Stability Research Of Grid-connected Inverters Based On The Mesured Impedance Under Weak Grid Condition

Grid-connected inverters usually adopt phase-locked loop(PLL)to realize the synchronous operation with the grid.However,the grid impedance will enhance the interaction between PLL and the system current control loop in the weak grid condition,which may cause the instability of the system.Therefore,researching and analyzing the stability of the “invertergrid” interactive system,exploring its instability mechanism and proposing the methods for improving system stability have important engineering application significance.This article takes the three-phase grid-connected inverter as an example,and conducts research from the following aspects:Firstly,the d-q frame impedance model of the grid-connected inverter system and the grid are established.Based on the impedance stability analysis method,the established multi-input and multi-output model is reasonably simplified into two single-input single-output models by using the bode diagram analysis and the generalized Nyquist stability criterion.On this basis,the influence of PLL parameters and grid impedance on the system stability are analyzed,and the mechanism of system instability under weak grid condition is also researched.Secondly,in order to further verify the accuracy of the above model,the impedance measurement of the grid-connected inverter system is carried out based on the frequency sweep method.Taking passive components and grid-connected inverters as objects,the impedance measurement technology based on single-frequency injection and multi-frequency injection are compared and analyzed.Based on the discrete Chirp signal disturbance voltage source injection,the grid-connected inverter system is measured accurately,based on which the influence of injection voltage amplitude,injection signal duration,PLL parameters and grid impedance on the measurement results are explored.Thirdly,based on the established model,a widely applicable PLL parameters optimization design method is proposed.The relationship between the parameters of the PLL are established,and based on the open-loop transfer function of the system,the parameters constraint region is obtained by introducing the system phase margin and harmonic rejection conditions.Finally,the stability impact analysis of the interactive system determines the selection of the parameters.And three commonly used PLL(SRF-PLL,LSRF-PLL and DSOGI-PLL)design cases are tested to verify the effectiveness of the proposed method based on the hardware-in-the-loop(HIL)test system and the experiment platform..Finally considering the situation that parameter optimization design under weak grid condition is difficult to meet all the requirements of the stable operation of grid-connected inverters,a control method based on the system q-axis grid voltage feedforward control is proposed to improve the system stability.The proposed control strategy,with a detailed design of the feedforward controller,effectively increases the phase angle of the inverter’s q-axis input impedance in the low frequency band,which improves the stability of the system as the phase margin of the interactive system has been sufficiently increased under weak grid condition.Finally,the effectiveness of the proposed improved feedforward control method is verified based on simulations and experimental results.