Impact of Dead-Time on Resonance in Doubly Fed Induction Generator Under Weak Grid

Subsynchronous resonance (SSR) and high-frequency resonance (HFR) are two phenomena that exist in a doubly fed induction generator (DFIG) system operated in a weak grid. Besides the aforementioned resonance, dead-time in the converter (rotor side converter (RSC) and grid side converter (GSC)) of the DFIG system can cause resonance at middle frequencies. The aforementioned resonance at middle frequencies can greatly degrade dynamic performance and cause instabilities in the DFIG system. Nevertheless, there is not enough research available in existing literature on resonance in the DFIG system, considering dead-time impact. This paper proposes a new method of resonance analysis in a DFIG system, taking dead-time effect into consideration and utilizing small-signal impedance modeling. Dead-time of the RSC and GSC will introduce distorted voltage into the rotor-and stator-side of the DFIG. Then, rotor-side impedance is modified by adding the effect of transfer function (impedance) from the distorted voltage to rotor current. Likewise, the stator-side impedance is modified by adding the effect of the transfer function (impedance) from distorted voltage to stator current. Thus, overall DFIG system impedance, as seen from the point of common coupling is changed. Finally, resonance is revealed by plotting the DFIG system impedance and weak grid impedance on the Bode plot. Additionally, resonance mitigation is impedance on the Bode plot. Additionally, resonance mitigation is exposed through the dead-time compensation (DTC) technique using an adaptive second-order bandpass filter. With the proposed technique, the resonance at middle frequencies is damped with zero probability of resonance reoccurrence. Finally, the validity and the accuracy of the proposed DTC techniques are validated using mathematical analysis and simulations in Matlab.