| In recent years, the technology of grid-connected photovoltaic(PV) generation system develops rapidly, making tremendous contributions to solving the global energy crisis. With the increasing demand of electrical equipment, the requirement for the capacity and reliability of the grid-connected PV generation system becomes more and more urgent. More and more attention has been paid to grid-connected PV generation system with multi-inverter operated in parallel because of its high reliability, high capacity and good redundancy. Among them, loop current suppressing and MPPT control are its two key technologies.In order to ensure the balance of the output power and improve the efficiency of the generation system, the two key technologies are studied in this paper. The main research contents of this paper are as follows:1. The structure and the control strategy of multi-inverter operated in parallel for PV system are studied.The system framework of the topological structure is determined, and the principle, advantages and disadvantages of several common control strategies are analyzed in detail.2. The mechanism of the loop current in multi-inverter operatied in paralle is analyzed, traditional droop control is improved to suppress the loop current in the system, and the simulation experiment under Matlab/Simulink environment is done. The loop current will cause the malfunction or rejection of the switch, seriously damage the equipment and affect the quality of the power grid. Through building the simulation model, this paper verifies the relationship between the output voltage amplitude difference, the phase difference and the loop current of the inverter. In this paper, an improved droop control method is proposed to suppress the loop current in the parallel multi-inverter system, and is proved feasible by simulation experiment in the Matlab/Simulink environment.3. In the light of the power efficiency of the PV grid-connected generation system, this paper established a circuit model of photovoltaic cells, and analyzed its output characteristics. Based on the traditional perturbation and observation method, this paper puts forward the improved Maximum Power Point Tracking(MPPT) control algorithm which is based on the size of |dP/dU| variable step. At the same time, the MPPT coordinated control method for parallel PV inverters operation is proposed.4. The general disturbance observation method and the improved MPPT control algorithm which is based on the size of |d P/dU| variable step are simulated by experiments. By comparison, it is proved that the improved MPPT algorithm in this paper has more advantages. At the same time, MPPT coordinated control technology of multi parallel inverters is also verified the correctness by using the experiment simulation. |
