Study On The Stability Of Photovoltaic Grid-connected Inverter In Weak Grid

Developing new energy distributed generation has already becomed the only way for human sustainable development along with an increasingly prominent energy problem.Grid-connected inverter receives more and more attention as it is the key equipment for new energy to realize grid-connected power generation.Small-capacity power system will present weak grid property for a large amount of grid spurious impedance after several step-down transformers and a long transmission in transmission and distribution network.Grid-connected inverter may has the risk of reducing stability or even losing its stability influenced by weak grid changing impedance.Firstly,this paper analyzes the reciprocal influence caused by grid impedance along with local loads at grid connection points from the perspective of frequency domain,thereby draws a conclusion that grid parasitic inductance is the main reason which leads to the decrease of the system stability.On the basis of this,adaptive control based on online measurement of grid impedance as the main thought is clarified in this paper.In addition,the existing problem in grid impedance measurement based on typical Fourier transform is analyzed and the new algorithm is proposed which based on the windowed Fourier transform.Secondly,this paper analyzes the compensation strategy of grid impedance under weak grid condition and the impact it caused to the system controlled object's frequency characteristic.The method and steps of parameters tuning in PI and PR current controllers is proposed on the basis of it.This paper gives the contrastive analysis of PI and PR controllers' performances under weak grid condition and proposes adaptive control strategy based on PR controller,then simplifies this algorithm and deduces the difference equation of controller so that the digital controller is running well.Then,it introduces single-phase photovoltaic grid-connected experimental platform's structural components along with the design and realization of software and hardware based on the controller of TMS320F28335.At last,Matlab/Simulink simulation and experimental confirmation show that the main theoretical analysis is correct and the algorithm is effective.