Mechanism Analysis of Near Fundamental-Frequency Oscillation in an Actual DFIG-Based Wind Farm Connected with MMC-HVDC

Recently, near fundamental-frequency (NFF) oscillation phenomena in the frequency range of 40 ~ 60 Hz occurred in practical doubly-fed induction generator (DFIG) based wind farms connected with modular multilevel converter based high-voltage DC (MMC-HVDC) transmission system in China. The mechanism of the NFF oscillations is more complex than those oscillations in other frequency ranges, as all the control loops could impact the NFF characteristics. In this paper, the impedance models of doubly-fed induction generator (DFIG) based wind turbine generator (WTG) and wind farm side MMC (WFMMC) considering positive/negative-sequence control loops are established based on harmonic state-space method, which are then validated by frequency-scanning. On this basis, the mechanisms of the positive/negative-sequence NFF oscillations of the MMC-HVDC connected DFIG-based wind farm are revealed from the perspective of impedance characteristics. Then, the key influencing factors of the NFF oscillation stability of the interconnected system are quantitatively analyzed using the parameter phase margin sensitivity. Finally, the NFF oscillation phenomena in the practical system are reproduced, and the effectiveness of the mechanism and key influencing factors analysis is verified by time-domain simulations.