Reasons: distribution efficiency and LV placement

(i) Maximum efficiency of a distribution transformer at loads much smaller than full load

• A distribution transformer is connected to the supply for all 24 hours, so its iron loss is present all the time.
• Its load varies widely: high only for a few peak hours, light at night and during much of the day. The average load is often only 50–70% of the rating.
• Copper loss varies with the square of load, so it is large only during the short peak periods.
• Efficiency is maximum when copper loss equals iron loss, at load fraction

$$x = \sqrt{\frac{P_i}{P_{c,FL}}}$$

• The useful measure is all-day efficiency (kWh output / kWh input over 24 h). To make it high, the iron loss is kept small by using a lower flux density and good grain-oriented steel, while a higher copper loss at full load is accepted.
• So the designer makes $P_i$ about one-third to one-half of $P_{c,FL}$, which puts maximum efficiency at roughly 50–70% of full load, matching the actual load pattern. Energy loss and running cost over the day are then minimum.

Power transformers, in contrast, run near full load and are designed for maximum efficiency near full load.

(ii) LV winding near the core and HV winding outside

  core | ins | LV | duct | HV |
       |thin |    | thick|    |

1. Insulation saving: the core is at earth potential. Placing LV next to it needs only thin insulation between core and LV. If HV were placed next to the core, thick insulation for full HV would be needed there, increasing the core-winding gap, the mean turn length and cost.
2. Less leakage and copper: with the smaller gap, the core diameter and winding diameters stay small, giving shorter mean turns, less copper and lower leakage reactance.
3. Tappings and repair: tappings for voltage control are taken on the HV winding. On the outside they are easy to bring out, and the HV winding is easy to inspect and repair.
4. Lower current: HV winding carries smaller current with thin conductors, which are easier to wind on the outside on a large diameter.