Research On System Structure And Control Strategy Of Series-connected Optimizer In Photovoltaic Grid-connected System

As the problems of environmental pollution and energy crisis become severe with each passing day, the new energy industry is of great development potential. In the grid-connected solar system of series-connected power optimizer structure, every solar panel is linked to an optimizer, the output ports of the optimizers form the DC bus and the central inverter transforms the energy from DC bus to power grid. The structure can utilize the energy of each panel with high efficiency and modularization, according to the requirement of distributed solar system, becoming one of the major research areas. This dissertation focuses on the system and control strategy.The first chapter introduces the background of series-connected solar system, and briefly analyses and summarizes the achievement of others.In chapter2, the working condition of series-connected solar system has been described, that is the-latter-stage inverter balances power flow and maintains DC bus voltage, it has no need to track the maximum power point(MPP). The-former-stage optimizers accomplish MPPT of each panel to solve the problem of hot pot effect and power loss. Based on that, the cascaded Buck-Boost is suitable as the topology of the optimizer and the design of circuit parameter, small signal model and double-loop controller is completed.In chapter3, based on the analysis of common-mode current, the parallel dual-Buck inverter has been chosen for the central inverter with input-voltage outer loop and output-current inner loop. Utilized of a novel control method to compensate voltage ripple in voltage loop brings the decrease of output current THD.In chapter4, the control strategy of the series-connected power optimizer structure has been proposed to accomplish the stabilization without communication between each module. Based on the control strategy by fixed and unfixed bus voltage, a novel structure with part of panel connected with the optimizer has been proposed. It can cut down the cost and maintain high output power.