A control strategy for an offshore wind farm with the generating units connected in series with a VSC-HVDC transmission link

This paper proposes a control strategy for an offshore wind farm with the generating units connected in series with a DC transmission link. This configuration presents the advantage of not requiring an offshore substation with a power transformer to increase the power plant terminal voltage since the generators are connected in series through the DC terminals of the interface rectifiers. On the other side, the energy transferred by the subsea DC cable is processed by a modular multilevel inverter before it is injected into the mains. The modular multilevel inverter regulates the current through the DC link cable instead of the voltage of the DC system. This choice allows all the generating units to transfer maximum power through the DC link cable without the need of any communication channel to exchange information between the offshore units and the onshore substation. The system controllers are designed to assure the stable operation of the proposed strategy. Digital simulations results, obtained with PSCAD/EMTDC, are used to validate the effectiveness of the designed controllers and to demonstrate the performance of the system for different operational conditions.