Grid-Current-Feedback Control for LCL-Filtered Grid Converters With Enhanced Stability

This paper proposes a second-order-generalized-integrator (SOGI)-based time-delay compensation method for extending the stable region of a dual-loop grid-current-feedback (GCF) control system. According to the analysis, stable region of the dual-loop system should be designed below a certain critical frequency, before time-delay compensation method can be applied. To always meet the requirement, relationship between single-loop converter-current-feedback and dual-loop GCF control is clarified, before a robust inner-loop gain for the dual-loop GCF scheme is determined. Enforcing this gain allows the converter to remain in its stable region, regardless of how its LCL-filter parameters and grid impedance vary. The SOGI-based delay compensation method can then be applied for widening the stable region of the dual-loop GCF scheme, as proven through s-domain Bode diagrams and z-domain root loci. These theoretical proofs are eventually validated by experimental results obtained in the laboratory.