Review on DC collection grids for offshore wind farms with high-voltage DC transmission system

Traditionally, the network composition of offshore wind farms consists of alternating current (AC) grid; all outputs of wind-energy conversion units (WECUs) on a wind farm are aggregated to an AC bus. Each WECU includes: a wind-turbine (WT), a generator and a power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus. The transformer in each WECU is replaced by a converter which is more compact and smaller in size compared with the transformer, thus simplifying the wind farm structure. The use of AC offshore grids instead of DC offshore grids is mainly motivated by the availability of protection devices. Efficient solutions to protect DC grids have already been addressed. Presently, there are no operational DC wind-farms, only small-scale prototypes are being investigated worldwide. Therefore, a suitable configuration of the DC collection grid, which has been practically verified, is not available yet. This study discussed some of the main components required for a DC grid including: the WT-generator models, the control and protection methods, the platform structure, and the feeder configurations. The key component of a DC grid is the converter; therefore, this study also reviews some topologies of converter suitable for DC-grid applications.