The Grid-Connected Inverter Stability Enhancement Control Based on Impedance Instability Region Elimination

The weak grid and high Phase-Locked Loop (PLL) bandwidth can easily cause instability issues in the Grid-Connected Inverter (GCI) system. The present methods mainly enhance system stability by increasing the magnitude and phase of the GCI output impedance, but it cannot completely eliminate the “negative impedance” behavior of the GCI. It means that there is still a risk of instability in the system. Therefore, this paper aims to propose a more general stability analysis method, revealing the instability conditions under negative impedance and providing a new insight for stability analysis and control strategy design. First, this paper considers the broadband oscillation characteristics exhibited during GCI instability and proposes a new stability analysis method. This method indicates that the system instability is caused by the interaction between fundamental voltage and non-fundamental voltage. This interaction can exacerbate the oscillation of system voltage, ultimately leading to instability issues. This method provides a clear physical explanation for the mechanism of system instability. Secondly, a new impedance-based stability discrimination method is obtained, determining the conditions for GCI instability. This method can provide a clearer explanation of the present instability phenomena. Then, a compensation method based on q-axis voltage is proposed. This method achieves complete decoupling between the GCI impedance and voltage, effectively eliminating the instability region. Compared with existing methods, the method proposed can better ensure the stability of GCI under weak grid and high PLL bandwidth. Finally, the effectiveness of the proposed methods is verified through simulations and experiments.