A Non-SOGI Single-Phase Frequency-Locked Loop Based on Synchronous Frequency Tracking Filter with Enhanced Immunity to Sensor-Induced DC Offsets

In single-phase grid-connected converters, a second-order generalized-integrator-based frequency-locked loop (SOGI-FLL) is widely used to meet synchronization requirements due to its structural simplicity. However, since the quadrature-phase form of the input signal in this method exhibits a low-pass filter characteristic, its immunity to harmonic distortion and sensor-induced DC-offset errors is limited. To address these problems and improve synchronization accuracy under weak grids, this study proposes a non-SOGI frequency-locked loop (FLL) method based on a synchronous frequency tracking filter (SFTF), hereinafter referred to as SFTF-FLL. The most significant advantage of the suggested method compared to the standard SOGI-FLL is that its quadrature component exhibits a bandpass filter characteristic. This characteristic enables the SFTF-FLL to attenuate harmonic components and, more importantly, significantly suppress sensor-induced DC-offset errors in the input signal, thereby improving the accuracy of frequency estimation. To facilitate the parameter design of the suggested method, its small-signal model is derived. Comparative experimental results are provided with SOGI-FLL and complex bandpass filter-based FLL (CBF-FLL) to demonstrate the effectiveness and performance enhancement of the suggested SFTF-FLL. The numerical results demonstrate improved amplitude and phase estimation under harmonically distorted, sensor-induced DC-offset, and measurement noise conditions.