Research On Phase-locked-loop Technique Of Grid-connected Inverter Under Weak Grid

With the development of distributed generation,inverters are widely used in the grid connected system.The phase locked loop is used as a part of the inverter phase reference for the grid connected current.Especially under the condition of weak grid,the grid impedance is high.If the phase of the grid doesn't be accurately locked,the injected current of the weak grid will be distorted and even affect the stability of the system.In this thesis,under the condition of weak grid,the problems existing in the phase locked loop of the grid connected inverter is studied,and the corresponding phase locked strategy for the weak grid is designed.In this thesis,first three kinds of different and representative structure of the phase locked technologies are selected,according to the characteristics of each kind of phase locked loop,three structures are analyzed under the ideal and non-ideal grid condition.Secondly,the high impedance condition of weak grid is introduced,the linear and nonlinear mathematical model of synchronous reference frame PLL are derived,and self-synchronization phenomenon is analyzed based on the model under the condition of weak grid.At the same time,the possible problems in the PLL of grid connected inverter,including other two PLL structures,are analyzed theoretically.Then three phase locked structures are applied to the grid connected inverter respectively by using the simulation platform.According to the characteristics of high impedance of weak grid and the phase locked problem,impedance cancellation strategy of PLL is designed.The common grid impedance estimation method,and the structure of positive voltage component extraction based on dual two order generalized integral are analyzed,then they applied to the phase locked strategy.Finally,the simulation platform of grid connected inverter under the condition of weak grid is built.The basic performance of the PLL is simulated,and the feasibility of the phase locked loop strategy is verified by comparing the simulation results with the phase locked loop impedance control strategy.