Phase-Locked Loop Applications Under Weak Grid Conditions Assisted with Dual-Layer Filtering

The multi-energy grid integrated with renewables and large-scale nonlinear loads poses new challenges to the grid synchronization. The phase-locked loop is widely used in grid-tied inverters but is rarely used in extraction of grid frequency, phase, and voltage components under weak grid conditions, such as unbalanced grid voltage, DC offset, and harmonic injection. This paper proposes a grid-connected phase-locked loop with dual-layer filtering for accurate extraction of grid synchronization signals under weak grid conditions. The voltage components of the weak grid are first analyzed, and a dual-layer filter is constructed to allow the phase-locked loop to filter out disturbances under various weak grid conditions. The transfer function of the filter is derived, and the frequency response characteristics are analyzed. A mathematical model based on the dual-layer filtering phase-locked loop is built, and the control parameters are designed. An experimental platform is built to verify the performance of the proposed phase-locked loop technology. Data analysis and comparison confirm the feasibility of the proposed technology, achieving phase extraction in non-ideal grid situations such as three-phase asymmetry drop, DC offset, and harmonic injection, with the phase accuracy of the fundamental wave being higher than 99.5%.