Abstract
Traditionally, the internal network composition of offshore wind farms consists of alternating current (AC)
collection 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 plus mechanical parts, a generator including electronic controller, and a huge 50-or 60-Hz
power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus; consequently, the transformer
in each WECU is replaced by a power converter or rectifier. The converter is more compact and smaller in size compared to
the transformer. Thus reducing the size and weight of the WECUs, and also simplifying the wind farm structure. Actually, the
use of offshore AC collection grids instead of offshore DC collection grids is mainly motivated by the availability of control
and protection devices. However, efficient solutions to control and protect DC grids including HVDC transmission systems
have already been addressed. Presently, there are no operational wind farms with DC collection grids, only theoretical and
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 paper discussed some of the main components required for a DC
collection grid including: the wind-turbine-generator models, the control and protection methods, the offshore platform
structure, and the DC-grid feeder configurations. The key component of a DC collection grid is the power converter; therefore,
the paper also reviews some topologies of power converter suitable for DC grid applications.