Unstable Limit Cycle Induced by Outer Control Loops and Stability Boundary of Voltage Source Converter

This paper illustrates the phenomenon of oscillatory instability under large disturbance of grid-connected voltage source converter (VSC) with the outer control loops. A 3rd-order nonlinear differential-algebraic equations (DAE) model with DC and AC voltage outer control loops is developed, and the existence of an unstable limit cycle is verified. The unstable limit cycle is induced by the outer control loops and is a component of the stability boundary of the VSC system. The characteristics of the unstable limit cycle and the stability boundary are also analyzed. The conclusions are verified with simulation results of both the reduced-order DAE model and the full-order model.