Averaged modeling and analysis of multilevel converters

Multilevel voltage-source converters (VSC) are attractive for high-voltage and high-power applications. However, the increased part counts and control complexity also complicate the converter modeling and control. One particular control problem is the balancing of dc capacitor voltages. Conventional voltage balancing control design has been based on periodic steady-state or line-cycle averaged models of the converter that don´t correctly predict the capacitor voltage dynamics within a line cycle and under dynamic conditions. This paper develops an averaged model for three-level neutral-point clamped (NPC) converters that is valid under all conditions and involves switching-cycle averaging only. Unlike previous works, no assumption is made about the balance of ac line voltages, making the resulting model suitable for analysis and simulation of large, possibly unbalanced electric power systems. Applications of the model include more efficient system simulation, controller analysis and design for ac current regulation, dc bus voltage regulation and dc capacitor voltage balancing. Simulation results are presented to validate the proposed models.