Frequency-locked Loop With Kalman-based OSG Immune To The DC-Offset
A frequency-locked loop synchronization with an inherent orthogonal signal generator is one of the key enablers of reliable synchronization for grid-connected power converters. An accurate and robust grid synchronization is essential for stable control and power regulation in those applications. Conventional structures, commonly based on second-order generalized integrators, may operate irregularly in the presence of measurement noise, harmonic distortion, frequency changes and a DC-offset component. To address this issue, a modified Kalman-based generator with the DC-offset immunity is integrated into the conventional frequency-locked loop. The proposed approach utilizes a state-space oscillator model equipped with an additional state for DC-offset component estimation, enabling generation of orthogonal signal with DC-offset rejection. The effectiveness of the proposed loop is verified in the MATLAB/Simulink environment under numerous tests such as DC-offset introduction, sudden amplitude and frequency changes. The shown results demonstrate accurate grid voltage estimation and ripple-free frequency estimation with providing robustness and accuracy against conventional structures.