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

Saeed Golestan, Ali Akhavan, Josep M. Guerrero, Abdullah Mohammad Abusorrah, Muhyaddin Jamal Hosin Rawa, Juan C. Vasquez

Research output: Contribution to journalJournal articleResearchpeer-review

6 Citations (Scopus)


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.

Original languageEnglish
JournalIEEE Industrial Electronics Magazine
Issue number3
Pages (from-to)23-35
Number of pages13
Publication statusPublished - 1 Sept 2022


  • Information filters
  • Low-pass filters
  • Phase locked loops
  • Power harmonic filters
  • Pulse width modulation
  • Software
  • Synchronization


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