Improved SOGI-PLL model for grid distortions and THD measurement capability

• A symmetrical SOGI-PLL (SSOGI-PLL) is proposed to improve quadrature signal estimation under distorted grid conditions. • An inner low-pass filter tuned at the system frequency enables symmetrical band-pass filter behavior for enhanced stability. • Accurate phase, frequency, and THD estimation are achieved under DC-offsets and subharmonics, outperforming SOGI-PLL and cascaded SOGI-PLL. • The proposed THD measurement method is validated against SOGI-FLL-THD, with results confirmed by MATLAB/Simulink and Typhoon HIL. With the increasing complexity of modern power systems, synchronization is becoming more and more challenging. Some of the issues that have emerged include DC-offsets, subharmonics, voltage perturbations, and inefficient grid monitoring; yet, current approaches fall short of fully addressing these issues single-handedly. This paper proposes a modification of a second-order generalized integrator phase-locked loop (SOGI-PLL) using a low-pass filter (LPF) tuned at the system frequency to make the quadrature output of the SOGI to function like a band-pass filter (BPF). As a result, the system can operate effectively in the presence of subharmonics and DC-offset. Moreover, the low-pass filter (LPF), tuned to the grid frequency, facilitates accurate measurement of the total harmonic distortion (THD) in the grid voltage. The effectiveness of the proposed technique is evaluated by comparing its stability against SOGI-PLL and cascade SOGI-PLL, and its THD measurements capability against SOGI-frequency-locked loop THD (SOGI-FLL-THD), using MATLAB/Simulink simulations under various grid disturbance conditions. In addition to successful software validation, real-time implementation is conducted using a Typhoon Hardware-in-the-Loop (HIL) platform to ensure practical applicability.