An accurate and robust adaptive notch filter-based phase-locked loop
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ORIGINAL ARTICLE
An accurate and robust adaptive notch filter‑based phase‑locked loop Jinbo Li1 · Qin Wang1 · Lan Xiao1 · Zehao Liu1 · Qunfang Wu1 Received: 20 February 2020 / Revised: 22 July 2020 / Accepted: 28 July 2020 © The Korean Institute of Power Electronics 2020
Abstract The use of an adaptive notch filter (ANF) is one of the most popular methods to block harmonics in a phase-locked loop (PLL). Existing ANFs suffer from drawbacks such as a low harmonic rejection capability, complex structure or instability. To improve the performance of ANF-based PLLs, an enhanced ANF (EANF) is proposed in this paper. Its structure, filtering characteristics and frequency-adaptive method are analyzed. Multiple EANFs can be easily cascaded and achieve adaptation through a common frequency feedback loop (FFL). A small signal model is provided, and its control parameters are tuned through the symmetrical optimum method. The harmonic rejection capability and stability of the proposed method are confirmed through simulation and experimental results. Keywords Adaptive notch filter (ANF) · Phase-locked loop (PLL) · Synchronization · Phase detection · Frequency estimation
1 Introduction The phase and frequency of grid voltage are crucial pieces of information for many grid-connected devices. With the growth of renewable energy sources, the grid synchronization task has become more and more challenging [1–4]. Many techniques have been applied to deal with this challenge and fruitful results have been produced. Among the existing synchronization techniques, the PLL may be the most popular one, since it has a simple structure and is easy to control [5, 6]. A PLL usually includes three parts: a phase detector (PD), a loop filter (LP) and a voltage-controlled oscillator (VCO) [1, 7–9]. Generally, they are sensitive to the voltage amplitude variations. Thus, an amplitude normalization scheme (ANS) can be used to improve performance [5, 10, 11]. When the grid encounters an accident, a traditional PLL may suffer from a large frequency oscillatory. In [12] and [13], EPLLs are suggested to alleviate such transient frequency oscillations. When the grid voltage is ideal, a PLL can easily obtain accurate values for the phase and frequency. In a case with harmonic components or an unbalanced voltage, * Jinbo Li [email protected] 1
College of Automation Engineering, Nanjing University of Aeronautics, Astronautics, Nanjing, China
additional filters are indispensable for the PLL. The function of these additional filters is to block the voltage disturbance. If this is not done, the PLL can suffer from output errors or even instability [14, 15]. In PLLs, widely used filters include the second-order generalized integrator (SOGI) in [16], the delayed signal cancellation (DSC) operator in [5], the moving average filter (MAF) in [17], the notch filter (NF) in [18–20], etc. The SOGI exhibits a strong filtering capability for high-order harmonics, but a limited filtering capability for low-order harmonics [21, 22]. To enhance filtering capabi
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