Investigation and Mitigation of Common-mode Voltage in Four-level NPC Converters Modulated by Redundant Level Modulation

Redundant level modulation (RLM) has emerged as a powerful modulation-based voltage balancing method for a range of multilevel converters. For four-level neutral point clamped (4L-NPC) topologies, utilizing redundant voltage levels is the mandatory solution in principle to enable the single-end 4L-NPC converters to keep a voltage balance under all operating conditions, without requiring auxiliary circuits. However, the side effects of RLM caused by the extra switching actions have not been fully studied, with the common-mode voltage in particular. Hence, this work investigates the worsened common-mode voltage problem induced by RLM through theoretical analysis and simulations, followed by a simple carrier-based mitigation method that is verified in experiments.