MMC Impedance Modeling and Interaction of Converters in Close Proximity

This article develops a small-signal impedance model of modular multilevel converters (MMCs) using the harmonic state-space (HSS) method and studies the stability in a multiple converter scenario. In order to simplify analysis of the coupling characteristics between different frequencies in MMCs, the proposed model is developed in the positive–negative–zero (PN0) sequence frame, where the zero-sequence current in a three-phase three-wire system is directly set to zero without introducing a complicated method. A simple 2
$\times $
2 admittance matrix in the PN0-frame is extracted from the MMC small-signal model for ease of system stability analysis. Using the developed impedance model, the multi-infeed interaction factor (MIIF) measure is adopted to analyze the most significant interactions for multi-infeed converter systems to be prioritized. Different outer-loop controllers are adopted and compared in the analysis to illustrate the effect of different control modes on converter impedance and system stability. Analytical studies and time-domain simulation results are provided to validate the proposed model and stability analysis.