Stability Analysis Of Grid-connectedinverters Based On Grid Impedance Identification

Under the increasing of distributed renewable energy generation and the development trends of smart grid, the stability of power system is getting more likely to be influenced as the composition and dynamic characteristics of it are getting more complex. Most renewable energy generation get connected with grid through inverters, and there are varying difference between them, which causing various compatibility issues. Among them the resonance caused by impedance mismatch is particularly critical, and in severe will cause paralysis of the entire power network. For a separate inverter in the system, the impact on its stability from the overall impedance of other inverters can be reflected by grid impedance. Therefore, based on grid impedance identification, the stability of grid-connected inverters will be analyzed in this paper, which is significant to ensure the stability of the grid system.At first, the implementation scheme of online grid impedance identification has been studied, in which the basic principle of the impulse response method has been analyzed. In addition, the impulse injecting and response analysis have been detailed designed, and the characteristics about short identification cycle and wide band have been verified. The scheme has been verified in simulation, furthermore the response waveform and the influencing factors on the identification results have been analyzed.The small signal model of phase locked loop and current control in inverter control system have been established using harmonic linearization method. Combining the average model of inverter main circuit, the inverter output impedance model has been obtained, moreover it has been verified in simulation with the method of frequency sweep. In addition, the influencing factors on the inverter output impedance have been defined for the subsequent stability analysis.Using grid impedance and the inverter output impedance model, the impact of grid impedance on the stability of grid-connected inverters has been analyzed through impedance based method. According to the stability analysis results and the influencing factors on the inverter output impedance, the method to solve the resonance problem is determined. The theory has been connected with the time domain response of inverter system through simulation, which verified the adopted analysis method. Finally, the experimental platform for grid-connected inverters stability analysis has been established, and the experiments of online grid impedance identification and stability analysis have been carried out to verify every study in this paper.