A Comparison of Voltage Balancing Versus Energy Balancing Approach for Modular Multilevel Converters

The multilevel submodules (SMs) bring the added advantages of compact size and improved efficiency over standard submodules to the modular multilevel converter (MMC). These SMs have more than one floating capacitor, which stores the arm energy. Each arm of MMC has several SMs connected in series to meet the required operating voltage. The SM capacitors voltage or energy must be controlled to ensure the safe and reliable operation of MMC. This objective can be achieved by using either voltage balancing or energy balancing approaches. When all the SMs have equal capacitance value, then the voltage and energy balancing approaches have identical performance. However, the SM capacitance deteriorates over a long run, which affects the performance of MMC. This paper presents the performance comparison of voltage and energy balancing approaches for MMC with equal and unequal SM capacitance. The performance of balancing approaches is evaluated in terms of active and reactive power loading of individual SMs, rating of IGBT devices and SM capacitors, and output voltage and current harmonic distortion.