A frequency-responsive power-smoothing scheme of a doubly-fed induction generator for enhancing the energy-absorbing capability

Continuous wind speed variations fluctuate the active power of a wind turbine generator (WTG), thereby causing grid frequency fluctuations. Difficulty arises in alleviating the frequency fluctuations during normal operation, if WTGs continue to operate maximum power point tracking even for a high wind penetration level. This paper proposes a novel frequency-responsive power-smoothing scheme for a doubly-fed induction generator (DFIG) that can maintain the frequency deviations within a narrow range during normal operation of a power grid. To decrease the frequency in the overfrequency section, the proposed scheme decreases the DFIG active power, thereby absorbing energy into the rotating masses of a DFIG. To increase the frequency in the underfrequency section, the proposed scheme increases the active power, thereby releasing energy from the rotating masses. In the medium and high rotor speed regions, to improve the frequency-regulating capability, the proposed control gain increases as a fourth-order polynomial of the rotor speed. To avoid overdeceleration in the low rotor speed region, the control gain decreases as a sixth-order polynomial of the rotor speed as the rotor speed decreases. The simulation results clearly demonstrate that the proposed scheme significantly mitigates the frequency fluctuations under various wind conditions even in high wind penetration levels.