Current control and energy balancing of a square-wave powered 1AC-3AC modular multilevel converter

This paper presents a new control method for a Modular Multilevel Converter (MMC) fed by a push-pull converter via a medium frequency (MF) transformer. Application of this topology is a universal high-precision 3AC voltage source for a Power Hardware-in-the-Loop Emulator with a frequency range from DC up to almost the medium frequency. Advantages are high efficiency, very low harmonic distortion and high dynamics of the output voltage which can be used e.g. for simulation of electrical machines. Due to the medium frequency input of the MMC the transformer size for the galvanic isolation is much smaller compared to a low frequency input. Additionally, the required cell capacitance for the MMC is reduced which saves cost and space. The delivered medium frequency square-wave voltage requires an alternating input current of the MMC. For the square-wave powered 1AC-3AC MMC an energy and current control is proposed. Due to very high current dynamic requirements a dead-beat controller as subordinated controller is used to achieve a trapezoidal input current which allows zero current switching (ZCS) at the push-pull-converter. The design of the superposed energy and balancing controller is also shown. For the coupling between the superposed and subordinated control loop the arm power is analyzed and a calculation scheme is given.