MMC Equivalent SISO Sequence Impedance Model for Stability Analysis of Interconnection System

When the modular multilevel converter (MMC) station is interconnected with weak power grids, wind farms etc., broadband oscillation is serious and stability problems are prominent. The AC-side Mingle-Input Mingle-Output (MIMO) impedance models are the basis of the impedance analysis method. They are highly accurate, but the Generalized Nyquist Criterion (GNC) needs to be used during analysis, which is very cumbersome. In this regard, this paper derives the Single-Input Single-Output (SISO) sequence impedance model of MMC to stability analysis, and it is more simpler to apply. Firstly, the d-q impedance model of MMC is developed, and based on it, the SISO sequence impedance model is further derived. The correctness of the above model is verified by frequency sweeping. Finally, the models are applied to the stability analysis of MMC station and weak grid interconnection system. Both theoretical analysis and simulation results show that the SISO sequence impedance model has the same analytical accuracy as the MIMO model, and the analysis process is simpler.