Mechanism analysis and mitigation of instability in grid-connected Voltage Source Inverter with LCL filters based on terminal impedance

Grid-connected Voltage Source Inverter (VSI) with LCL filters, controlled by proportional-resonant compensator, is very popular in utility application for its better power quality feature, while it is prone to instability especially under weak grid condition. This paper deals with this issue utilizing the terminal impedances of the inverter and the grid. Firstly, the instability mechanism is revealed by modeling the inverter output impedance, and it is observed that the item, contributing to the instability, is a sudden peak in the magnitude of the inverter output admittance. Moreover, it is found that this peak is inevitable, and is introduced by a stable designed inverter with high current control loop bandwidth. Secondly, to dampen out this peak for mitigating the instability, a novel control method based on adding lag compensator directly into the current control loop is proposed without extra needs on the current and voltage sensors. Lastly, the effectiveness of the proposed instability mitigation scheme is verified both by the simulation and experimental results.