Three single-phase, two-winding transformers, each rated 450 MVA, \(20 \mathrm{kV} / 288.7 \mathrm{kV}\), with leakage reactance \(X_{\text {eq }}=0.12\) 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 windings are connected

(a) in \(\Delta\) with American standard phase shift or

(b) in \(\mathrm{Y}\) with an open neutral. Use the transformer ratings as base quantities. Winding resistances and exciting current are neglected.