Single phase vs 3-phase distribution transformer

Distribution transformers may be single-phase (11 kV/230 V, or SWER/phase-to-phase primary) or three-phase (11/0.4 kV). Each suits a different type of load and area.

Single-phase distribution transformer

Suitable where:

• Loads are small, scattered and mainly domestic (lighting, fans, TV): rural villages, hilly settlements.
• Load per cluster is small (5–25 kVA), so a three-phase unit would be lightly loaded with high no-load loss.
• The HT line can be a single-phase tap (two wires or SWER), much cheaper than three-phase.

Advantages: low cost, light (easy pole mounting and transport in hills), lower no-load loss, can be placed very close to small clusters, reducing LT length.

Disadvantages: cannot supply three-phase motors; may unbalance the three-phase feeder unless taps are distributed across phases; lower capacity.

Three-phase distribution transformer

Suitable where:

• Load density is high (urban, semi-urban areas, market centres).
• There are three-phase loads: motors, mills, pumps, workshops, industries, irrigation.
• Larger capacities (25–500 kVA) are needed.

Advantages: supplies both 3-phase (400 V) and 1-phase (230 V) loads; about 15–20% less material and cost per kVA than three single-phase units; balanced loading of the HT system; better efficiency at large ratings.

Disadvantages: higher cost for small loads, heavier, needs a three-phase HT line, high no-load loss when lightly loaded.

In practice, single-phase units are used for rural electrification of small, dispersed settlements, and three-phase units for urban, commercial and industrial areas.