Power quality improvement by means of AC/AC choppers

In this paper a solution to current unbalance is proposed. A three-phase grid feeds a single-phase load that causes current unbalance. This is a typical situation for AC railway substations. By employing AC-Choppers it is possible to obtain a variable Steinmetz Circuit, and define a control law able to compensate a variable current unbalance. At this effect, a theoretical analysis of load unbalance has been performed and a graphical analysis allows determining easily the constraint that the single-phase load has to respect. A circuit composed by interleaved AC-Choppers involves as variable impedance with good power-quality performances. A control law with a feedback action has been employed in order to have a dynamic compensation of unbalance. Numerical simulations validate the theoretical analysis.