Research On Circulation Suppression Method Of Multiple Parallel Inverters In Microgrid

Microgrid is an important part of the new type of power supply system which utilizes new energy sources as the main body in China.A large number of inverters are connected in parallel to the microgrid in order to expand the capacity of inverters and thus a parallel inverter system is formed.However,circulating current is inevitably generated in the actual operation of the parallel inverter system,and is harmful to the stability and safety of the system.In this thesis,the formation causes of circulating current are analyzed.The strategies are proposed to suppress fundamental wave and harmonics which are the two components of the circulating current in the parallel inverter system and are verified by simulation.Firstly,the control strategy of a single inverter is introduced briefly as well as the structure of inverter parallel system.The formation causes of circulating current are analyzed by means of the analysis of circulating current path,the circulation of simplified circuit and the analysis of power of circulating current.Secondly,in terms of the fundamental wave,the power distribution in the system is improved by using droop control,and the limitations of droop control are compensated by means of virtual impedance.The fixed-value virtual impedance method is improved and an adjustable virtual impedance method which is based on the circulating current feedback is designed.The initial value of the adjustable virtual impedance and the adjustment step size are elaborated.The simulation results show that the adjustable virtual impedance method realizes the suppression of the fundamental wave.Lastly,in terms of the harmonics,FFT is used to extract the main sub-harmonics.A harmonic droop controller is designed to reduce the magnitude of the output voltage harmonics of each inverter in the parallel system and to improve the accuracy of harmonic power distribution.The droop factor in the harmonic droop controller is set.The simulation results show that the harmonic droop controller realizes the suppression of harmonics.