This paper discusses a control method for grid-interactive inverters named “control-sync.” This method incorporates two pairs of control paths to provide voltage amplitude and phase-angle references: one pair is dedicated to grid-forming (GFM) operation and the other to grid-following (GFL) operation. A distinctive feature of the control-sync approach is that the background paths remain synchronized with the active paths. As a result, inverters within a microgrid – capable of operating in either islanded or utility grid-connected conditions – can seamlessly transition between GFM and GFL modes without any communication with a synchro-check relay – a device for controlling the microgrid’s connection to the utility grid. This paper employs the control-sync method with the minimum number of control loops. It also provides communication-free islanding and grid reconnection detection schemes, rendering a fully autonomous capability to the inverters in a microgrid. The efficacy of the control-sync method is validated through a hardware setup consisting of a 30-kVA grid emulator and two 5-kVA, 208-V three-phase inverters.
