Impedance Modeling and Stability Analysis of MMC Flexible DC System at the DC Side

Abstract

Aiming at the stability of modular multi-level converter (MMC) on the dc side, this paper adopts harmonic linearization method to establish impedance model of MMC dc side. This method ignores the higher-order components of voltage and current, and considers the influence of the control loops to guarantee high accuracy at the same time. The high-frequency band of impedance presents inductive characteristics, and the mid- and the low-frequency band is related to the control loop. The parameters of the current loop have a greater influence on the impedance characteristics of the mid-frequency band while the voltage loop parameters have a greater influence on the low-frequency band. For the back-to-back transmission system based on MMC, the voltage at the dc side has obvious oscillation when the impedance mismatch or the stability margin is small. The risk of dc side voltage instability can be eliminated by optimizing the controller parameters and phase angle margin can be improved.