Design And Research On Three-phase Grid-connected PV Inverter Of DC-bus System

With the rapid development of the world economy, energy shortages and environmental issues become more prominent, photovoltaic(PV) power generation gets more and more attention around the world. In the PV power generation systems, grid-connected inverter is the key equipment which is connected with grid and power generation systems. In this paper, an in-depth research is conducted about three-level grid-connected inverter of PV power generation system with DC-bus.In this thesis, the architecture of PV power generation system with DC-bus is described. The working principle of diode-clamped three-level inverter is explained. In order to simplify the conventional three-level space vector pulse width modulation(SVPWM) algorithm, the algorithm based on 60 degree coordinates is introduced. The sectors judgment and vectors synthesis principle of algorithm is analyzed. Off-grid inverter simulation model has been built in Simulink to verify the feasibility of the algorithm. The algorithm of feedforward current decoupling control is studied. The principle of single synchronous coordinate system software phase locked loop(SSRFSPLL) is analyzed. Since the single synchronous coordinate system software phase locked loop can not track the phase when the voltage imbalance leads to second order generalized integrator software phase locked loop(SOGI-SPLL). The phase locking capability of the two phase locked loops in the voltage balance and the voltage drop is compared. The model which controlled by grid-connected current closed-loop of inverter is established according to the above research content. The output current of the inverter with L filter and LCL filter is analyzed. The harmonic content of the output current of the grid-connected inverter with L filter and LCL filter is analyzed, and the filtering effect of the two filters is compared.Based on the above theories and simulation studies, the main circuit, sampling circuit and control circuit of diode-clamped three-level inverter system is designed and calculated. The procedures of main program and interrupt procedures are described. A prototype of diode-clamped three-level inverter is built, and SVPWM algorithm and grid control algorithm was experiments on the prototype. According to the experimental results and simulation results, the correctness of the algorithm and the feasibility of the design of the diode-clamp three-level grid-connected inverter system is proved.