| Parallel inverters can improve the reliability and increase the capacity of the system.In islanded micro-grids,the load current is distributed proportionally according to the total output impedance of each inverter.Due to the existence of uncontrollable factors such as line impedance,it is difficult to determine the total output impedance of each inverter,which affects the current-sharing performance of the system and leads to circulation in the parallel inverter system.The existence of circulation affects the performance and the life of switching devices.Therefore,how to realize current equalization of parallel inverters is an important problem.The specific research process and results are as follows:(1)The mathematical model of double closed loop control(voltage outer loop and current inner loop)for the single inverter is established.In order to improve the steady state performance of inverter,a cascaded repetitive control method is proposed.(2)Simulation analysis is carried out on parallel repetitive control and cascaded repetitive control of the single inverter.Compare the performance of two repetitive controllers under different conditions.The experimental results verify the effectiveness and correctness of the proposed method.(3)The mathematical model of parallel inverter system is established.In order to solve the circulation problem caused by line impedance,droop control and virtual impedance are introduced.The droop control method with virtual impedance can reconstruct the output impedance of the system to achieve the effect of current equalization.(4)The mathematical model based on droop control and virtual impedance is established,and the effectiveness of the scheme is verified by simulation analysis.At the same time,the parallel experimental platform of two inverters was built for experimental verification.The experimental results show that the system can achieve good current sharing effect due to the existence of droop control and virtual impedance. |
PDF Full Text Request