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.

Detailed analysis of a 4-phase tapped-inductor boost DC-DC converter operating in continuous conduction mode is presented.Expressions needed to evaluate converter performance are derived. These are then used to plot input-and output-side current ripple characteristics. It is shown that inputand output-side ripple vary in proportion and the converter operates with low ripple over certain ranges of duty ratio. The performance of a 1-phase converter is seen to represent the worst-case scenario for 3-and 4-phase converters. For a given per-unit load and type of snubber, 1-and 4-phase converter efficiencies are identical. Converter is suitable for applications with wide input voltage range, requiring low input-and outputside ripple, high efficiency and boost ratios. Theoretically derived characteristics are validated through simulations and there is good agreement between analytical and simulation results.

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