Control-oriented modeling and robust control of a three-phase buck+boost PWM rectifier (VRX-4)

The modeling and control of a three-phase, three-switch buck-type rectifier system with integrated boost output stage (VRX-4) is analyzed in this paper. A cascaded multi-loop control structure is presented that has the following features: constant output voltage for a wide input voltage range and step-wise changes of load; sinusoidal input currents in phase with the mains voltages that also remain sinusoidal in case of asymmetric mains conditions; and active damping of the input filter resonance. For the control design an equivalent DC-DC small-signal model of the converter is derived and verified by simulations and measurements. Based on this and the identification of the critical operating point the controllers for the inner DC current and outer voltage control loops are selected The stability of the closed loop system is discussed and the robust operation of the system is verified by measurements on a 5 kW prototype.