A Critical Cross-Examination on Load-Balancing Transformers for Distribution Systems

A load balancing transformer (LBT) has already been suggested for improving the unbalance of three-phase primary currents. Each phase includes one extra pair of coupling windings in addition to the usual primary and secondary windings. Two coupling windings, located on two different phases, are made in series, where the resulting circuit is then reversely paralleled with the secondary winding placed on the third phase. Under unbalanced conditions, the load currents are distributed between the coupling and secondary windings that are supplied through different primary phases. This paper proposes a method in order to define all possible LBTs, which paves the way to cross-examine and select the best connections of windings for the LBT. Starting from the original LBT, it is shown that the best LBT is practically connected, such as a zig-zag winding. Then, a novel idea is suggested in which a controller and some four-quadrant semiconductor switches contribute to the improvement of the performance of the LBT. These switches should control the amount of current transfer from one phase to another. A combinatorial selection problem is arranged to find the best way of switching modulation. Both simulations and experimental works (using a designed 12-kVA laboratory prototype) verify the studied examination and proposals, showing the switch-mode zig-zag LBT as having the best performance.