Selecting distribution transformer size and location

The distribution transformer steps 11 kV (or 33 kV) down to 400/230 V for consumers. Its size and location decide the voltage at consumers' terminals, the LT and transformer losses and the cost of the scheme.

Considerations for size (rating)

1. Design-year peak demand: rating is based on the forecast peak (usually 5th year, sometimes 10th) in kVA, including diversity, contribution factors, power factor and loss allowance: $S = P_{peak}/\cos\phi$.
2. Standard ratings: choose the next standard size (e.g. 10, 16, 25, 50, 100, 200, 300, 500 kVA) to keep spares and stock limited.
3. Load growth: allow for future growth, but avoid gross oversizing. A unit loaded at about 50–80% of rating at peak is usual.
4. Losses: an oversized unit has high no-load (core) losses throughout the year, which matters when load factor is low (rural). An undersized unit has high copper loss and overheating. Loss capitalisation (total owning cost) decides the economic size.
5. Overload capability: short-time overloads allowed by IEC 60076-7 loading guides, considering ambient temperature.
6. Type of load: motor starting, single- vs three-phase load, harmonics.
7. Number of units: one large unit vs several small units; small units near loads reduce LT length.
8. Transport and mounting: pole-mounted units are limited to about 200–300 kVA; larger ones need plinth mounting.

Considerations for location

1. Load centre: at or near the centre of gravity of the load, $\bar{x} = \sum P_i x_i/\sum P_i$, to minimise LT length, voltage drop and loss.
2. Voltage drop limit: all consumers should be within the allowable LT feeder length.
3. Proximity to HT line: short 11 kV tap line.
4. Accessibility: near roads for installation, maintenance and replacement.
5. Safety: away from schools, playgrounds and crowded places; proper clearances, fencing and earthing.
6. Site conditions: firm ground, free from flooding, landslide and waterlogging; space for earthing pits.
7. Land availability and public acceptance.
8. Future expansion of the settlement.

   11 kV line
 ======+=========================
       | HT tap (short)
      [T] at load centre
   ____|____________
  |    |       |    |   LT feeders
  o o  o o   o o  o o   consumers

The final choice balances capital cost against losses and voltage quality.