Passivity-Based Control of Single-Loop Grid-Forming Inverters

This article addresses the passivity-based control of grid-forming inverters with an emphasis focused on the single-loop control system. It is elaborated that the closed-loop control system could be stabilized even without damping the resonance of the
LC
filter, and just by introducing the negative phase to the system. This negative phase is introduced using an all-pass filter that is adopted in series with the voltage regulator, and therefore, the system structure keeps its simplicity. It is also revealed that, unlike the traditional single-loop control system in which the stability region is higher than one-third of the sampling frequency, the proposed method is able to widen the stability region to frequencies lower than one-sixth of the sampling frequency. Then, the output impedance of the inverter is modified using the feedback of the output current to have a passive behavior up to the Nyquist frequency, which assures the stability of the system in different grid impedances and load conditions. The controller design procedure is elaborated in each part to provide the passivity of the system. The proposed control system is examined by experimental tests in grid-tied and off-grid modes to validate the theoretical concepts.