Effect of voltage-loop controllers on bus voltage fluctuation and recovery time in voltage source converters

In applications employing AC-DC power converters, the DC-bus capacitor plays the role of de-coupling the instantaneous interaction between the DC-side load current and the AC-side source current. In most practical applications, the magnitude of the DC-bus ripple current maximum allowable voltage fluctuation and the voltage hold-up time determines the DC-bus capacitance with the ripple current being a primary consideration. For a power converter to operate with a small bulk capacitor, it is important for the capacitor to be able to handle the ripple current. This paper investigates the effect of voltage-loop controllers on both the transient (DC-bus voltage recovery time, and magnitude of voltage fluctuation) and the steady-state (magnitude of ripple and harmonic spectrum) response in voltage source power converters with small bulk capacitors. Emphasis is placed on power converters employing current loop controllers based on the synchronous reference frame model and voltage-loop controllers incorporating load power feedforward. It is shown that a power converter employing an inner-loop controller based on the synchronous reference frame and an outer-loop controller that includes load power feedforward, results into lower bus ripple voltage during steady-state. Additionally, a power converter operates with reduced voltage fluctuation and faster voltage recovery time following a step change in load, compared with one employing a conventional three-phase current-loop controller