Improved Control of DFIG Systems During Network Unbalance Using PI-R Current Regulators

This paper presents a new control strategy for a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. Coordinated control of the grid- and rotor-side converters (GSC and RSC, respectively) during voltage unbalance is proposed. Under an unbalanced supply voltage, the RSC is controlled to eliminate the torque pulsation at double supply frequency. The oscillation of the stator output active power is then compensated by the active power output from the GSC, to ensure constant active power output from the overall DFIG generation system. In order to provide precise control of the positive- and negative-sequence currents of the GSC and RSC, a current control scheme consisting of a proportional integral (PI) controller and a resonant (R) compensator is presented. The PI plus R current regulator is implemented in the positive synchronous reference frame without the need to decompose the positive- and negative-sequence components. Simulations on a 1.5-MW DFIG system and experimental tests on a 1.5-kW prototype validate the proposed strategy. Precise control of both positive- and negative-sequence currents and simultaneous elimination of torque and total active power oscillations have been achieved.