Wind turbine drivetrain for direct MVDC connection

This paper presents the concepts of platformless DC connections of offshore wind turbines. It describes the possible and preferred wind turbine drivetrain arrangements capable of producing sufficiently high DC voltage. The preferred DC drivetrain comprises of a medium speed permanent magnet synchronous generator (PMSG), a step-up transformer and a modular multilevel converter (MMC) based rectifier. The focus of this paper is on the detailed modeling of medium speed PMSG, which is a critical component of the preferred wind turbine drivetrain enabling desired operating frequency range for conventional line frequency transformer and MMC based rectifier. Due to large inductance of medium speed PMSG, conventional Zero d-axis current control (ZDAC) method for MPPT may produce a significant steady-state overvoltage at the PMSG terminals. Instead, the constant mutual flux linkage (CMFL) method is implemented for torque control. Simulation results are presented which demonstrate the desired performance of the PMSG for the full range of wind speeds.