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This study presents control strategies of offshore wind farms with dc series collector systems and modular multilevel converter based high voltage direct current (MMC-HVDC) transmissions. Series connection of dc wind turbines enables the collector system to establish the HVDC transmission voltage level with neither transformer nor centralized converter platform, indicating huge savings on offshore wind farm investment. However, severe wind variation imposes overvoltage on the wind turbines and may induce a cascade of failures of other series connected units. This paper first explains the wind farm design constrains bound by overvoltage limitation of wind turbine units. Then two kinds of control strategies, classical control and proposed HVDC global control, are presented to ensure every unit operating within safety limit. Wind farms with both control strategies are simulated. Finally, the two kinds of strategies are compared in terms of technical feasibilities, economical benefits and annual energy production rates. The results demonstrate that DC series offshore wind farms with proposed HVDC global control strategy imply competitive economic advantages of future integration of remote energy generation.

to ensure the operation of each wind turbine under a safe voltage level while no power production is lost in the wind farm. This paper first introduces the whole wind generation system. Then the steady state analysis of the dc series offshore wind farm and the overvoltage phenomenon are described, which lead to the proposal of the global control strategy. The detailed realization of the global control strategy by use of the MMC is given after a short recall of the MMC arm average model.