Analysis of the Lower Limit of Allowable Energy in Modular Multilevel Converters

The operation of Modular Multilevel Converter (MMC) relies on the storage of a certain amount of the energy in the stacked sub-modules (SMs). With appropriate control, this internally stored energy provides an additional degree of freedom, which does not exist in conventional Voltage Source Converters (VSCs). However, due to the internal energy pulsations and their strong non-linearity, the minimum required amount of this energy to ensure its proper operation had not been fully understood. In this paper, a thorough analysis of the converter arm behavior is presented, which gives an analytic expression of the lower limit of the energy as a function of the converter operating point and the connected grid conditions. The relation between the lower energy limit and the operating power is analyzed by using the practical MMC specifications of an HVDC application. An experimental test of a small-scale MMC mock-up demonstrates the validity of the theoretical analysis.