AC-Voltage Controller for Grid-Forming Converters

This article presents a voltage controller designed to operate grid-forming (GFM) converters under different grid impedance scenarios demonstrating good performance in conjunction with external power-synchronization loops (PSLs), e.g., droop control. GFM converters are required to have fast, accurate and stable voltage-frequency response, self-synchronization, power balance, grid supporting and fault ride-through (FRT) capabilities; either they are connected to the grid or to other generators. State-of-the-art voltage controllers are mainly focused on achieving either good voltage tracking and disturbance rejection or wide stability range and robustness. This proposal fulfils both tendencies as, first, it shows fast dynamic response, accurate voltage tracking, and total cancellation of output current disturbances at both positive-and negative-sequences of fundamental frequency because the controller presents zero-output impedance. Second, the controller shows a good sensitivity function and a wide stability range, which permit its operation in grid-connected and stand-alone mode. Finally, the proposed controller exhibits a good performance combined with a PSL under a wide range of grid impedance scenarios, showing seamless transition between grid-connected and stand-alone mode and vice versa, demonstrating also FRT capabilities preserving GFM operation under symmetrical and asymmetrical grid-fault events.