How is the line current expressed on the secondary side of a delta-star (Δ-Y) transformer connection in terms of secondary phase current IL and transformation ratio 'a' (where a = N1/N2)?

In which unit is the power rating of a transformer expressed?

In a transformer, it is generally preferred to wind the low-voltage winding close to the core and the high-voltage winding over the low-voltage winding. What is the primary advantage of this type of arrangement?