Research On Parallel Control Of Low Power Three Phase Inverters

With the reduction of traditional energy and the increase of pollution problems,new energy is getting more and more people's attention.In the wind power and photovoltaic power generation and other new energy development,inverters play a more significant role in power generation systems.The inverter can realize the need of high power and grid connection.In order to increase the system capacity and reliability,the parallel connection of the inverter is more and more widely used.Parallel inverter of redundant function has become one of the main research directions of the high power conversion technology.Firstly,the topology of three phase four leg inverter is selected in the article,which explains the reason of choosing it.Its mathematical model under three different coordinates is analyzed,and its design in three-phase stationary coordinates is described in detail.The LC filter design is described.The fourth bridge arm is used with a third harmonic injection.A single inverter is used with dual closed-loop control,which the outer ring with PR control and the inner ring with proportional control.In order to control the nonlinear load,repeated control is added.The double-loop controller and the repetitive controller are designed by using Bode diagram,and the discretization of the PR controller is described.The simulation of a single four-leg inverter is carried out to verify that the system has good dynamic performance and steady state performance.The parallel inverter is designed by selecting the distributed control mode based on the single inverter.The formula of circulating impedance is deduced,and the influence of loop impedance on circulation is explained.The circulation and mathematical model of the parallel three-phase four-leg are analyzed in the article.Then the effect of virtual impedance is explained.Influence of virtual impedance location on the loop impedance is described.Several different virtual impedances are compared with each other,and the current sharing loop,average circuit and synchronous circuit are designed.In the case of parallel module synchronization,in which the current sharing loop is used to adjust the reference voltage value so that change the size of the reference voltage to change the circulation size.The design process of the current-sharing loop is illustrated,and the influence of different proportion of current-sharing loop on the system is verified.Two modules of the communication transport by CAN to ensure the real-time transmission of information.Several conditions for the generation of circulation are analyzed in the article.Simulink is used to simulate the parallel system.It is proved that the current-sharing loop can control the size ofthe circulating current well and ensure the normal operation of the parallel unit.Finally the hardware used in the experiment is designed in the article,and the software part is described.Experiments on parallel inverters are carried out.The experimental results show the correctness of the control strategy.