Coordinated Design of Extended-State Observer-Based SOGI-PLL and Optimized Voltage Feedforward for Enhanced Disturbance Rejection

Under weak-grid conditions, phase-locked loops (PLLs) are essential for synchronizing grid-connected inverters with the utility grid. However, conventional PLLs suffer from insufficient DC-offset and harmonic rejection capabilities, and the coupling between PLL dynamics and grid-voltage feedforward (GVF) may lead to system instability under weak-grid conditions. To address these issues, this paper proposes an extended-state third-order generalized integrator PLL (ES-TOGI PLL) with an enhanced voltage feedforward path. The ES-TOGI structure integrates a DC-offset estimation module and an inertial compensation loop, which suppresses the DC component from 0.0355% to 0.0135% and attenuates the third harmonic from 0.161% to 0.137% under distorted grid conditions. Meanwhile, the enhanced feedforward path, implemented via all-pass and high-pass filters, improves the system phase margin and ensures stable operation under weak-grid conditions with a short-circuit ratio (SCR) of 2. Experimental results demonstrate that the ES-TOGI PLL reduces the output current of total harmonic distortion (THD) from 7.70% (without PLL) to 5.75%, and the enhanced feedforward further lowers the THD to 4.63%, outperforming conventional SOGI, ESOGI, and AFAPLL structures. These results confirm that the proposed ES-TOGI PLL with enhanced feedforward provides an effective solution for robust grid synchronization under non-ideal grid conditions.