Improved Weak Grids Synchronization Unit for Passivity Enhancement of Grid-Connected Inverter

The weak grids may be unstable due to the negative resistance behavior of the grid-connected inverter (GCI) caused by the synchronization unit with the phase-locked loop (PLL). This article proposes a passivity enhancement method to attain the positive output resistance of GCI in the qq channel. Initially, the impedance decomposition is utilized to reveal the reason for the negative output resistance brought by PLL. Secondly, with decomposed impedance, a novel PLL by adding a prefilter and an impedance phase regulator with a normalized amplitude to the input of traditional synchronous reference frame PLL (SRF-PLL) is proposed. Afterward, the effect of the critical system parameters and GCI operating conditions on the GCI's passivity is discussed based on the established small-signal impedance model. Theoretical analysis manifests that the proposed PLL does not affect the system's stability under rectifier mode and brings the passivity of GCI's output impedance in the qq channel in broad system parameters under inverter mode. Lastly, the experiments are implemented, verifying that using the proposed PLL, the instability caused by the intersection between grid and GCI impedances is removed due to the passivity, and the system can operate at 1.0 per unit (pu) active power reference under the very-weak-grid conditions.