A study on minimum required capacitance in flying capacitor multilevel converters for grid-connected applications

Multilevel converters are expected as a fundamental solution to harmonics and EMI problems caused by power converters. In general, the multilevel converters generate the output voltage using voltages of internal capacitors or floating voltage sources. From the practical point of view, implementation of the source of multilevel voltage without increase in the volume of the converter is strongly desired. In this context, flying capacitor multilevel converters are the most promising candidates for the practical multilevel converters. In this paper, the minimum required capacitance of the flying capacitors is investigated for the case of grid-connected inverters in which the reduction of the harmonics and EMI is strongly required. The minimum required capacitance is determined considering the allowable range of voltage variation in the flying capacitors to ensure proper operation both in the steady state and transient conditions. For the determination of the minimum capacitance, theoretical expression of the voltage ripple in the flying capacitors is derived. The validity of the theoretical expression is confirmed experimentally. In addition, the transient operation under severe disturbances such as a sudden change in power flow and a short-time power line fault is confirmed experimentally.