Research On Parallel Inverter Control Strategy And Circulation Suppression In Micro-grid

At present,the conventional droop control strategy has the problems of low power sharing accuracy and power coupling.How to control the coordinated operation of micro-inverters in micro-grid,to realize precise power distribution and circulation control between micro-sources with different capacity and same capacity,to improve power supply quality in micro-grid cluster power supply mode,to make up for droop control defects to improve performance of power system has important theoretical value and practical significance.The parameter differences between the parallel inverters lead to the circulation.Circulation not only increases the power loss of the system,but also makes the output current waveform distorted severely and the power distribution is uneven.Therefore,this article has carried on the thorough research on the control strategy and the basic characteristic of the inverter in the droop system and the power distribution of the parallel operation of the same capacity or different capacity inverters.This paper mainly did the following work:(1)The mathematic model of three-phase full-bridge inverter was established,and the double closed-loop control strategy of voltage and current was designed under the dq rotating coordinate system.The mechanism of inhomogeneity and circulation of inverter parallel power was deduced.The traditional droop control strategy was studied and its defects were analyzed.An improved droop control strategy was proposed for the same capacity inverter instead of the conventional droop control.An improved droop control curve was constructed by introducing AC bus voltage compensation.Combined with the basic principle of improved droop control and the stable working conditions of the system,the parameter selection range that took into account the compensation effect and the characteristics of the output of the inverter was analyzed.The validity of the proposed inverter control strategy was verified by simulation.(2)Since the previous research was based on the consistent impedance of parallel inverters,it was difficult to achieve the same impedance of inverter in practical applications.Generally,the method of adding a line inductance was adopted to ensure the consistency of the connection impedance,resulting in the increase of the size and cost of the inverter Therefore,in this paper,a virtual impedance was based on the composite filter.It had harmonic suppression and avoided the noise introduced by current differential.When Flexible features of the virtual impedance control was token in,an adaptive virtual impedance strategy was presented.The virtual impedance value of the inverter was adjusted with the output power to reduce the difference of the impedance of the inverter connection and improve the steady performance of the system.The simulation results verify the correctness of the proposed control strategy.(3)The influence mechanism of equivalent output impedance on power distribution and the limitation of traditional droop control method were studied.A multi-loop feedback controller with voltage-current loop was designed.By constructing an integrator and redesigning the equivalent output impedance,reactive power control and equivalent output impedance of the relationship between the checks and balances was eliminated,so that the improved droop controller had strong robustness in the application of parallel system with different capacity micro sources.It improved the accuracy distribution of load power and effectively reduced the system circulation.The validity and correctness of the parallel control strategy of different capacity inverters proposed were verified by simulation.