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Three-phase voltage source converters (VSCs) are commonly used to convert ac power from a three-phase grid to a regulated dc voltage with unity input power factor. The control of the VSC is normally achieved by an outer voltage feedback loop and a sinusoidal pulsewidth-modulated inner current loop. However, the nonideal power grid and the presence of other interacting loads give rise to nonlinear operation and drive the VSC to enter an irreversible instability region. In this paper, an irreversible bifurcation phenomenon in a three-phase VSC connected to a power grid with an interacting load is reported. The converter can also be regarded as exhibiting a catastrophic bifurcation in which the input current expands to impose undesirable component stress. A large-signal analysis is adopted to identify the physical origin of the phenomenon and to locate the boundary of the instability. Experimental results on a 4-kVA ac/dc converter prototype provide verification of the instability phenomenon.