A Series Flexible Transfer Converter with Reduced Cost while Keeping Functions of Instant Power Support and Auto-Synchronization for Microgrid

AC microgrids are normally required to work in both islanded mode (IM) and grid-connected mode (GCM), while the key step to transfer from IM to GCM is the presynchronization. For its realization, active methods are preferred but communication to coordinate numerous distributed generation (DG) units is usually required. To fully achieve autonomous control of microgrids, the flexible transfer converter (FTC) has been proposed to facilitate the auto-synchronization without communication. A general-purpose FTC (back-to-back type) has been verified to be effective but with drawbacks like higher cost and loss. This paper proposes a series FTC (SFTC) to overcome the disadvantages above. It consists of a series transformer, a voltage-source converter, a phase-selection switch to constrain voltage stress and auxiliary switches for precharging and bypassing. The steady-state model is established, and main parameters are designed. Through theoretical analysis, active power exchanged between grids can be adjusted by controlling the output reactive power of SFTC. The pre-charge, control mode transfer, auto-frequency control for instant power support and auto-synchronization control are illustrated in detail and simulated through a comprehensive case study. Finally, the experiment verifications are carried out to show its value in practical applications.