Selecting load centre and transformer, rural/urban

A load centre is the point from which a distribution transformer feeds a group of consumers through LT feeders. Its location and the transformer size decide the LT voltage drop, losses and cost of the scheme. The considerations differ for rural and urban grid-extension schemes.

Selecting the load centre

• Centre of gravity of load: place the transformer at or near the load-weighted centre, $x = \dfrac{\sum P_i x_i}{\sum P_i}$, so that LT feeders are short and balanced.
• LT feeder length limit: keep feeder lengths within the voltage drop limit (about ±5–10% in Nepal; LT reach typically 0.5–1 km urban, up to about 1.5–2 km rural).
• Accessibility: near roads/tracks for transport, installation, maintenance and replacement.
• Short HT tap: close to the 11 kV line route, so the HT tap line is short.
• Safe and clear site: away from schools, playgrounds, flooding, landslide areas; land availability (government or community land).
• Future growth direction of the settlement.

Selecting the distribution transformer

• Design-year peak demand: rating ≥ forecast 5th-year (or 10th-year) peak kVA including losses; choose the next standard size (10, 16, 25, 50, 100, 200, 300, 500 kVA).
• Loading for minimum loss: avoid gross oversizing (high no-load loss) and overloading (high copper loss, ageing).
• Phase: single-phase for scattered small domestic loads; three-phase where motors or larger loads exist.
• Standardisation: few sizes to reduce spare stock.
• Efficiency/loss capitalisation: low-loss (e.g. amorphous core) units where load factor is low.

Rural vs urban

In rural schemes, it is often cheaper to extend the 11 kV line and use several small transformers near each cluster than one large transformer with long LT lines; in urban schemes, transformers are placed so that each covers a compact block within thermal and voltage limits.