Stability Analysis of a Direct-synchronized Single-phase Grid-tied Inverter

A single-phase grid-tied inverter uses the second-order generalized integrator (SOGI) filter to create an orthogonal channel for the synchronous reference frame phase-locked-loop (SRF - PLL). A direct synchronization approach synchronizes to the LCL filter capacitor voltage employing two alternative strategies: using the capacitor voltage and SOGI quadrature output as the inputs to the PLL or using both SOGI outputs as PLL inputs. This work analyzes the stability of these two direct synchronization methods. A synchronous reference frame small-signal model is developed by introducing fictitious orthogonal quantities, and Floquet theory is applied to perform stability analysis of the developed model. The stability predictions are validated in two ways: using a DSIM co-simulation of a single-phase grid-tied inverter, and experimentally using a 5-kW grid-tied inverter.