Low-Capacitance Operation and Its Hierarchical Control for STATCOM Based on T-Type Converter With MMDTC

For conventional operation, the submodules of a static synchronous compensator based on a T-type converter with modular multilevel dc-link require high capacitance to suppress capacitor voltage ripple. Being different from that, an intriguing discovery reveals the capacitor voltage ripple is in phase with arm voltage in capacitive reactive power mode and opposite in inductive reactive power mode. As a result, a low-capacitance operation is proposed by exploiting this intriguing discovery. To better accommodate the proposed low-capacitance operation, this article presents a hierarchical control strategy that integrates a single PI controller, a model predictive control with a single objective, and an enhanced algorithm for sorting capacitor voltages. Finally, simulation and experimental results validate the effectiveness of the proposed low-capacitance operation and its hierarchical control strategy.