Dynamic Modeling And Stability Analysis For Islanded Microgrid With Multiple Inverters

Microgrid has been an effective way to integrate distributed energy resources.A large amount of distributed energy resources are interfaced to islanded microgrid by inverters,which makes the electromagnetic transient model of islanded microgrid more complicated.Moreover,inverter-based micro-sources have small inertia and complex nonlinear characteristics so that the stability of islanded microgrid is vulnerable to disturbances.In order to research on the dynamic characteristics of microgrid under islanded operation mode and to analyze the factors that affect the stability of the system from mechanism,it is necessary to establish microgrid dynamic model and analyze its stability.Based on the comparison of several typical models of voltage source inverter,the dynamic model of three phase voltage source inverter is established by adopting dynamic phasor method.The dynamic phasor model is compared with the corresponding electromagnetic transient model by the simulation model in MATLAB/Simulink.And the simulation results show both models have same results.So dynamic phasor method can be applied to depict the stability and dynamic characteristics of three phase voltage source inverter,and also can be used to simulate and analyze islanded microgrid with multiple inverters.Then,the dynamic phasor models of three-phase balanced islanded microgrid with multiple inverters are completed by dynamic phasor method.And based on different time scales,the reduced order dynamic phasor models of the islanded microgrid are derived by singular perturbation theory.The dynamic phasor simulation models of the islanded microgrid are constructed in MATLAB/Simulink.The waveforms of voltage and frequency show these models can accurately describe the dynamic characteristics of the system.The complete dynamic phasor model is compared with the corresponding reduced order model,simulation results verify the correctness of the reduced order model.Numerical simulations show that the larger the droop control coefficient is,the faster the response speed of the system is,but when the coefficient exceeds a threshold,the system will become unstable.In order to analyze the small signal stability of these full order dynamic phasor models,these models are linearized by Lyapunov's first method,and then its stability is judged in terms of eigenvalue criterion.Theeigenlocus plotted for increasing frequency droop control coefficient is presented to obtain the range of P/f droop control parameter keeping the system stable.The reduced order dynamic phasor models can predict the instability of the system.The range that keeps microgrid stable can be applied to design P/f droop controller.