Control method of an AC-AC MMC for open-end stator winding wind turbine generation system

Abstract

With the increasing of power rating and voltage level of wind turbine, the traditaional low voltage converter is being replaced by multilevel converter. This paper proposes a new ac-ac modular multi-level converter (MMC) connecting the open-end stator winding permanent magnet synchronous generator (OW-PMSG) to AC power grid. The working principle of the proposed topology is analyzed. Under different grid conditions, a unified capacitor voltage balance control method and a current control method are proposed. Aiming at the low grid voltage ride through problem, a distributed braking chopper circuit and the corresponding control strategy is given. Simulation is conducted for verification. Compared with the back to back (BTB) MMC for OW-PMSG, the number of capacitors and arm inductors of the proposed topology are reduced by 43% and 33%. Moreover, the number of the state variables of the average capacitor voltages and currents of the proposed topology are reduced by 33% and 47%, reducing the control complexity. The proposed topology also has higher efficiency than the BTB MMC for OW-PMSG. Compared with the other ac-ac MMC topologies for OW-PMSG, the combined power rating of the power devices of the proposed topology is much lower and thus the proposed topology has higher power density.