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In this paper, an overview on the existing electrical collection and transmission systems for large offshore wind farms, using both a.c. and d.c., is briefly given focusing on the criteria like the energy production cost, reliability & availability, energy efficiency and power density. A HV d.c. transmission and MV d.c. distribution system structure for offshore wind energy is discussed. Several practical issues like: MW DCDC converters, MV d.c. cables, d.c. protection, N+m redundancy, parallel modules, etc., are discussed. Finally the paper will show how a d.c. distribution system integrates into a point to point or multi-terminal HVDC system. When d.c. power collection and HVDC transmission are integrated, it can provide advantages like higher efficiency, availability and power density over existing AC/DC HVDC schemes.