Sequence Impedance Modeling Method for LCL-Type Grid-Connected Inverters Considering Frequency Coupling

The broadband oscillation problem of large-scale renewable energy systems exhibits new characteristics such as multi-point oscillation coexistence and mutual coupling. These frequency coupling characteristics must be considered when analyzing the stability of the grid-connected inverter (GCI). For engineering applications, GCIs are usually LCL type with DC voltage loop and grid voltage feedforward loop to stabilize DC-bus voltage and improve the dynamic response. This paper proposes a sequence impedance modeling method considering frequency coupling for widely used LCL-type GCIs. The model involves the DC voltage loop, phase-locked loop, current loop, and grid voltage feedforward loop, and deeply investigates the influence degree of each control loop on frequency coupling and impedance characteristics.