Improved Direct Torque Control for a DFIG under Symmetrical Voltage Dip With Transient Flux Damping

This paper proposes a new reference-generation-strategy for direct torque control (DTC) of doubly fed induction generator (DFIG), under symmetrical voltage-dips. Since DTC has no current-control loop, it cannot prevent overcurrent during voltage-dips. The proposed method prevents overcurrent in the rotor side converter, by damping the transient-flux and modifying the references of rotor-flux and torque, during voltage dip and recovery. The analysis of DTC for DFIG is presented by a λ-i equivalent circuit, which is decomposed into forced and natural circuits. Based on the natural λ-i circuit, the method adds a transient compensation term to the rotor-flux reference, which is obtained by multiplying the stator-natural-flux with a proposed decaying-ramp function. Moreover, this method reduces the forced component of rotor-flux proportional to the grid voltage. As a result, this method adjusts the transient-flux damping time and ensures overcurrent prevention in the rotor and stator windings. The torque reference is modified to maintain the torque-angle (between rotor and stator flux vectors) constant, which results in stable control of the DFIG during grid faults. The effectiveness of the proposed method is confirmed by experimental results of a 3-kW test set-up and simulation results of a 2-MW DFIG.