Decoupled Control of a Three-Phase Modular Multilevel Cascade Converter Based on Double-Star Chopper-Cells

This paper presents a theoretical analysis and control of a three-phase modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC). The analysis and control are characterized by applying the so-called “αβ0” and “dq0” coordinate transformations to the converter. Existing analysis and control methods were unable to use any coordinate transformation because the three-phase converter was considered as three sets of single-phase converters that were analyzed and controlled independently. As a result, it was difficult to avoid mutual interferences among the three-phase ac currents, circulating currents inside the converter, and dc current. Power-flow analysis using the two coordinate transformations succeeds in decoupling these currents, as well as in deriving lucid relationships between the power flow and the circulating currents. Moreover, the power-flow analysis makes it possible to design a decoupled digital controller for balancing and regulating the dc mean voltages of all the floating dc capacitors. The validity of the design is confirmed by computer simulation using a software package of PSCAD/EMTDC.