Admittance-Based Stability Analysis of LCL-type Grid-connected Inverter Considering AC-side and AC-DC Frequency Coupling Effects

With the advancement of distributed generation (DG) technology, numerous grid-connected inverters are being integrated into the power system. The interaction between inverters and the power grid has triggered complex resonance and stability issues in the power system. Currently, the research models for LCL-type grid-connected inverters do not simultaneously consider the AC-side and AC-DC frequency coupling effect, which could lead to misjudgment in the system's stability. To address this deficiency, this paper formulates a third-order admittance matrix model for an LCL-type grid-connected inverter that accounts for both AC-side and AC-DC frequency coupling using the harmonic linearization. Furthermore, this paper presents a simplified modeling approach for the inverter. Taking a two-stage photovoltaic system as an example, this paper establishes the Generalized Nyquist Criterion (GNC) for the system, and employ GNC to perform stability analysis on the photovoltaic system. Results indicate that when the outer control proportional coefficient of the DC-bus voltage is relatively high, the AC-DC frequency coupling will significantly impact the system's stability. The proposed third-order admittance matrix model's effectiveness in stability analysis was verified through simulations on the PSCAD/EMTDC platform.