Sequence-Frame Coupling Admittance Analysis and Stability of VSC Connected to Weak Grid

This paper develops small-signal admittance of grid-connected voltage source converter (VSC) in the positive-negative (pn) sequence-frame for ease of stability analysis. Based on the converter admittance in pn frame, the stability problems associated with the coupling admittance created by the phase-locked loop (PLL) and outer loop controllers are studied. To overcome the disadvantage of the traditional outer loop controller and improve system stability, an improved outer loop controller that can reduce the coupling admittance is proposed. VSC system connected to a weak grid is modelled in MATLAB-SIMULINK to validate the theoretical study and demonstrate the effectiveness of the improve outer controller.