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

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 for 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. In this work, it is presented a design methodology to control the dc-link and the three-phase currents of the modular multilevel inverter in the synchronous reference frame. The relation between the dc link and the q-axis currents allows to design an outer control loop and the controllers to regulate the dc link current. Results of digital simulations carried out with PSCAD/EMTDC software are used to validate the proposed system and demonstrate the performance of the controllers.