Research On The Control And Parallel Strategy Of Isolated Micro-grid Inverter

With the development of power electronic technology,low voltage AC microgrid composed of inverters and other power generation equipment is widely used as backup or replacement of main grid.In micro grid,the mode that power supply equipment does not connect to the main grid and works independently is called isolated microgrid mode.Unbalanced and nonlinear loads in isolated microgrid can lead to voltage distortion and power quality reduction.Therefore,the inverter voltage control technology has put forward higher requirements.Because of the lost of main grid voltage support,the inverters of isolated microgrid need to work with other inverters or power generation devices to provide the load voltage in voltage source mode.It is necessary to study the parallel operation of voltage source of inverter.In this paper,the advantages and disadvantages of three-phase inverter topologies are analyzed,and the three-phase full bridge inverter based on single phase independent control is chosen to be the converter.The topology has the advantage of high utilization ratio of DC voltage and can effectively cope with unbalanced output voltage caused by unbalanced loads.This paper aiming at the control strategy of three phase full bridge inverter,the shortcoming of the current loop in the traditional series double closed loop control system under high power condition is analysed.And a single voltage close loop control strategy is presented instead.By comparing the performance of single closed loop control and double closed loop control,the design requirements of controller based on phase forward compensation and enhancement of stability are proposed.On this basis,a quasi-proportional resonart(PR)and repetitive compound control strategy for three-phase full bridge inverter is proposed.Quasi-PR controller improves the fundamental control performance by phase forward compensation.A model cancellation filter is designed for repetitive controller to improve stability and restrain voltage harmonics.A controller switching strategy based on voltage and current amplitude detection is proposed in this paper in order to solve the problem of current limiting under over-load or short-circuit condition.The influence of passive damping of the inverter system is analyzed and series capacitive resistor is used to enhance the stability of voltage and current control mode.This paper study on the power droop control strategy under the parallel work condition of three phase full bridge inverter.A mathematical model is established to analyze the dynamic characteristics of the power regulation process.The improvement strategy of droop control is put forward to improve stability of paralleled inverter system.Finally,the experiment platform of three-phase full bridge inverter based on ARM controller STM32F407 is designed,and experiments is performed by software programming.On the experimental platform,it is proved that the proposed control strategy could improve voltage under unbalanced nonlinear load condition of isolate microgrid and could limit output current by control mode switching.