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 x 2 admittance matrix in the PNO-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 he 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.