A Phase-Locked Loop Design Method Based on Impedance Remodeling of Grid-Connected Inverter under High Permeability

Under high permeability, the phase-locked loop (PLL), grid impedance, and grid-connected inverter are coupled, seriously impacting the stable operation of the grid-connected inverter system based on synchronous control of PLL. Firstly, for the PLL control structure, grid impedance leads to the introduction of additional feedback loops in the PLL structure. When the grid impedance changes in a wide range, the PLL system will generate the right half plane pole, decreasing the stability of the PLL or even instability. Secondly, for the grid-connected inverter system, when PLL is considered, the additional negative impedance will be introduced into the output impedance of the system, which directly affects the amplitude and phase of the output impedance of the system, decreasing the stability margin of the grid-connected inverter system. Therefore, this paper takes the single-phase LCL filter grid-connected inverter under the weak current network as the research object and proposes a new PLL control structure with the dot connection current and dot connection voltage as input signals. The new PLL control structure uses the grid impedance measurement value and differential link to dynamically adjust the grid-connected current branch in the new PLL, limiting the grid impedance component in the PLL input signal. Meanwhile, the advance corrector is used to reshape the phase characteristics of the system output impedance. The influence of additional negative impedance on system stability was reduced. Then, the output phase Angle of the new PLL was corrected using an all-pass filter to ensure that the fundamental frequency phase was consistent before and after impedance remodeling. The robustness analysis of the grid-connected inverter system based on the new PLL control shows that the new PLL structure has high robustness and can reshape the output impedance of the inverter system to improve the phase angle of the output impedance at the intercept frequency. When the system is in a weak grid, the phase margin is greater than 30o, and the system robustness is guaranteed. In addition, the simulation and experimental verification show that the quality of the grid-connected current of the system is improved when the new PLL control is adopted. The voltage and current of the grid connection point do not have phase deviation, which meets the requirements of the grid connection point unit power factor. Moreover, the system has good dynamic performance, and effectively expands the adaptive range of the grid-connected inverter electrical system to the grid impedance under high permeability. In the next step, the grid voltage feedforward function, which can eliminate the influence of PLL, will be obtained by an equivalent transformation of the control block diagram of the grid-connected inverter. The phase characteristics of the output impedance of the inverter will be reshaped using grid voltage feedforward control to improve the stability of the grid-connected inverter system under high permeability. © 2024 China Machine Press. All rights reserved.