Adaptive control method for enhancing the stability of grid-connected inverters under very weak grid condition

摘要

In grid-connected inverter applications, the phase-locked loop (PLL) and the feedforward of the grid voltage at the point of common coupling (PCC) were commonly used to ensure a high-quality grid current with unity power factor and low grid current distortion. However, when the inverter is linked to the PCC with a large grid impedance (specifically, with a low short-circuit-ratio (SCR) less than 5), the inverter performance and the stability were seriously endangered. Through theoretical analysis, it has been found that the cause of such phenomenon was that the inverter output impedance was improperly shaped because the grid impedance voltage was involved in the input single of PLL. Therefore, this study proposes an adaptive control method for the PLL. Based on online grid impedance estimation, the signal used as the input of the PLL is carefully modified to solve the instability caused by the large grid impedance. The principle of the proposed method is briefly explained, and selected waveforms are provided to demonstrate the effectiveness of the proposal.