Direct torque control and virtual-flux based direct power control of current source converter in wind turbine application

Proposed control algorithm for current source converter (CSC) in wind turbine application is based on direct torque control (DTC) of permanent magnet synchronous generator and a virtual-flux based direct power control (VF-DPC) for grid side converter. State of novelty of the control algorithm rely on the fact that DTC and VF-DPC are not yet implemented in CSC applications. Furthermore, in proposed solution only sensors in DC link are required, since machine and grid side currents and voltages are reconstructed from DC link measurements and current state of active switches in rectifier and inverter, respectively. DTC for generator side converter is based on constant switching strategy developed for CSI induction motor drive. Instead of hysteresis torque and flux controllers it calculates stator flux reference from d and q reference currents and uses PI controller to determine reference DC link current. Modified switching table assures that switches are activated properly to generate lower torque ripples. On the grid side, DPC algorithm is used for control, assuming that transformer and line inductances are replaced with as a virtual AC motor model. Integration of phase-to-phase voltages of virtual motor yields virtual flux vector, necessary for estimating grid active and reactive power. Similarly to the basic DTC, VF-DPC uses hysteresis comparators to determine switching states in inverter. As a final result, proposed control algorithm assures independent active and reactive power control while maintaining the maximum converter efficiency and extracting the maximum power necessary for windgenerator systems. Simulation results confirm proposed control strategy.