Chapter 3, Problem 3.36P

Three single-phase two-winding transformers, each rated $\text{25 MVA},\text{ 34}.\text{5}/\text{13}.\text{8}\text{kV}$, are connected to form a three-phase $\Delta -\Delta$ bank. Balanced positive-suence voltages are applied to the high-voltage terminals, and a balanced, resistive Y load connected to the low-voltage terminals absorbs 75 MW at 13.8 kV. If one of the single-phase transformers is removed (resulting in an open $-\Delta$ connection) and the balanced load is simultaneously reduced to 43.3 MW ( $\text{57}.\text{7}\%$ of the original value), determine (a) the load voltages $V_{a\eta },V_{b\eta },$ and $V_{c\eta };$ (b) load currents $I_a,I_b,$ and $I_c;$ and (c) the MVA supplied by each of the remaining two transformers. Are balanced voltages still applied to the load? Is the open $-\Delta$ transformer overloaded?