Abstract
The phase-locked loop of grid-connected inverters affects their output impedance characteristics and stability. Based on the NPC, three-level LCL grid-connected inverter, this article first analyzes the frequency characteristics of the improved moving average filter phase-locked loop (IMAF-PLL). Then, analyzing the influence of the neutral point control on the inverter impedance modeling, the harmonic linearization impedance modeling method combined with the symmetrical component method is used to establish a positive and negative sequence impedance model of the NPC three-level LCL grid-connected inverter. Furthermore, the frequency characteristics of the IMAF-PLL and the impedance model of the NPC three-level LCL grid-connected inverter are verified by simulations. Finally, the impedance-based analysis method is used to study the stability of the grid-connected system under different grid impedance conditions when using different phase-locked loops, and the theoretical analysis was validated experimentally.
Highlights
With the widespread application of distributed power generation technology, an increasing amount of renewable energy sources are connected to massive power grids through power electronic devices [1]
When short circuit ratio (SCR)=2.6, the positive sequence phase margin of the grid-connected system using the synchronous reference frame phase-locked loop (SRF-PLL) is insufficient, and the grid-connected current may oscillate, while the grid-connected system using the improved moving average filter (IMAF)-PLL has a sufficient phase margin and the grid-connected system is stable. These results show that under weak grid conditions, the IMAF-PLL is more helpful to the stable operation of grid-connected systems than SRF-PLL
In this paper, the harmonic linearization method is used to analyze the frequency characteristics of the IMAF-PLL and establish the impedance model of the NPC three-level LCL grid-connected inverter, and the stability of the gridconnected system was studied
Summary
With the widespread application of distributed power generation technology, an increasing amount of renewable energy sources are connected to massive power grids through power electronic devices [1]. Some references have studied the influence of the frequency characteristics of the phase-locked loop on the stability of the grid-connected inverter [21,22,23,24,25,26,27,28,29]. Little consideration has been given to the improved moving average filter phase-locked loop (IMAF-PLL) with better dynamic response and robustness under a distorted power grid and its interaction stability with three-level grid-connected inverters. The remainder of this paper is organized as follows: The frequency characteristics of the improved moving average filter phase-locked loop (IMAF-PLL) are analyzed in Section II as a foundation for the subsequent analysis. The SRF-PLL has the disadvantage of a slow dynamic response Aiming at this shortcoming of the MAF-PLL, an improved moving average filter phase-locked loop (IMAF-PLL) is obtained by introducing a correction link. The transfer function of the improved moving average filter (IMAF) is GIMAF (s) GMAF (s)Gc (s)
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