---
title: "Short note: Power supply system"
subject: "Basic Electrical and Electronics Engineering"
chapter: 4
key: "semester_1/basic_electrical_and_electronics_engineering:short-note-power-supply-system"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/short-note-power-supply-system
site: BCT Engineering
---

# Short note: Power supply system

Basic Electrical and Electronics Engineering · Chapter 4: Electrical Installations

- Asked in: 2081 Baishakh (new course), 3 marks
- Video: [The Electrical Distribution System](https://www.youtube.com/watch?v=Fqk0G1yDjeY) — TEGG Service
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/short-note-power-supply-system
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_and_electronics_engineering/chapter-4

## Question

Write a short note on Power Supply System.

## Short answer

A power system consists of generation (e.g., 11kV), step-up transmission (132-400kV), step-down to 33/11kV for primary distribution, and final step-down to 400/230V three-phase four-wire for consumers, minimizing losses. In Nepal, standard is 230V/50Hz.

## Answer

A power supply system carries electrical energy from generating stations to consumers. It has four main stages.

```text
 Generating  -> Step-up   -> Transmission -> Step-down  -> Distribution -> Consumer
 station        transformer    line           substation     (11 kV/400 V)
 (11 kV)       (132/220 kV)   (HV)           (33/11 kV)
```

1. **Generation:** power is generated at about 11 kV, 3-phase, 50 Hz by synchronous generators in hydro, thermal, solar and other plants (in Nepal, mostly hydro).
2. **Transmission:** transformers raise the voltage to 66, 132, 220 or 400 kV, so that the current and the $I^2R$ loss are small over long distances on overhead lines.
3. **Sub-transmission/primary distribution:** step-down substations reduce the voltage to 33 kV or 11 kV for towns and industries.
4. **Secondary distribution:** distribution transformers step the voltage down to 400 V (line) / 230 V (phase) on a three-phase four-wire system, with 230 V single phase supplied to homes.

**Systems of supply:** AC (usually) or DC; single-phase two-wire; three-phase three-wire (transmission); three-phase four-wire (distribution). The distribution layout may be radial, ring or interconnected, to improve reliability.

In Nepal, the Nepal Electricity Authority (NEA) operates the grid, and the standard consumer supply is 230 V single phase / 400 V three phase, 50 Hz.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the correct order of a power supply system?
   A. Distribution, transmission, generation, consumer
   B. Transmission, generation, distribution, consumer
   C. Generation, transmission, step-down and sub-transmission, secondary distribution, consumer
   D. Consumer, generation, transmission, distribution
   Answer: C. Energy is generated, transmitted at high voltage, stepped down at substations, and distributed to consumers. (Key point: Stages of supply)

2. Why is voltage raised by step-up transformers before transmission?
   A. To increase the current and loss
   B. To make the power single-phase
   C. To reduce frequency
   D. So that current and I^2R loss are small over long distances
   Answer: D. For the same power, higher voltage means lower current, so I^2R loss is much smaller. (Key point: Why high transmission voltage)

3. What is supplied by the secondary distribution in a three-phase four-wire system?
   A. 400 V line and 230 V phase
   B. 11 kV line and 400 V phase
   C. 230 V line and 400 V phase
   D. 33 kV line and 11 kV phase
   Answer: A. Distribution transformers step down to 400 V between lines and 230 V per phase; homes get 230 V single-phase. (Key point: Distribution voltage)

## Related in Basic Electrical and Electronics Engineering

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