Robust state control with narrowband disturbance rejection for wind driven DFIG under grid voltage unbalance

This paper introduces a direct power control (DPC) method with narrowband disturbance rejection capability for the grid-connected doubly fed induction generator (DFIG). The method is suited for variable speed operation of DFIG, especially in wind energy conversion systems connected to weak networks where voltage unbalance is common. The control framework is based on the linear quadratic regulator theory with a frequency shaped cost functional method. The DFIG is represented in the synchronous reference frame as a multi-input, multi-output controlled plant. The control objective is to bring the stator active and reactive power errors to zero despite plant modeling errors and disturbances. A supplementary compensation loop is provided to alleviate the operational problems associated with grid voltage unbalance. The supplementary compensator is optimally designed by considering the whole closed-loop system dynamics. The proposed DPC operates at constant switching frequency and there is no need for rotor flux estimation and for subsynchronous or supersynchronous speed detection. Stability analyses together with simulation studies on 2 MW DFIG are provided to confirm the viability of the method.