Novel Hybrid Cross-Connected Submodule-Based MMC for MV Applications

Modular multilevel converter (MMC) utilization in medium voltage (MV) DC applications can be limited by two factors. First, a lower sub-module (SM) counts leading to reduced output voltage levels. Second, using silicon (Si) IGBT restricts the switching frequency, ultimately impacting the THD of the output voltage. Mitigating these issues with additional filters can increase the overall size and cost of the converter. Replacing all Si IGBTs with silicon carbide (SiC) MOSFETs enables high-frequency switching, but their availability in low blocking voltage rating necessitates more SM count, hence increased cost. This paper presents a novel hybrid MMC to address the cost and efficiency limitations of the conventional MMC. The innovative design achieves 2N+1 level in output voltage while utilizing only N number of SMs per arm. The proposed converter employs a combination of N-1 Si IGBT SMs which are made to operate at relatively low frequency and a single Si/SiC based cross connected SM where all high frequency switching is concentrated on SiC MOSFETs. A novel control scheme is also proposed for balancing the SM capacitor voltages and ensuring a fine balance between the switching and conduction loss by strategically distributing the switching pulses among the SMs. The operation of the proposed converter with the novel capacitor voltage balancing scheme is validated using both simulation and experimental studies.