Simulation study of FACTS devices based on AC-AC modular multilevel hexagonal chopper

This study proposes a new range of flexible AC transmission system (FACTS) device based on direct AC-AC conversion, where the modular multilevel AC hexagonal chopper (M2AHC) is employed. The M2AHC is operated in quasi-two-level mode; and the heterodyne modulation is used to decouple voltage amplitude regulation from the phase-shift; thus, independent control of active and reactive powers is achieved. Then, a family of FACTS devices based on M2AHC that offers voltage, active power and reactive power flow control as both shunt and series compensators is analysed. The use of AC cell capacitors instead of DC capacitors in M2AHC makes its footprint much smaller and lighter than conventional AC-DC or DC-AC voltage source converter-based FACTS devices; hence, high reliability and extended service-life could be expected. The system modelling and controller design of the proposed FACTS devices are illustrated in a unified reference frame, considering different control modes, transient and unbalanced conditions. Simulation results are used to verify the feasibility of the proposed M2AHC-based FACTS device. These FACTS devices will be preferred over conventional counterpart for confined spaced applications such as the grid access of large-scale offshore wind farms and resolution of loop flow in megacities.