Compensation of Magnetizing Current for Enhanced Operation of DFIG Under Grid Unbalance

In the context of a doubly fed induction generator (DFIG) connected to the utility grid under unbalanced voltage conditions, the controller design needs to ensure additional challenges such as restricting the total harmonic distortion (THD) in grid current, minimizing the pulsations in generated power, torque, dc-link voltage, etc., apart from facilitating the generator power control. In this study, a negative-sequence compensation scheme for the magnetizing current in a DFIG is investigated. The reference currents are modified to include the negative-sequence compensating component for the magnetizing current. A rotor-side converter is employed to compensate the pulsations in magnetizing current thereby minimizing the ripple in power and torque. A grid-side converter is used to maintain unity power factor and a constant dc-link voltage. It is revealed that controlling the magnetizing current as a single control target enables simultaneous reduction of pulsations in torque, power, and dc-link voltage and also minimizes unbalance in currents. The effectiveness of the scheme is validated through detailed analysis and numerical simulations in PSCAD/EMTDC for a practical 250 kW DFIG wind generation system and further, through experimental results for a 2.3 kW DFIG test setup. Typical experimental results for the laboratory test setup show an improved performance.