Multiscale Modelling And Passivity Enhancement Regulation Methods For Islanded Microgrid

With the growing need to smoothly integrate renewable energy sources into the grid and to green the energy structure,distributed generation and microgrid（MG）technologies are being developed extensively.Due to its flexibility,precision and fast response in electrical energy conversion,power electronic converters are widely used in the source-gridload of MG systems and profoundly influence the dynamic characteristics of the system.Compared to the traditional main grid,MGs are smaller in capacity and less resistant to disturbances,and the multi-timescale dynamics of the system are interactively coupled under a nonlinear structure,leading to a more complex system instability mechanism.To investigate the dynamic characteristics and instability mechanisms of MG systems on multiple time scales and optimize system stability,this paper focuses on islanded AC MGs dominated by voltage source inverters（VSI）,with the following main research elements.（1）For islanded AC MGs dominated by VSIs,a detailed small-signal state-space model of the system including the inverter,network and load is first developed.Based on this model,a small-signal eigenvalue analysis was carried out,and a participation factor analysis tool was also utilized to identify the relevant dynamics that have a strong impact on the dominant mode of the system.To simplify the system modelling process and improve the efficiency of the model calculation,a coupled reduced-order model of VSI is also derived,which effectively preserves the interaction information between the innerloop control and the low-frequency modes dominated by the droop control.Finally,the accuracy of the reduced-order model is verified by comparing it with the detailed model and the traditional reduced-order model through eigenvalue distribution and time-domain simulation analysis.（2）For islanded AC MGs with VSIs operating in parallel,the large-signal nonlinear stability of the system is investigated based on Lyapunov stability theory.Considering the existence of intermittent characteristics of renewable energy generators in AC MGs and transient disturbance problems such as load throwing,the dynamic stability margin of the system is studied from both the DC-side and load-side of the inverter.To estimate the stability domain of the system,a fuzzy algorithm based on Takagi-sugeno multimodeling（TSM）is introduced to quantify the effect of system parameters and load conditions on the stability domain.The validity of the stability domain estimation is also verified in time-domain simulations.（3）For the VSI,one of the key devices of the islanded AC MG system,a damping injection-based passivity enhancement regulation strategy is designed and the related control parameters design method is also presented.A well-designed passivity-based controller can ensure the convergence stability of the system under various load conditions or certain LC filter parameter drift.In addition,each control loop has good dynamic performance and relatively independent control bandwidth.The robustness analysis and case simulation have verified the effectiveness of the regulation strategy and parameter design.