Alternating current and the National Grid

The National Grid

Main features of the national grid from power station to step-up  transformers then transmission lines, step-down transformers and finally consumers

The National Grid distributes electricity across the country. The National Grid connects power stations to homes, workplaces and public buildings all around the country. The electricity may be produced by a conventional power station turning a generator or by another method.


An iron core transformer, with the primary coil on the left hand side and secondary coil on the right hand side.

Transformers are used to change voltages and currents in transmission lines. A transformer is formed from two coils of wire around a magnetic core. The number of coils determines whether the transformers will step-up or step-down the voltage.

As the power transferred must stay the same:

  • increasing voltage decreases current
  • decreasing voltage increases current

In the National Grid, a step-up transformer is used to increase the voltage and reduce the current. The voltage is increased from about 25,000 Volts (V) to 400,000 V causing the current to decrease. Less current means less energy is lost through heating the wire.

To keep people safe from these high voltage wires, pylons are used to support transmission lines above the ground.

Before reaching the end user, a step-down transformer, reduces the voltage from the transmission voltage to the safer voltage of 230 V for home use.

Transmission lines

As an electric current flows through the thick cables held up by the pylons, they will get hotter and dissipate energy to the surroundings. The electrical power dissipated depends on current and resistance:

power = current2 × resistance

P = I^2 \times R

This is when:

  • power (P) is measured in watts (W)
  • current (I) is measured in amps (A)
  • resistance (R) is measured in ohms (Ω)

To ensure that the minimum amount of power is lost from the cables:

  • the cables are thick so that their resistance is low
  • high voltages are used to reduce the current through the transmission lines

A low resistance and a low current mean that the transmission wires will not heat up as much. As a result, most of the power is delivered to the consumer, and not lost through the wires.

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