Dynamic Modeling Method And Stability Analysis Of AC Microgrid

In recent years,human have begun to develop and utilize renewable energy in a large scale to solve global issues such as energy shortages and environmental pollution.Microgrid is an effective solution to effectively use renewable energy and reduce the power transmission loss.The inverter-interfaced distributed generators(DGs)has obvious differences with the synchronous generator in power system in terms of operation mechanism,control dynamics,over-current capability and response speed.The microgrids have the following characteristics in the operation mode and dynamic characteristics: the dynamic characteristics are dominated by the control behavior of the grid-connected inverters;the limited capacity,the small inertia,and the large fluctuation range of the frequency and voltage;the coupling among the DGs in wide time-scale.Therefore,the microgrid presents complex dynamic behaviors under disturbances such as load fluctuations and the changes of renewable energy output.It is necessary to study the mathematical modelling and dynamic behaviors of microgrid,and analyze the dynamic characteristics and system stability of the microgrid.In this paper,the mathematical modeling,model reduction and stability analysis of inverter-based microgrid are studied.Based on the developed model,the dynamic stability of microgrid under common conditions such as the connection of multi-type loads and three-phase unbalance are studied.The innovation work of this paper are as follows:(1)The modeling method of microgrid with multi-type loads is studied,after obtaining the accurate mathematical expression of each component in microgrid,the subsystems are combined to obtain the complete model.Based on the developed model,the numerical bifurcation analysis is carried out to study the parameter region of the microgrid stable operation.The effects of main control parameters and loads on the stability region of the microgrid are investigated.The proposed microgrid model is the basis of this thesis.(2)A time-scale model reduction of microgrid based on singular perturbation theory is proposed.Under the premise of accurately describing the dominant dynamic behavior of microgrid,the order of mathematical model is sufficiently reduced.It solves the problem that the microgrid model has high order and complex analysis when performing system level stability analysis.(3)The special scale model reduction of multi-microgrid system based on dynamic equivalent theory is proposed.When studying the dynamic behavior of microgrid in study area,the external area can be replaced by dynamic equivalent model.The developed reduced model can accurately describe the dynamic behavior in the study area,but the model of the microgrids on the eternal area are greatly simplified,the complexity of the system-level analysis is further reduced.It solves the problem that the numerical calculation takes a long time when analyzing the transient characteristics of a large-scale multi-microgrid system.(4)A mathematical modeling method for unbalanced microgrid based on dynamic phasor theory is proposed,which accurately describes the transient response characteristics of unbalanced microgrid.Compared to the detailed switching model,the time-domain simulation time is greatly reduced.The traditional mathematical model of microgrid cannot solve the problem of describing structural asymmetry and obtaining equilibrium points for stability analysis.Based on the developed dynamic phasor model,the small-signal analysis of the microgrid with unbalance compensation control is carried out,and the oscillation problem that may occur when the grid-connected inverters are equipped with typical unbalance compensation method is found.An improved unbalance compensation strategy is designed to improve the compensation effect with enough stable margin.(5)Based on the research results of mathematical modeling and stability analysis,the parameter optimization design of microgrid control system is carried out for an island 10 kV microgrid project.The frequency and voltage response performance of the microgrid is optimized for different operating conditions,ensuring the safe and stable operation of the microgrid under disturbance.Based on the RT-Lab hardware-in-the-loop simulation platform,the microgrid project was built to verify the effectiveness of the parameter design.Based on the safe and stable operation of microgrid,this paper proposes a dynamic modeling and model reduction method for microgrid with multi-type loads and under threephase asymmetry.The dynamic response characteristics and system stability of the microgrid under various operating conditions are studied,and the influence of main control parameters on system stability is obtained.The mathematical model studied in this paper is the basis of the system-level design of the microgrids.The analysis conclusions obtained in this paper can provide reference for the overall parameter selection of the microgrid.At the same time,the modeling method of this paper can be extended to distribution system,renewable energy generation and independent power systems such as ships and aircraft,which promotes the development of power electronic power system.