Research On Stability Of Single-Phase Grid-Connected VSI System Considering Frequency Coupling Effect

The single-phase voltage source inverter(VSI)system has periodic and nonlinear characteristics,and often exhibits frequency coupling effect(FCE),especially in the weak grid.The frequency coupling effect not only affects the control performance of the system,but also aggravates the stability of the system,making the grid-connected system more prone to instability.In order to deeply understand the cause of the frequency coupling effect,and to eliminate or suppress the adverse effects of the FCE on the system,this paper studies the single-phase gridconnected VSI system from the aspects of impedance modeling considering FCE,stability analysis,and frequency coupling suppression,and following work has been done.1.The multiple-input multiple-output(MIMO)and single-input single-output(SISO)impedance models of single-phase grid-connected inverter system considering FCE are established,and the accuracy of the model is verified by impedance measurement.The impedance models cover the necessary sub-modules such as synchronous reference frame phase locked loop(SRF-PLL),current loop,delay link,and main circuit.Based on the small signal model and the frequency domain dynamic propagation diagram of SRF-PLL,the main cause of FCE is clarified.By comparing the impedance models with or without FCE,the necessity of accurate impedance modeling considering FCE in the weak grid is clarified.Finally,the frequency scanning method is used to verify the accuracy of the obtained impedance models.2.The stability of the single-phase grid-connected system under the synchronous frame DQ controller is analyzed,and the accurate stability boundary is determined.Based on the MIMO and SISO impedance models,the Nyquist stability criterion is used to analyze the stability of the system under different operating scenarios and different control loop parameters.In addition,the differences in the stability analysis results when considering FCE or not are compared,and the comparative analysis shows that considering FCE is crucial for correct stability prediction in the weak grid.Finally,the analysis results of stability are verified by experiments.3.The influence of frequency coupling effect on grid-connected system stability is revealed,and a frequency coupling suppression strategy based on band-pass filter is proposed.The suppression strategy does not rely on the phase-locked loop with special functions,and can effectively suppress FCE only based on ordinary SRF-PLL,which greatly weakens the influence of FCE on the system,and at the same time significantly broadens the stability range of the system.Therefore,when modeling the exact impedance of the system,FCE does not need to be considered,which significantly reduces the difficulty of system stability analysis and controller design.At the same time,the experimental results demonstrate the effectiveness of this strategy in suppressing FCE and broadening the system stability range.In addition,the strategy hardly changes the dynamic characteristics of the original control scheme,and at the same time has a good suppression effect on the harmonics of the grid and the sudden change of the grid phase.