Nonlinear Modeling and Analysis of Grid-Connected Voltage-Source Converters Under Voltage Dips

In power systems, voltage-source converters (VSCs) are gradually replacing traditional devices and dramatically changing power system dynamic behavior, as the VSCs intrinsic high-order nonlinearity and multiple time-scale characteristics. As a simple case of transient stability for power-electronic-based power systems, in this article, we study a VSC system with a fifth-order nonlinear model, with the aid of nonlinear dynamics analysis. For the transient behavior, we find that both the amplitude and the phase of the terminal voltage of VSC show discontinuity when the fault occurs or is cleared. The behavior of time-scale separation is also generally observable, namely, the fast time-scale dynamics damps and disappears quickly, and the system bulk behavior is dominated by the slow time-scale dynamics. In addition, we find that the system transient stability is uniquely determined by whether the fault-clearing state is within or out of the basin of attraction of the post-fault equilibrium point. Finally, simulations and hardware-in-the-loop experiments verify the analytical results. We expect that all these interesting findings could provide an improved physical insight on our understanding of transient stability of power-electronic-based power systems.