The diode-clamped half-bridge MMC structure with internal spontaneous capacitor voltage parallel-balancing behaviors

Abstract

In order to significantly reduce the number of voltage sensors and computation boards in the modular multilevel converter (MMC), this paper proposes a diode-clamped half-bridge MMC with full capability of capacitor voltage parallel-balancing. The newer generation of MMC structure can be re-constructed from the traditional half-bridge MMC by integrating one clamped diode and damping resistor into each submodule (SM), and four auxiliary voltage balancing circuits across all three phases. The proposed MMC topology can naturally balance its SM capacitor voltages regardless of modulation and triggering techniques. This is due to the fact that all the SMs in the MMC belong to 6 Spontaneous Capacitor Parallel Behaviors (SCPBs), through which the voltages of SM capacitors are automatically balanced. The inserted SM capacitor is now parallel with the adjacent bypassed SM capacitor. By designing proper RC constant, the adjacent SMs in the proposed MMC will have equal capacitor voltage ripples. Although voltage balancing is naturally guaranteed, in order to average the switching losses, open loop nearest level modulation (NLM) and carrier phase shifted sinusoidal PWM (CPS-SPWM) techniques are used in the proposed MMC. The electromagnetic transient (EMT) simulations on PSCAD/EMTDC and the down-scaled prototyping results will validate the new topology.