Operation Control Strategy Of The Off-grid Three-phase Combined Inverter With Unbalanced Loads

In recent years,the development of the microgrid has been paid high attention in the world.The important component of the microgrid is the distributed power supply,which can effectively improve the utilization efficiency of the intermittent renewable energy,realize the optimization and reduce the power loss of the distributed power supply.The distributed power supply can ensure the power supply for the critical load and improve the reliability of power supply during the grid fault,and it can also be used to solve the problem of power supply in the remote areas.Unbalanced three-phase loads in the microgrid bring about adverse effects on the voltage balance of public AC bus,it affects the quality of power supply in the off-grid mode.Based on this,the prime target of this project is to use a reasonable circuit topology and design the corresponding control strategy to improve the three-phase AC-bus voltage balance in the microgrid.According to the characteristics of a large number of single phase loads in the microgrid,the three-phase combined inverter is used.There is no coupling relationship among three phases.It is easy to realize the independent control of each phase.The power control loop,voltage control loop and current control loop are adopted for the three-phase combined inverter.Control strategy of P-f curve based on S-function and improved Q-V curve is presented for power droop control loop.The balanced degree of the three-phase inverter reference voltages may be ensured to be high under the conditions of three-phase balanced or unbalanced loads.Meanwhile,the influences of inconsistent system parameters on the balance of three-phase output voltages are analyzed and the no-load voltage adjustment based on improved droop control scheme is studied so that output voltage of the three-phase inverter is still balanced when system parameters are not consistent.In order to verify the design of the system,simulation model of a three-phase combined inverter system using PSIM simulation software is established,and the control method of two parallel inverters is studied.The correctness of the presented control strategy is confirmed by simulation results.strategy is confirmed by simulation results.Experimental platform of a three-phase combined inverter based on Infineon XE164FN as control core is built.Software programming and debugging are completed.The control strategy designed is verified.The feasibility and effectiveness of the presented strategy are confirmed by experimental results.