Three-Phase AC–AC Hexagonal Chopper System With Heterodyne Modulation for Power Flow Control Enhancement

This paper proposes a three-phase ac chopper system for the interconnection of various distributed generation (DG) farms or main utilities to enhance the active and reactive power flow control. The absence of large energy storage component in direct ac-ac converter makes the system footprint small and reliable. As the interface for different ac sources, the presented converter can be configured as star or delta. However, delta connection is preferred as it can trap the potential zero-sequence current and reduce the current rating of the switching devices. In this way, the proposed converter resembles the hexagonal chopper, and it offers an inherent degree of freedom for output voltage phase shifting. Considering the scalability in high-voltage applications, a new version of the hexagonal chopper with half-bridge cell modular multilevel structure is developed. The modular multilevel ac hexagonal chopper (M2AHC) is operated in quasi-two-level mode to suppress the electromagnetic interference (EMI) caused by high-voltage switching. Quasi-two-level operation divides the voltage level transition into multisteps, diminishing the voltage rising and falling rates (dv/dt) in high-voltage condition. Then, heterodyne modulation is adopted for the presented chopper system, supplying a new degree of freedom to decouple the phase and amplitude regulation. Based on this idea, system control strategy is developed in synchronous reference frame (SRF). Simulations and experimentations have confirmed the validity of the proposed approaches.