Small Signal Stability Analysis Of Grid-Connected Converter System In Current Control Time Scale

With the progress of power electronics technology,driven by environmental protection,economic development and other factors,power system presents the trend of being dominated by power electronics.Because of flexibility and controllability,Voltage source converter(VSC)is widely used in new energies generation,flexible AC transmission and other scenarios.Actually,VSC has controllers with different response speed,which makes the system’s dynamic behavior have multiple timescale characteristics.In recent years,oscillations in current control(CC)timescale caused by VSC integrating into the weak grid have occurred and threaten the stable operation of power system.However,the three-phase grid-connected converter system is a multivariable system,in existing studies,the generalized Nyquist stability criterion is often used for stability analysis,which increases the complexity of analysis.Therefore,small signal stability analysis method and small signal stability analysis of grid-connected converter system are studied in this paper.The details are as follows:(1)Choosing VSC as the research object,the control structure of VSC is introduced.Basing on the modeling idea of motion equation concept,considering phase locked loop’s(PLL)dynamics,controller parameter asymmetry in dq axis,terminal voltage feedforward and delay,VSC’s amplitude frequency motion equation model in CC timescale is established.The correctness of the model is verified by comparing characteristic roots and time domain simulation waveforms.Finally,a network linearization model is established.Because the VSC and the network are modeled independently,the linear model of the grid connected VSC system can be easily obtained.(2)Based on the theory of equivalent open-loop process(EOP),a single variable equivalent method for multivariable systems is proposed.The complex multivariable system can be equivalent to a single variable system which can retain the relationship between the key variables of the original multivariable system.It is helpful for researchers to use the simple,intuitive,physically significant and mature single variable frequency domain control theory to study the small signal stability of grid-connected converter system.Finally,the reliability of the stability analysis based on the proposed single variable equivalent method is verified by comparing the eigenvalues of the equivalent single variable system and the original multivariable system.(3)The linear model of grid-connected converter system in CC time scale is established,and the equivalent single variable system model is obtained based on the single variable equivalent method.Based on the equivalent single variable system model,the influence of power grid strength,PLL’s bandwidth,current dq-axis controller proportional/integral coefficient,output power and terminal voltage feed-forward filter parameters on the small signal stability is analyzed.Finally,the accuracy of the stability analysis is verified by matlab/simulink time domain simulation.