Control Strategy For Paralleled LCL-type Grid-connected Inverters

Energy crisis and environmental problems become serious due to the traditional energy sources such as oil, natural gas, or coal. Renewable energy such as solar energy and wind energy, has been drawing more and more attentions. Distributed power generation systems(DPGSs) is the effective utilization approaches of the renewable energy. Paralleled grid-connected inverters are coupled due to the grid impedance in the point of common coupling(PCC). The system may unstable by changing the grid impedance. In this thesis, the stability of paralleled grid-connected inverters system is analyzed.First, the average switching model of the digital controlled single grid-connected inverter is built. Since the inverter is controlled to inject current into the grid, the inverter is represented by a Norton model. And the model of the multiple system is getted according to the Norton equivalent model. Then, the impedance-based stability criterion applying in the multiple system is described.Second, taking into account that the 1.5 beats delay that brought in by digital control, the parameters of a grid-connected inverter with capacitor current feedback active damping are analyzed.Following, The instability phenomena causing by the grid impedance or increasing grid-connected inverters is discussed in this thesis. In order to improve the stability of the LCL-type paralleled grid-connected inverters system, a new method that shunt resistor and capacitor(RC) in series branch circuit in the point of common coupling(PCC) is proposed in this paper. Then, the parameters of the RC circuit is designed.Finally, a prototype of two 3-kW digital controlled LCL-type paralleled grid-connected inverters system has been built and test in the lab. And theoretical stydy proposed in this paper is validated by the experimental results.