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 $\alpha \beta $ -frame (called $\alpha \beta $ MAF). Compared with the standard dq -frame MAF ( dq MAF) used as an in-loop filter, $\alpha \beta $ MAF 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 $\alpha \beta $ MAF 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.
Highlights
With the growth of renewable energy sources and nonlinear loads, the grid synchronization task becomes more challenging, and the synchronization approach may significantly influence the performance of grid-connected equipment
Even harmonics and DC offset are eliminated by a special αβ delayed signal cancellation (DSC) operator, and odd harmonics are eliminated by a dq-frame MAF (dqMAF)
The window length of dqMAF is shortened to a half of the fundamental period, but a phase delay is still introduced into the control loop
Summary
With the growth of renewable energy sources and nonlinear loads, the grid synchronization task becomes more challenging, and the synchronization approach may significantly influence the performance of grid-connected equipment. In [25], to improve the dynamic response, the dqMAF with a short window length is proposed to eliminate only high-frequency harmonics, and a delay operation period filter (DOPF) is added to eliminate fundamental frequency negative-sequence (FFNS) component. Even harmonics and DC offset are eliminated by a special αβ DSC operator (acts as a prefilter), and odd harmonics are eliminated by a dqMAF (acts as an in-loop filter) In this structure, the window length of dqMAF is shortened to a half of the fundamental period, but a phase delay is still introduced into the control loop.
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