PLL with MAF-Based Prefiltering Stage: Small-Signal Modeling and Performance Enhancement

In three-phase applications, the synchronousreference frame phase-locked loop (SRF-PLL) is a standard PLL, which benefits from a simple structure and satisfactory performance under symmetrical and undistorted grid conditions. Under unbalanced and harmonically distorted conditions, however, it suffers from a very poor performance in the detection of grid voltage parameters. To deal with this challenge, incorporating different filters inside its control loop or before its input has been proposed. Recently, using the moving average filter (MAF) as the SRF-PLL prefiltering stage has been suggested in several works. The MAF is a linear-phase filter that can behave like an ideal lowpass filter under certain conditions. The main aim of this letter is to derive the small-signal model of the SRF-PLL with MAF-based prefiltering stage (briefly called the PMAF-PLL), which has not been presented before. This model enables the designer to simply analyze the stability condition and dynamic behavior of the PMAF-PLL. After developing the model, a simple modification to enhance the PMAF-PLL performance under frequency varying environments is presented. Finally, the equivalence of PMAFPLL
and the space-vector Fourier Transform based PLL (SVFTPLL), which is a well-known PLL in three-phase applications, is proved. This equivalence implies that the small-signal model of the PMAF-PLL and the method presented to enhance its performance are valid for the SVFT-PLL.