Control Of Grid-connected Inverter With LCL Filter During Grid Short-circuit Faults

Energy shortage, environment pollution help new energy generation technology gain more and more attention. Distributed PV generation is the major trend of new energy generation in the future, because it's clean, noise-free and independent. Interface devices are needed when DC power generated by PV array being connected to AC grid. Mostly used interface device is photovoltaic grid-connected inverter with LCL current filter. But as LCL current filter is third-order system, which makes it complex to control, so appropriate control strategy is needed. In order to fulfill the control of grid-connected inverter, it is necessary to ensure that the phase of grid-connected current is the same with that of grid voltage, so the study of synchronization technology during grid faults becomes particularly important. As a supplement to large grid, PV generation system should not only provide power to the grid in normal grid condition, but also support the voltage during grid faults and improve power quality. So this paper mainly focuses on control of grid-connected inverter and grid-connected reference current setting during grid faults.In this paper, principle of LCL current filter is analyzed. According to its transfer function, LCL current filter's control model is built and parameters are designed. According to the mathematical model of LCL current filter, its simulation model is set up and influence of the capacitor current feedback to the control system is analyzed. Characteristic of PR control and QPR control is studied and advantages and disadvantages of them is compared. System transfer function of QPR control using dual closed-loop capacitive current feedback is established. According to automatic control principle, by selecting appropriate system parameters, accuracy of this model is verified.Voltage sags and grid harmonic that may arise during grid short-circuit faults is discussed. Three synchronization technology that is suitable for complex gird condition is given and their principles and characteristics is analyzed. These three methods are direct decoupled synchronization method, phase-locked loop based on crossing decoupled adaptive complex filter and frequency-locked loop based on second order generalized integrator. In situation of grid faults, simulation of three synchronization method is done, and their simulation data in normal grid condition and grid fault condition is obtained. After analyzing the simulation results, suitable synchronization method is chosen.Situation during grid short-circuit fault is discussed, and a kind of current control strategy that changes voltage of PCC by changing active and reactive current injected to the grid is proposed, and its principle of voltage supporting is analyzed. Finally, simulation of normal and fault grid condition is done. Through the simulation of LCL PV grid-connected system based on QPR control, validation of the control system and parameters is verified. And the simulation during grid fault verifies that reference current algorithm can support grid voltage and eliminate active power fluctuation in weak grid during faults.