Research On Parallel Operation Control Of Inverters In Low Voltage Microgrid Based On Improved Droop Control

The parallel inverter operation in the micro grid is an important way to expand the system capacity.In actual work,the difference in the electrical parameters of the inverters causes a circulation phenomenon between the inverters,which is detrimental to the normal operation of the system.In severe cases,it will cause damage to the inverter.As one of the main control strategies of distributed generation in micro grid,droop control can also effectively control the distributed generation of micro grid without communication.However,the micro grid system is mainly located in the middle and low voltage side of the distribution network,and the impedance of the middle and low voltage lines is generally resistive.At this time,the traditional droop control strategy can not effectively suppress the power circulation of the system,and the power distribution accuracy of the parallel operation system of inverters is greatly reduced,and the stable operation of the system can not be guaranteed.In view of the problems of poor power distribution effect and large frequency fluctuations in the parallel operation system of inverters in the low-voltage micro grid when disturbed,this thesis takes the parallel operation of two inverters as the research object,and aims to improve the power distribution accuracy of the system and suppress the frequency fluctuation of the system.The parallel operation system of the inverters is analyzed and researched.The main research work and achievements of this thesis are summarized as follows:1.The operating characteristics of parallel inverter system in micro grid and the relationship between circulating current and steady-state voltage of parallel operation system of n inverters in S-domain are studied.The mathematical model of parallel operation system of n inverters in S-domain is established.It is proved that the parallel operation system of n inverters is a multi-input multi-output system.In this thesis,the mathematical expression of the circulating current of the parallel operation system with n inverters of different grades in S-domain is proposed.It is proved that the output voltage of the inverter in the system is directly related to the circulating current of the system,and the load and line impedance are also related to the circulating current of the system.2.The power distribution problem of inverter parallel operation system under low voltage micro grid is studied.A parallel operation system of two inverters is built in the Matlab/Simulink simulation platform,and the traditional droop control strategy and the droop control strategy with virtual impedance are used to control it.The results show that the inverter parallel operation system adopting the traditional droop control strategy has poor power distribution accuracy when the load/inverter is switched on and off;the inverter parallel operation system adopting the virtual impedance control strategy can operate stably when the load is switched on and off,and when the inverter is switched on and off,the system does not have good power distribution capabilities.3.Aiming at the deficiencies of the traditional droop control strategy and the droop control strategy with virtual impedance,an improved droop control strategy is proposed: based on the introduction of virtual negative resistance,the impedance angle of the line impedance is introduced into the droop control as a variable.Small-signal modeling and load/inverter switching simulation modeling are performed on the improved droop control.The results show that the application of this strategy can not only maintain the stable operation of the system,improve the power distribution accuracy,but also effectively smooth the frequency fluctuations.