An αβ-Frame Moving Average Filter to Improve the Dynamic Performance of Phase-Locked Loop

Abstract

The moving average filter (MAF) is one of the most widely used methods to block harmonics in a phase-locked loop (PLL). This paper proposes a simple structure for MAF to work directly in the αβ-frame (called αβMAF). Compared with the standard dq-frame MAF (dqMAF) used as an in-loop filter, αβMAF acts as a prefilter and significantly improves the PLL dynamic response. Its derivation process and implementation is described in detail in both continuous and discrete time domains. In an adaptive prefilter-based PLL, a frequency feedback loop (FFL) is indispensable, but it is not easy to design. Therefore, nonadaptive αβMAF is suggested, and a corrector is added to eliminate the phase error at off-nominal frequencies. In this paper, the small signal model of the proposed PLL is carried out through the perspective of frequency shift, which simplifies the derivation process. The effectiveness of the proposed technique is confirmed through simulation and experimental results.

Keywords

• Moving average filter (MAF)
• phase locked loop (PLL)
• prefilter
• grid synchronization