Power Transmission and Three-Phase

Introduction

When you switch on a light, electricity may have travelled hundreds of kilometres from where it was generated. Moving it efficiently requires some clever physics. This lesson is conceptual. Working on any part of the power system requires specific training, authorisation and safety procedures.

The grid in stages

1. Generation: power stations turn other energy into electricity, using turbines driven by steam, gas, water or wind, and solar panels that produce it directly.
2. Step-up transformers raise the voltage for transmission.
3. Transmission: high-voltage lines carry power long distances.
4. Substations step the voltage down in stages.
5. Distribution: medium-voltage lines carry power through towns.
6. Local transformers reduce it to the voltage used in buildings.
7. Consumers: homes, shops and factories.

Supply and demand must match at every moment. Grid operators balance them continuously with controllable plants, storage and demand management.

Why high voltage?

Power is P = V × I. To deliver a given power, you can use high voltage with low current, or low voltage with high current. But the loss in the cables is the power wasted as heat:

P_loss = I² × R

Current is squared, so reducing current has a huge effect. Example: deliver 10 MW through lines of the same resistance.

• At 10 kV: I = 10,000,000 ÷ 10,000 = 1000 A
• At 100 kV: I = 10,000,000 ÷ 100,000 = 100 A

The current is 10 times smaller, so the loss is 100 times smaller. That is why transmission uses very high voltages, and why transformers (which work with AC) were so important to power systems.

Three-phase power

Single-phase supply, as in most homes, is one alternating voltage. Three-phase uses three AC voltages of the same size, each 120 degrees apart in time. At any moment the three waves add up in a balanced way, so the total power delivered is smooth rather than pulsing.

Advantages:

• More power through the same amount of copper than single phase
• Smoother power, especially for motors
• Self-starting induction motors with simple, rugged designs

Three-phase is used for transmission, industry, large buildings and big motors. Homes in many countries get one phase, and some get all three.

Line and phase voltages

In a typical star (wye) connection, the voltage between a phase wire and neutral is the phase voltage. The voltage between two phase wires is the line voltage, which is larger by a factor of √3 (about 1.732):

V_line = √3 × V_phase
230 V × 1.732 ≈ 400 V

That is why a European three-phase system is described as 230/400 V.

Distribution safety and standards

• Utilities follow detailed standards and safety rules for design, earthing, protection and operation.
• Only trained, authorised people work on the network. Fallen power lines are dangerous, so stay well away and call the emergency services or the utility.
• Never fly kites or drones, or climb or place ladders near overhead lines.

Try it yourself

A town needs 5 MW delivered over a line. Calculate the current at 20 kV and at 200 kV. By what factor does the line loss change? Check your answer: the current drops tenfold, so the loss drops by about a hundred times.

Summary

• The grid links generation, transmission, substations and distribution, with supply and demand matched continuously.
• High-voltage transmission cuts current, and loss is I²R, so it falls dramatically.
• Three-phase uses three voltages 120 degrees apart, gives smoother, more efficient power and has V_line = √3 × V_phase.