Improved Current Control for a Quasi-Two-Level PWM-Operated Modular Multilevel Converter

Quasi-Two-Level PWM Operation of modular multi-level converters enables a significantly smaller amount of installed module capacitance, even with low output frequencies required for machine drives. This article presents an implementation of this operation mode with an improved current control that leads to fast and precise current transitions. The proposed current control combines a deadbeat controller with a simple hysteresis controller that uses a predictive estimation of branch currents, which are both implemented using an FPGA. The enhanced performance of the proposed control is demonstrated by a comparison to the original one. Experimental results on a laboratory prototype prove the feasibility of Quasi-Two-Level PWM Operation as a machine drive and validate the advantage of reduced overvoltages with long machine cables in comparison to those of conventional two-level inverters.