figure 2).
And that could result in cost savings and more efficiency compared to conventional platforms: “The volume of the platform structures is reduced by four-fifths, and the weight is cut by two-thirds. As a result, costs are reduced by more than 30%. At the same time, the new solution enables transmission capacity to be increased by one-third, while transmission losses fall by one-fifth,” Siemens said.
Siemens has commissioned 13 offshore grid connections to date, four of which have been DC connection projects. It says the use of DC instead of AC transmission to connect wind farms to the grid is made cost-effective by high-voltage DC transmission technology, which can be used for cable lengths of more than 80 km. The new solution also provides for a 20% shortened installation time, it said.
—Sonal Patel, associate editor