Impedance reshaping control of LCL grid-connected inverters under weak grid conditions: digital delay compensation and robustness enhancement

Abstract

Abstract Under weak grid scenarios, wide variations of grid impedance distort resonance characteristics of LCL-type grid-connected inverters. Digital control delays introduce phase lag, which easily causes damping polarity reversal in conventional capacitor-current-feedback active damping strategies. From the perspective of impedance stability, this paper reveals that control delays produce frequency-dependent resistive components in equivalent damping impedance. The analytical boundary of positive–negative resistance transition is derived, which dominates the weak-grid adaptability of inverters. Accordingly, an impedance reshaping strategy based on phase-lead delay compensation is proposed. Embedded in the feedback loop, the phase-lead network extends the valid positive-resistance frequency region and decouples the inherent coupling between LCL resonance frequency and sampling frequency. The critical frequency is lifted from $$\frac{{f}_\mathrm{s}}{6}$$ to above $$\frac{{f}_\mathrm{s}}{4}$$ , and the system maintains a stability margin over 45° within 0–10 mH grid inductance range. A quasi-proportional-resonant cascaded current regulator is further designed to suppress background harmonic interference. Simulation and experimental tests on a 5 kW prototype verify the superior performance. When grid inductance steps from 0 to 8 mH, grid-connected current THD remains below 2.8%, and transient response completes within two fundamental cycles. This study provides theoretical guidance and practical solution for stable grid integration of high-penetration renewable energy systems.