Performance Evaluation of Modified SOGI-Based PLLs with DC Offset Rejection Capability

The appropriate synchronization of grid-connected Renewable Energy Sources (RES) in a three-phase system is a serious challenge these days. Key elements, including the utility voltage's frequency, amplitude and phase are essential for the proper synchronization of grid-connected inverters. These factors are correctly determined in a Synchronous Reference Frame Phase Locked Loop (SRF PLL) under a perfect grid condition. However, its performance gets worse in the presence of unbalanced or distorted grid conditions. As a result, an efficient and accurate Dual Second Order Generalized Integrator PLL (DSOGI PLL) was proposed for grid synchronization but it also failed to precisely estimate the phase when there is a Direct Current (DC) offset present in the grid voltage, which calls for additional modifications. The presence of DC offset in the grid voltage cause fundamental frequency oscillations leading to phase-locked deviation from incorrect extraction of the fundamental voltage component. In order to find a better PLL with improved performances regarding adaptive filtering, DC rejection, and quickest transient response, this work explores enhanced PLLs based on SOGI filter modifications and evaluates their benefits and drawbacks. The effectiveness of these enhanced PLLs is analysed and validated through MATLAB/Simulink simulations under grid conditions affected by a DC offset.