Fractional‐Order Impedance Reshaping and Integral Finite‐Time Control for LCL Grid‐Connected Inverters in Weak Grids

Abstract

ABSTRACT Under extremely weak grids (short‐circuit ratio, SCR < 2), LCL grid‐connected inverters suffer severe wideband resonance and degraded transient performance due to voltage feedforward phase lag and phase‐locked loop (PLL) negative impedance. To circumvent the trade‐off between frequency‐domain stability and time‐domain tracking speed, this paper proposes a composite dual‐layer control strategy. At the linear level, a dual‐channel fractional‐order low‐pass filter (FOLPF) is applied to the feedforward path and the PLL to reshape system impedance and enhance the phase margin. At the nonlinear level, an integral finite‐time control (IFTC) is designed for the current loop to accelerate disturbance rejection. Global stability across various grid strengths is verified via the describing function method. Hardware experiments at SCR = 1.7 demonstrate that the proposed method ensures stable operation where conventional control fails, maintains the current total harmonic distortion (THD) below 5%, and reduces settling time by over 60% without overshoot.