Under a transient disturbance, sustained oscillation may emerge in the grid-forming converter. Such sustained oscillation is not generated from the supercritical Hopf bifurcation as the location and the stability status of the equilibrium points remain unchanged before and after the disturbance. Therefore, the linear analytical approaches such as impedance-based method cannot be used for studying the mechanisms leading to instability. In this chapter, the physical origins of sustained oscillations will be explored, and a new type of bifurcation associated with the emergence and vanishment of such oscillations will be exposed.