Low-Frequency Hopf Bifurcation and Its Effects on Stability Margin in Three-Phase PFC Power Supplies Connected to Non-Ideal Power Grid

Three-phase power-factor-correction (PFC) power supplies 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. However, the non-ideal power grid may drive the converter to enter a low-frequency instability region. In this paper, a low-frequency instability phenomenon in three-phase PFC power supplies is reported. The converter can also be regarded as exhibiting a Hopf-type bifurcation in which the dc output voltage oscillates at 150 Hz, and a large amount of harmonics appear on the line current. We develop an averaged model of the grid-converter system to predict the low-frequency instability. The effects of circuit and grid parameters can be studied with this model. Additionally, as a result of the emergence of low-frequency oscillation, the stability boundary leading to a catastrophic bifurcation changes dramatically. Simulation results also provide design-oriented boundaries of operation. This phenomenon is confirmed by experimental measurement.