Stability Analysis Of Grid-Connected DC Microgrid Based On Lyapunov Stability Theory

The grid-connected DC microgrid under the background of high penetrations of renewable energy sources and high penetrations of inverter-interfaced devices presents the characteristics of strong nonlinearity,low inertia and poor antidisturbance ability,which leads to the problems of large-scale voltage fluctuation and instability when the microgrid is disturbed,which seriously affects its safe and stable operation.Therefore,in this paper,the small-signal and large-signal stability analysis based on Lyapunov stability theory is carried out for the grid-connected DC microgrid system.The main research work is as follows:(1)The grid-connected DC microgrid system model including photovoltaic power generation unit,energy storage unit,grid-connected VSC,DC-DC converter and load is built.The main circuit and control circuit equations of the system are established.On this basis,the state space model of the system is constructed.The Jacobian matrix based on Lyapunov indirect method is derived,and the influence of system parameter variation,constant impedance and constant power load fluctuation on the small-signal stability of grid-connected DC microgrid system is analyzed according to the root locus curve of Jacobian matrix.The accuracy and limitation of the above small-signal stability analysis method are verified by electromagnetic transient simulation,which lays a foundation for the subsequent large-signal stability analysis method.(2)Firstly,for the grid-connected DC microgrid system,a large-signal stability criterion based on the mixed potential function theory under the ideal reduced-order equivalent model is proposed.By comparing the error region between the stability criterion of the mixed potential function theory and the stability boundary obtained by simulation,the conservative characteristics of the mixed potential function theory are revealed.Then,the internal mechanism of conservatism is analyzed from three aspects:the basic principle of mixed potential function method,the construction of equivalent model and the derivation process of stability criterion.The rationality and correctness of the above analysis are verified by simulation,which paves the way for the stability criterion of low conservative mixed potential function.(3)Aiming at the problem of strong conservatism of large-signal stability criterion of mixed potential function theory under ideal low order equivalent modeling method,the large-signal stability criterion of high-order mixed potential function method based on detailed modeling of distributed power interface converter is proposed.According to the low conservative stability criterion,the influence of VSC controller parameters,main circuit parameters,control parameters and parasitic parameters of DC-DC converter,and line fault on the large-signal stability of grid-connected DC microgrid system is judged,and the large-signal stability region of the system under different parameters is divided.The validity and accuracy of the proposed method are verified by comparing the stability criterion of the mixed potential function theory under the high-order equivalent modeling,the stability criterion of the mixed potential function theory under the low-order equivalent modeling and the stability region obtained by simulation.Finally,the correctness of the above analysis is verified by electromagnetic transient simulation.