Modeling And Control Of MW Permanent Magnet Direct-driven Wind Power System Connected To The Grid

Permanent Magnet Direct-driven Power System (PMDPS) has beenwidely used in modern society. Modeling and control analysis to twotopologies (back-to-back topology and boost-based topology) of PMDPS ispresented. Operating conditions of two topologies under full ranges ofwind speeds are compared, and Low Voltage Ride Through (LVRT)capacity of the system is evaluated.Derivation of models for each part of PMDPS, including turbine,drive chain, Permanent Magnet Synchronous Generator (PMSG),stator-side and grid-side converter, DC-link and unloading circuit, ispresented. Derivation of p.u. models is also presented for the sake ofgenerality.Control strategies of two topologies of PMDPS are analyzed. Forcontrol strategy of turbine, Maximum Power Point Tracking (MPPT)methods based on tip-speed ratio, power and torque are compared. Addingpitch angle control method above rated wind speed, general controlstrategy is presented under full ranges of wind speeds. For control methodsto stator-side and grid-side of back-to-back topology, Zero d-axis Current(ZDC) control and Voltage Oriented Control (VOC) control are adoptedrespectively, while DC current control and VOC control are adoptedrespectively for control methods to stator-side and grid-side ofboosted-based topology.Simulations for two topologies of PMDPS under full ranges of windspeeds based on PSCAD platform are conducted. Three phases ofoperating conditions and responses for two topologies are compared carefully. Simulating results show that large ripples exist in both DCvoltage and output active power for boost-based topology, although controlstrategy of this type is much easier than back-to-back topology. However,angular speeds of generator and response capabilities for two topologiesare similar by virtue of similar control strategy.Unloading circuit is exclusively analyzed under short circuit fault ofgrid. The importance of unloading circuit to improve LVRT capability ofPMDPS is proved by comparing output curves of DC voltage, active andreactive power output to grid between unloading-circuit-based system andwithout unloading-circuit system.LVRT capacity for PMDPS under short circuit faults of grid indifferent extents is analyzed. Simulating results show that more serious theshort circuit faults of grid are, stronger the voltage oscillation will be, andlonger the system will keep for recovering back after faults disappear.