Research On Control Strategy For Inverters And Islanding Detection Of Grid-connected System

With the rapid development of the global economy and industry, the demand for energy is soaring. The gradual exhaustion of traditional energies leads to the development of renewable, clear and pollution-free energies. The research of new energy now mainly focus on the grid-connected control of the solar power generating system and other new energy power generating systems.In the system of grid-connected power system, the grid-connected inverter is the core. This thesis chooses the voltage source inverter as the grid-connected inverter. Firstly the thesis analyzes the working principle and topology of the three-phase voltage source inverter, and then establishes a mathematical model for it. Finally, on the basis of the above analysis and modeling, the thesis conducts a research on the control methods of grid-connected system.The dual-close-loop control is adopted in the thesis. The outer loop is in the control of direct voltage loop. Through the adjustment of PI, it can receive the inner loop instructive current/instructive active power. The inner loop control separately adopts direct current control and direct power control. The direct current control uses the current decoupling control, which can realize unit power factor control by the instructive current received in the outer loop. The objects of direct power control are active power and reactive power. Compared with direct current control, it has the advantages of a simple algorithm/structure, higher power factors and lower THD. The thesis uses the MATLAB/Simulink to simulate the two control systems and verify the effectiveness of the two methods of control.In the end, the thesis studies the islanding detection in the grid connected system, and analyzes the advantages and disadvantages of passive detection and active detection. Based on the active current disturbing a simulation experiment on three-phase grid-connected inverting system in d-q coordinate system is conducted. The result of the experiment demonstrates that this method can detect the islanding providing that the output power of the inverter is balanced with the load power.