Control scheme for DFIG converter system based on DC‐transmission

Abstract

This study proposes a novel control scheme for doubly‐fed induction generator (DFIG) in DC‐grid where its stator and rotor are connected to DC‐bus via insulated‐gate bipolar transistor (IGBT)‐based converters, namely rotor side converter and stator side converter (SSC). Maximum slip frequency strategy is proposed to determine stator frequency and improve stator voltage at low‐rotor speed, which could lower the design difficulty of SSC. Stator voltage decreases with stator frequency at low‐rotor speed, and keeps its rated value at normal rotor speed. The control strategy of constant stator voltage and constant rotor exciting current is adopted to make DFIG operate at normal and low‐rotor speed, respectively, and low‐speed wind energy is utilised accordingly. Furthermore hysteresis control strategy is adopted to avoid frequent switching in the strategy switching process. Experiment results validate the feasibility and effectiveness of the proposed control scheme.