Three single-phase, two-winding transformers, each rated 450 MVA, 20 kV/288.7 kV, with leakage reactance Xeq 1?4 0:12 per unit, are connected to form a three-phase bank. The high-voltage windings are connected in Y with a solidly grounded neutral. Draw the per-unit zero-, positive-, and negative-sequence networks if the low-voltage wind- ings are connected: (a) in D with American standard phase shift, (b) in Y with an open neutral. Use the transformer ratings as base quantities. Winding resistances and excit- ing current are neglected.