Differential-Mode Voltage Components as Degrees of Freedom in the Control of Modular Multilevel Converters

High-frequency common-mode voltage components are used as control degrees of freedom for the modular multilevel converter to compensate large branch energies at low system frequencies. In this paper, it is shown that high-frequency differential-mode system voltage components can alternatively be used as degrees of freedom to reduce branch energy variations if, for example, the star point of the connected machine or grid is earthed. The novel approach is derived using an analytical model and is further validated using a simulation model. Moreover, a required filter for reducing the high-frequency components of the system currents is designed.