Mimo impedance based stability analysis of DFIG-based wind farm with MMC-HVDC in modifed sequence domain
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The modular multilevel converter (MMC) based high-voltage direct current (HVDC) transmission is widely applied for the large-scale wind farm integration making its interaction stability analysis especially significant. However, only a few related researches have so far been done and most of them depict ac-side characteristics of the connected MMC station using simplified single-input-single-output (SISO) impedance, which is not able to reflect all harmonic couplings and sequence couplings. To solve this issue, based on the harmonic state-space method, the multi-input-multi-output (MIMO) impedance model of the MMC in modified sequence domain are established to better characterize the MMC's off-diagonal coupling effects. Also, the accurate MIMO impedance model of the doubly-fed induction generator (DFIG) based wind farm is used in this paper. Subsequently, the generalized Nyquist criterion is applied to analyse the stability of such MIMO interconnected system. The accuracy of the established MMC's MIMO impedance model is verified via the frequency scanning, and the theoretical stability analysis of the interconnected system is validated by time-domain simulations in Matlab/Simulink.