Research On Power Sharing And Circulation Suppression Of Isolated Island Microgrid Based On Virtual Impedance

The emergence of microgrid provides a new way for the utilization of new energy,through the integration of distributed power sources,microgrid system can absorb new energy locally.As a control method of microgrid system,droop control has been widely concerned and studied.However,the traditional droop control operation of microgrid is limited by many factors such as lines and loads.In recent years,the development of virtual impedance provides a new method to solve the shortage of traditional droop control,by adding impedance,the power sharing and circulating current suppression of microgrid droop control can be better realized,but the introduction of virtual impedance will aggravate the voltage sag in the traditional droop control,and then affect the power supply quality of microgrid.In this paper,the parallel inverter system of isolated island microgrid is taken as the research object,and the above problems in traditional droop control and virtual impedance are studied.Firstly,the mathematical modeling model of three-phase voltage source inverter is analyzed,and the droop controller parameters are designed and selected based on the mathematical model and Routh criterion.At the same time,the circulation and power characteristics of the traditional droop control inverter parallel system are analyzed,and the reasons why load,impedance and voltage effect of reactive power sharing and reactive circulation are found out.The correctness of theoretical analysis is verified by simulation.For isolated island microgrid system with local load,the local load and line impedance are classified as the same equivalent impedance by normalization method.According to the normalized,the equivalent virtual power feedback adjustment is added to the equivalent virtual impedance,which realizes the dynamic adjustment of the equivalent virtual impedance,completes the power decoupling control,improves the reactive power sharing accuracy,and suppresses the circulating current.The influence of virtual inductance on the system is analyzed through microgrid operating conditions combined with small signal model,and the parameters are selected reasonably.In addition,in traditional droop control,the output voltage and frequency of inverter will drop with the increase of load,and the terminal voltage drop will be further aggravated after introducing virtual impedance.Therefore,the difference between reference frequency and output frequency is added to the active frequency droop control equation,and current deviation compensation and equivalent output impedance voltage drop compensation are added in reactive voltage link to improve the stability of frequency and voltage.Build a simulation model of microgrid parallel system in MATLAB/Simulink,and the effectiveness of this method is verified by comparing with the results of traditional droop control.Finally,an experimental system of three-phase inverter parallel hardware is built based on d SPACE platform,the improved droop control strategy of equivalent virtual impedance proposed in this paper is verified by experiments,the experimental results are compared with the simulation results to further verify the effectiveness of the improved method proposed in this paper.