Direct power control based torque oscillations cancellation in doubly fed induction generator operating with unbalanced grid

This paper is focused on the unbalanced grid operation of a doubly fed induction generator. The main target taken into consideration for rotor converter control strategy is to reduce electromagnetic torque oscillations and simultaneously keep sinusoidal, but not necessarily symmetrical, stator current. The second, alternative target is to obtain symmetrical stator current, for which the generated stator power can be higher than for first target, but small torque oscillations occur. Synthesis of the reference signal calculation method for classic DPC with hysteresis controllers was preceded by analysis of mathematical model of the machine. The control method for unbalanced grid voltage operation of DFIG is designed in stationary αβ frame and does not require signal decomposition into positive and negative sequence for the first target, whereas for the second target determination of positive sequence of symmetrical stator current vector components is required. Additional part analyzed and described in the paper is the reference signal limitation method allowing overcurrent protection under unbalanced grid voltage.