Line-frequency instability of three-phase PFC power supplies connecting to non-ideal power grid

Three-phase voltage source converters (VSC) 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 VSC to enter line-frequency instability region. In this paper, a line-frequency instability phenomenon in three-phase voltage-source converters is reported. The converter can also be regarded as exhibiting a Hopf-type bifurcation in which the dc output voltage oscillates at about 130 Hz, and large amount of harmonics appear on the line current. We develop an averaged model of the VSC and non-ideal power grid which can predict the Hopf-type bifurcation. The effects of circuit and grid parameters can be studied with this model, and are verified though cycle-by-cycle simulations. The simulations also provide design-oriented boundaries of operation.