AC Current Eletricity Step up Transformer

"Three single-phase step-up transformers rated at 10MVA, 4.6kV/13.8kV are connected in Wye-Delta configuration. The three transformers are used to feed a 20MVA load at 0.9 p.f. With an efficiency of 96%. Using per unit system, calculate:
a. the secondary line voltage.
b. the primary and secondary line currents when supplying this load.
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