Direct power control for wind-turbine driven doubly-fed induction generator with constant switch frequency

Compared with vector control, direct power control (DPC), without inner current control loops, is capable of simplifying the structure of control system and improving its dynamic performance when applied to the doubly-fed induction generator (DFIG) based wind power generation system. This paper proposes three DPC strategies with constant switch frequency by using space vector modulation (SVM) technique. These DPC strategies aim at controlling the DFIG rotor flux (RF-DPC), rotor current (RC-DPC) and electromagnetic torque (EMT-DPC), respectively, based on the relation between DFIG state variables and control variables. Simulation studies are carried out on a 2MW DFIG wind power generation system. The simulated results demonstrate that these control strategies can be used to regulate the active and reactive powers independently under both normal and grid voltage-dip conditions. Moreover the RF-DPC is able to reduce oscillating time, the RC-DPC is capable of depressing the oscillating magnitude of rotor currents, while the EMT-DPC is able to eliminate the vibration in electromagnetic torque, which, consequently, diminishes the mechanical stress on the turbine shaft.