A Novel Series Flexible Transfer Converter Enabling Autonomous Control of Microgrids

AC microgrids are normally required to work in grid-connected (GC) mode and stand-alone (SA) mode, while the key step to transfer from SA mode to GC mode is the pre-synchronization. For its acceleration, active methods are preferred but communication to coordinate numerous distributed generation (DG) units is usually needed. To fully achieve autonomous control of microgrids, the flexible transfer converters (FTCs), positioned at the interface between utility grid and microgrids have been proposed to facilitate pre-synchronizations without communication. A general-purpose FTC (back-to-back type) has been verified to be effective but with drawbacks like high cost and power loss. This paper proposes a novel series FTC to overcome the disadvantages above. It consists of a series-connected transformer, a voltage-source converter and some auxiliary switches to constrain voltage stress and facilitate the pre-charge of dc-side capacitor. Through theoretical analysis, active power exchanged between grids can be adjusted by controlling its output reactive power. The pre-charge, control mode transfer and pre-synchronization control of FTC are illustrated in detail and simulated through a comprehensive case study. Finally, the experiment verification is carried out to show its value in practical applications.
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