Instability of Grid Connected Converter under Weak AC Grid Conditions

Renewable energy sources like wind or hydrogen have been worked as a reliable option to traditional power sources. But for distributed power generation systems (DPGSs), the operation of the grid-connected converter under the weak ac grid condition has been a challenge for the stability and power quality issues. This paper shows the effect of the weak grid on the grid voltage-controlled inverters with LCL- filter and L- filter. The impedance-based analytical approach is employed to study the harmonic stability caused by the interaction between the power grid and the grid converter. A method of deriving the impedance ratios for different inverters is utilized to predict the harmonic stability of the power system through Nyquist diagrams and Bode plot. Time-domain simulations based on Matlab/Simulink are presented to validate the effectiveness of the theoretical approach and confirm conclusions from analytical studies. This study provides insight into the design of the controller under weak grid networks.