Two-coordinate decomposed SISO models-based stability analysis of three-phase grid-connected inverter systems with asymmetric control

In this paper, the Single-Input Single-Output (SISO) theory-based stability analysis method for weak grids with a three-phase grid-connected inverter (GCI) incorporating asymmetrical control is proposed in dq-frame and sequence domain, respectively. Initially, the paper presents two-coordinate decomposed models for Multi-input Multi-output (MIMO) systems. These models reveal that the stability of the MIMO system can be evaluated by analyzing two open-loop gains in the dq-frame or a single gain in the sequence domain using Nyquist/Bode plots. Next, the proposed analytical method is compared with existing state-of-the-art techniques. Theoretical analysis demonstrates that the proposed method enables the separate design of asymmetrical control in the d-axis and q-axis using the dq-frame approach. Furthermore, it enables the identification of the system's instability axes, namely the d-axis and q-axis. On the other hand, the proposed method in the sequence domain offers a lower computational complexity while accurately identifying the system's frequency-coupled resonances. Finally, the effectiveness of the proposed method is validated through simulations and experiments, consolidating its practical applicability.