A Hybrid Pre-loop/Post-loop Filtering Strategy Based Type-1 PLL for Synchronization Under Distorted Grid Conditions

Phase-locked loops (PLLs) are popular for the synchronization and control of grid connected equipment and systems. The type-1 PLL is the simplest PLL, which is characterized by having only one integrator in its control loop. Filters can be added, either inside the PLL's control loop as the in-loop filters or before the PLL's control loop as pre-loop filters, to improve the disturbance rejection capability of the type-1 PLL. However, the in-loop/pre-loop filtering strategy based type-1 PLL has a tradeoff between settling time and harmonic attenuation. To improve the performance of the type-1 PLLs, a novel filtering strategy of adding filters after the PLL's control loop as the post-loop filters is suggested, and a hybrid pre-loop/post-loop filtering strategy based type-1 PLL (HF-T1-PLL) is designed. Theoretical analysis and experimental results validate that the HF-T1-PLL can track the grid frequency and phase accurately in about 1.5 cycles under severe distorted grid conditions.