Back‐electromotive‐force observer (BEMF observer) based symmetrical PLL for grid synchronization stability enhancement under weak grid conditions

Abstract

The synchronous reference frame phase-locked loop (SRF-PLL) is widely utilized for grid synchronization in voltage-source converter (VSC) system. However, it fails to eliminate the effect of high grid impedance leading to instability in weak grids. To address this issue, this paper introduces the back-electromotive-force observer (BEMF observer), commonly being utilized in DC motor, to modify the input of the SRF-PLL. To be specific, the equivalent internal electromotive force calculated by the BEMF observer rather than the PCC voltage is used to capture the phase information. Since the BEMF observer is designed in α β $\alpha \beta$ -frame while the SRF-PLL is in d q $dq$ -frame, the conventional stability analysis method suffers from complicated process. Thus, this paper further reconstructs the SRF-PLL with the BEMF observer to be symmetrical for simplification. Apart from the symmetrical structure, the stability criterion is accordingly modified. As a result, the proposed back electromotive force based symmetrical PLL (BEFS-PLL) offers the following advantages: (i) it effectively enhances the grid synchronization stability and simplifies the stability analysis process simultaneously, (ii) it exhibits a strong robustness even with the calculation error up to 50%, and (iii) it is characteristic of clear physical meanings. Finally, the effectiveness of the BEFS-PLL is verified by simulation.