Research On Three-phase Photovoltaic System Grid-connecting Control

Currently, due to the global energy crises, a lot of countries begin to take the researches on the renewable energy resources seriously. So, based on the use of solar energy, the researchs on Photovoltaic (PV) generation is paid broad attention. But there are some differences obviously between the PV generation system and the traditional electricity generation system: the electronic characteristic of PV array is non-linear evidently, it is needed to track the maximum power point of the PV array (MPPT). As one of the distributed generation systems, PV generation system needs to transfer the power to the public power grid, so the power grid connecting technology is needed to be studied. Through theoretical analysis and experiment, this paper studid MPPT method and control technology of grid-connecting based on single-stage grid-connected PV generation system.The mathematic model of the controlled target is the basis of the study on control method. This paper theoretically analyzed the three-phase inverter with LCL flitter, and established the state-space mathematic model of the inverter. Then this paper proved that the three-phase inverter can be decompounded to two single-phase inverters through coordinate transform and controlled in the equivalent single-phase inverter. The open-loop character of the inverter is also analyzed.In order to full fill with the requirement of tracking the PV array's MPP during the single-stage grid-connected inverter works, this paper studied the commonly used MPPT methods through theoretical analysis and experiment, compared their advantages and weakness, and pointed out the reasons of MPPT error and the influence of sampling error. Based on these studies, this paper proposed a new MPPT method that is suitable for the single-stage grid-connected PV system. The experiment result proved that the new MPPT method can efficiently improve the character of the PV system during MPPT period.Through the analysis about Pole-assignment control and repetitive control, this paper designed a hybrid controller incorporating repetitive control and state feedback pole-assignment control based on single-phase inverter with LCL filter, and the experiment proved the validity of the controller. According to the requirement of single-stage grid-connecting system, this paper incorporated the MPPT control and the grid-connecting control, and achieved the MPPT using the proposed MPPT new method during the grid-connecting. The simulation proved that the control strategy can tracking the PV array's maximum power point correctly, efficiently reduce the current distortion and achieve the unit power factor grid-connecting.