Power-flow Analysis and Capacitor-Voltage Control of the Modular Multilevel Cascade Converter (MMCC-xSBC)

Abstract

The modular multilevel cascade converter (MMCC) family has been attracting attention from power electronics and power system engineers because it has been showing considerable promise as one of the next-generation high-power high-voltage/medium-voltage power converters. This paper provides an intensive discussion on power-flow analysis of three MMCC members with star configuration, focusing on dc-mean-voltage balance/regulation and ac-voltage mitigation control of all the floating dc capacitors. Most existing analysis methods are based on single-phase circuits, which make it difficult to derive capacitor voltage regulation/mitigation methods relying only on circulating currents and zero-sequence voltage. The power-flow analysis and regulation/mitigation methods developed in this paper are characterized by applying the αβ0 coordinate transformation(s). The validity of the power analysis and the regulation/mitigation method is confirmed by simulation using the PSCAD/EMTDC software package.