Control Strategy To Achieve Multiple Objectives For Input-Series-Output-Series Grid-Connected Inverter System

Due to problems of energy exhaustion and environmental pollution,human beings have begun to pay more attention to clean energy such as solar energy and wind energy.The requirements of the inverter capacity are more stringent,which increases the design difficulty of a single inverter.The input series output series(ISOS)inverter system is suitable for applications where the input voltage and output voltage are both high.By using this system instead of a single inverter for grid connection,the capacity of each inverter in the system can be reduced.The voltage of the single inverter will greatly be reduced,which will reduce the production costs,shorten the development cycle and improve system reliability.In addition,due to the large variation range of the transmission line impedance in practical applications,the requirements for grid-connected inverters are further improved,and the robustness of the system to the grid impedance cannot be ignored.For the ISOS inverter system,it is first necessary to ensure the input equalization and the output equalization,that is,the power balance between the modules.The topology is applied to the gridconnected system,so the high power factor of the grid-connected current is required.It is necessary to suppress the inherent resonant peaks with the LCL filter at the same time.Then,capacitor current and grid current are selected as control variables based on the multiple control targets and a distributed control strategy is proposed.Each module has its own control loop,which mainly includes three control loops: input voltage sharing loop(IVS loop),modular inner current loop,and the output current loop.The output current loop ensures that the grid-connected current tracks the grid voltage to achieve high power factor,and the capacitor current inner loop can effectively suppress the LCL resonance spike while ensuring the output voltage is in phase,and the power balance between the modules can be realized by the input voltage equalization.In order to further improve the reliability of the system,a redundant fault-tolerant scheme of the system is proposed.Bypass switches are added to the input and output terminals,and reasonable control timing is adopted between the control signal switches to realize the redundancy.In order to improve the robustness of the ISOS grid-connected inverter system to the impedance change of the grid,the phase-lead compensation method is applied to the control strategy of the system.With each module equivalent to a Norton model,the system's equivalent model is obtained.According to the impedance-based stability criterion,the system robustness against grid impedance with and without phase-lead compensation is compared and analyzed,and the corresponding simulation verification is given.On the basis of theoretical analysis,an ISOS inverter system consisting of three inverter modules is fabricated,both simulation and experimental results are presented to verify the effectiveness as well as the validity of the proposed compound control and the redundancy scheme.