Comparison of PLL-based and PLL-less control strategies for grid-following inverters considering time and frequency domain analysis

This article presents a comprehensive comparison of three control strategies used for grid-following inverters (GFLIs). The first strategy is the phase-locked loop (PLL)-based vector current control (VCC), and the other two PLL-less controllers are the voltage-modulated direct power control (VMDPC) and the linear-parameter-varying power-synchronized control (LPV-PSC). The VCC relies on the PLL to synchronize with the grid frequency to control the exchanged real and reactive power with the grid, which may result in instability issues under weak grid conditions. To prevent this, the VMDPC and LPV-PSC are proposed recently as PLL-less approaches to overcome the difficulties of the VCC in weak grid conditions. The performances of the VCC, VMDPC, and LPV-PSC are comprehensively reviewed in this article, considering the operations of the GFLIs under both strong and weak grids. In each operation mode, the provided tests investigate: 1) step response of active and reactive power, 2) frequency jump in the grid voltage source, 3) phase jump in the grid voltage source, and 4) voltage sag in the grid voltage source. In addition to evaluating the three controllers in the time-domain, the frequency-domain impedance-based stability analysis based on the generalized Nyquist criterion is also considered, which confirms the time-domain findings in terms of accurate predictions of the stable/unstable operations of the GFLIs that are equipped with these controllers. It is found that, compared to the conventional VCC and the VMDPC, the LPV-PSC has promising performance under various test conditions in both strong and weak grids, which may be a future solution for weak-grids connection of the GFLIs.