Mitigation of MMC High-Frequency Resonance by Narrowband Damping

Control delay is known to cause negative damping in the impedance of modular multilevel converters (MMC) that may lead to high-frequency resonances. To damp such resonance, this paper presents a narrowband damping method through MMC control that adds damping around the resonance frequency. The paper first explains the effects of phase current and voltage feedforward on the MMC impedance, then describes a narrowband damping scheme based on sliding DFT that can damp a single system resonance. Use of multiple such narrowband dampers to simultaneously mitigate multiple resonances is also presented. The proposed design method takes into account the coupling among the dampers to avoid unintended interferences among them. Case studies are presented to verify the proposed designs.