Research On Circulation Suppression Strategy Of Parallel Inverter For Microgrid System In Islanding Operation Mode

Most distributed power sources in microgrids are connected to the microgrid AC bus in parallel after being converted by inverters.Microgrids mainly have two modes of operation: grid-connected and isolated islands.When microgrids operate in isolated islands,parallel inverters usually use sagging Control,based on droop control,a micro-grid parallel inverter will generate a large circulating current,which will seriously affect the safe and stable operation of the micro-grid.Aiming at the problem of circulating current caused by parallel inverters of micro-grids based on droop control,corresponding inverter control methods are proposed for the circulating current suppression methods of parallel systems of the same capacity inverters and parallel systems of different capacity inverters in droop control.The main research contents are as follows:1?Establish mathematical model of micro-grid parallel inverters.According to the topology of the inverter,the mathematical model of the three-phase bridge voltage inverter in the abc static coordinate system is established.Through coordinate transformation,the mathematical model in the dq synchronous rotating coordinate system is obtained.Research and analysis lay the theoretical foundation.2?The mechanism of circulating current generation of micro-grid parallel inverters based on droop control is studied.The equivalent circuit of the island-based microgrid based on droop control is deduced.It is analyzed that the unequal line connection impedance results in circulating current of parallel inverters of the same capacity,and the mismatch of line equivalent impedance and capacity ratio causes circulating current of parallel inverters of different capacities;in MATLAB / simulink platform has carried on the modeling simulation of the micro-grid parallel inverter system based on the droop control,and has studied the influence factor and the variation law of the circulating current generated by the island micro-grid parallel inverter.3?A circulating current suppression method based on reactive relative error control virtual impedance is proposed.Aiming at the circulating current problem of parallel inverters of the same capacity,a circulating current suppression method based on virtual impedance of reactive relative error control is proposed.The reactive impedance is added to the voltage and current control loop of droop control to control the virtual impedance.According to the real-time reactive power The relative error adjusts the magnitude of the virtual impedance and changes the equivalent output impedance of the inverter to balance the impedance difference of the inverter line connection.4?A method of circulating current suppression based on reactive power deviation control of virtual inductor is proposed.Aiming at the circulating current problem of parallel inverters with different capacities,a control strategy of virtual inductor based on reactive power deviation control is proposed.According to the relationship between the inverter output reactive power deviation and the inverter's equivalent reactance,a virtual inductance link is introduced in the droop control to adjust the equivalent output reactance of the inverter,so that the equivalent reactance ratio of the parallel inverter is close The inverse ratio of the capacity ratio reduces the reactive power deviation of the inverter output and achieves the purpose of suppressing circulating current.5?Experimental verification and conclusion.The Simulink module in MATLAB is used to build a microgrid simulation model to verify the inverter control method proposed in this paper.The simulation results show that the same-capacity inverter adopts droop control and the circulating current in parallel is 1.35 A.After adding the virtual impedance,the circulating current size It is 0.15 A,and the circulating current is about 0.02 A after adding the virtual impedance based on reactive power relative error control;the ratio of output current when inverters of different capacities adopt droop control in parallel is 1: 1.5.The ratio of output current is 1: 1.83,and the ratio of output current after adding the virtual inductor based on reactive power deviation control is 1: 1.96.It can be seen that the inverter control strategies proposed in this paper can effectively suppress the circulating current.