Energy storage and circulating currents in the Modular Multilevel Converter

This paper examines the operation of the Modular Multilevel Converter (M2C) at variable frequency e.g. for drive applications using pulse width modulation (PWM). The converter's arm currents and specifically the included circulating currents as well as the distributed capacitor voltage harmonics are analyzed. The strong correlation of these quantities with the design of the energy storage devices, i.e. the distributed submodule capacitors and the arm inductors is discussed. In this context, the harmonic excitation of the M2C which depends on the choice of the resonant frequency resulting from the energy storage devices is shown.