This paper proposes a Linear Parameter-Varying loopshaping controller for a power-synchronized grid-following inverter (PSGFLI). This control strategy regulates the inverter output active and reactive power at the terminal instead of the point of connection and does not require a phase-locked loop (PLL) for extracting the voltage phase angle. Hence, the prevalent stability issues exhibited when GFLIs are connected to weak grids are not present, and the proposed PSGFLI control strategy can work under both very weak and strong grid conditions without being prone to instability. In this approach, the controller parameters are functions of the operating point and are changed during the real-time operation such that the closed-loop performance is preserved in all operating points. Furthermore, since the grid impedance is a factor in the design process, a robustness analysis against grid impedance estimation error is conducted, and it is shown that discrepancies in the estimated and real grid impedances are unlikely to make the system unstable. The performance of the proposed control design is validated in MATLAB/PLECS and experiments for both strong and weak grids.

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This article presents an enhanced power-synchronized grid-following inverter (ePSGFLI) for reliable and stable integration of renewable energy resources into the grid. By eliminating the use of a phase-locked loop (PLL) in the control loop, the ePSGFLI resolves instability issues commonly associated with conventional PLL-based grid-following inverters (GFLIs). Furthermore, in contrast to existing PLL-less strategies like linear parameter-varying control of power-synchronized GFLI (LPV-PSGFLI), which suffer from steady-state errors and instability during grid frequency deviations, the proposed ePSGFLI ensures stable performance without steady-state errors, regardless of the strength of the grid. To achieve robust and accurate tracking of grid frequency, even under significant deviations, while effectively decoupling the active and reactive power loops, the ePSGFLI employs a loop-shaping design procedure for real-time auto-tuning of the multivariable second-order 2-by-2 outer power controller. The effectiveness of the proposed ePSGFLI is validated via simulations in MATLAB/Simulink and experimental testing under both strong and weak grid connections. The results demonstrate its superior performance compared to existing PLL-based and PLL-less control strategies in terms of stable operation and accurate tracking of power and grid frequency.INDEX TERMS Adaptive control, phase-locked loop (PLL), PLL-less inverters, power-synchronized control, stability, weak grid.NABIL MOHAMMED (Member, IEEE) received the bachelor's degree (Hons.