In-Loop Filters and Prefilters in Phase-Locked Loop Systems: Equivalent or Different Solutions?

Generally speaking, a phase-locked loop (PLL) is a feedback circuit/algorithm that constantly adjusts its output to match in phase and frequency with a reference signal. In power/energy applications, the basis for developing the vast majority of available PLLs is the synchronous reference frame (SRF)-PLL, which is known to have a limited ability in filtering grid disturbances, particularly harmonics, interharmonics, and grid-voltage imbalance. Therefore, a common trend in designing more advanced PLLs is including additional filters within SRF-PLL control loops or before its input. Currently, the literature is full of contradictory remarks about the advantages/disadvantages of these filtering solutions, which are often collectively referred to as in-loop filters and prefilters. This article aims to take a deeper look at these filtering solutions and contribute to dispelling misconceptions about them.