Research On The Stability Control Technology Of LCL Type Grid-Connected Inverter Under Weak Grid

In order to cope with the energy shortage and environmental pollution,renewable energy distributed generation system,dominated by wind energy and solar energy,has developed rapidly.The grid-connected inverter,regarded as the core device of distributed generation system,has attracted widespread attention.Before the distributed generation system was merged into the grid,it needs a long transmission line and increased transformer links which due to its performance characteristics of the weak grid.However,because of the weak grid,the grid impedance has a negative impact on the stability of the grid-connected inverter system.This thesis intends to take the LCL type grid-connected inverter as the research object under weak grid and analyses the influence of grid impedance on the stability of grid-connected inverter system.Combined with the power grid impedance detection technology,a feed forward phase angle compensation method is proposed in order to improve the adaptability of the grid connected inverter system on grid impedance and improve the power quality of grid-connected current.The specific research contents are as follows:(1)In the condition of the ideal grid,the topology of the grid-connected inverter is used and the mathematical model of LCL inverter is established.Because of the resonance of the LCL type grid-connected inverter,the grid connected inverter system is unstable.Therefore,in this thesis,the capacitive current feedback active damping method is used to ensure the stability of the grid connected inverter system,by adding a constant feedback to suppress the resonant peaks in the feedback.(2)To the question that traditional PI regulator can not track the AC signals effectively,as well as its steady state error,the PR current regulator with zero static error tracking AC signals is adopted to ensure the system has enough stability margin in this thesis.(3)On the basis of its stability,the model of weak grid LCL type connected inverter system is established under ideal grid conditions.What’s more,from the prospective of frequency domain method,by analyzing the stability of the system influenced by grid resistance and inductance,it is revealed that the inductance increase is the main reason to affect the system stability.(4)To improve the stability of the grid-connected inverter system under weak grid,active damping method LCL grid-connected inverter output impedance model is deduced.With grid impedance detection technology,feed forward phase angle compensation method is proposed.This method can improve the phase angle at intersection between the grid impedance and gridconnected inverter output impedance to a largest extent,without changing the output impedance of the grid-connected inverter.The adaptability of the grid connected inverter system can be effectively improved by the feed forward phase angle compensation method under the change of impedance.At the same time,the parameter design procedure is given,too.The simulation results show that feed forward phase angle compensation method can effectively ensure the stability of the grid-connected inverter system under the condition of impedance change.