Design of dynamic voltage restorer and active power filter for wind power systems subject to unbalanced and harmonic distorted grid
Nowadays unbalanced voltage and harmonic distortion are in the majority of the power quality problems in wind energy conversion systems (WECS). To alleviate the impact registered by unbalanced and harmonic distortion, a new circuit topology comprising a dynamic voltage restorer (DVR) and an active power filter (APF) is presented. A frequency shifting technique based on coordinate transformation is employed to unify the positive and negative sequence harmonics into a resonant current controller. To improve the accuracy for harmonic detection, a second-order generalized integrator (SOGI), characterized with large bandwidth at specific frequency, is capable of separating harmonics from feeder current. The fundamental and harmonic current controllers can be individually realized by the resonant current controller and combined to form voltage command for a voltage-sourced inverter (VSI) based on superposition theorem. Not only the computing time but also the harmonic currents in the feeder can be effectively reduced along with the proposed approach. To share the dc-circuit with the DVR, the APF and DVR are in back-to-back connection in favor of unbalanced and harmonic compensation for specific grid bus. A 3kW squirrel cage induction generator (SCIG) driven by a permanent synchronous motor (PMSM) is performed for wind-turbine generator emulation. The experimental results validate that the grid-tied SCIG power quality can be refined when grid bus is subjected to unbalanced and harmonic distortion.