Quasi-Two-Level PWM-Operated Modular Multilevel Converter With Nonlinear Branch Inductors

Quasi-two-level PWM operation of modular multilevel converters enables very low module capacitance even at low machine speeds and high torques while the current distortion is tolerable. In this article, the application of nonlinear inductors together with this operation mode is investigated to further decrease the module capacitances. The requirements for the inductors are analytically derived and the design of the nonlinear inductors using a stepped air gap is demonstrated. The feasibility and the advantages of quasi-two-level PWM operation with nonlinear inductors are validated using simulations and a laboratory prototype.