Flux-based control of 3-phase Active Front End

Active Front End converters (AFE) connect a DC link to a grid, usually with three phases. They allow energy flow in both directions, reduce grid-current harmonics, needed short-circuit power and non-active power in comparison to diode or thyristor-based devices. One more advantage, with is in the center of this paper, is the dynamic reaction to load variations combined with the need for robust control under varying grid parameters. The worst-case variations of grid parameters, in this context, are voltage sags and voltage unsymmetry (e.g. phase faults). This paper presents a control concept employing an inner, virtual-grid-flux based control for fast current control and an outer DC-link voltage control with a conventional PI controller. The layout of the controllers is detailed. Simulations characterise the dynamic performance concerning load variation and grid parameter variation. A comparison of measured and simulated result of a step-wise load change proves the validity of the simulation and the control concept.