Effect of the load and short-circuit impedance unbalance on the operation of Δ-Δ transformer banks

This paper deals with the individual windings´ load unbalance, loss behavior and the asymmetry in the primary load currents of Δ-Δ three-phase power transformer banks frequently used in distribution networks. Four main affecting parameters are considered: (i) the magnitude unbalance of the load side currents; (ii) the phase angle unbalance of these currents; (iii) the unbalance in the magnitude of the windings´ short circuit impedances; and (iv) the unbalance in the phase angle of these impedances. The investigation allows for the simultaneous presence of the above four types of unbalance. The results demonstrates the limitations imposed by these kinds of unbalance, individually or collectively, on the proper utilization of the total kVA capacity of the three single-phase transformers used to form a three-phase power transformer bank. Diagrams are also given for the dependence of the losses in the transformer bank on the different kinds of unbalance. Moreover, the resulting line current unbalance on the high voltage side is discussed