Enhanced frequency control for power-synchronized PLL-less grid-following inverters

This paper 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.