Catastrophic bifurcation in three-phase voltage-source converters

Active pulse-width-modulated (PWM) voltage rectifiers are commonly used to convert ac power from a three-phase grid to a regulated dc voltage with unity input power factor. The output voltage regulation is normally achieved by an outer voltage feedback loop and a sinusoidal pulse-width-modulated (SPWM) inner current loop. Due to output voltage disturbances, such as those produced by a capacitive load, the inner current loop can be easily driven into over-modulation and possible six-step operating regions. Current research efforts have focused on extending the linearity of the modulated voltage in the over-modulation range to reduce the input current harmonics and on studying the closed-loop stability via conventional small-signal linearization around a fixed operating point. Such linear techniques obviously fall short of predicting large-signal transient stability. In this paper, catastrophic bifurcation of the three-phase voltage-source converter is reported. The physical origin of the phenomenon is identified. Boundaries of catastrophic bifurcation in the parameter space are derived. The phenomenon has been verified experimentally.