Design Methodology for the DC Link Current Controller of a Series-Connected Offshore Wind Farm With a Droop Control Strategy

The series-connected offshore wind farm is an emerging configuration where the wind energy conversion units are connected in series through the dc terminals of their converters. It allows reaching high-voltage levels without need an offshore substation with a bulky step-up transformer and a huge rectifier in an offshore substation. On the receiving-side of the dc transmission, in the onshore substation, a modular multilevel converter is used to control the voltage or current of the dc link before injecting the energy supplied by the wind farm into the mains. Regardless of the control strategy, there are two main issued: overvoltage at the output dc terminals of the generating units and overcurrents in the parallel connected arrays of generating units. In this work, a new droop control strategy is proposed for the studied system, in order to keep the currents in the arrays balanced. Results of digital simulations performed with PSCAD/EMTDC software are used to validate the proposed control strategy and demonstrate the performance of the system.