The National Grid

The UK's power stations, coal, gas, oil, hydro and renewable all supply electricity into the National Grid System. The electricity supplied is in the form of an alternating current (ac). In the UK, the mains electrical supply is generated at a frequency of 50 Hertz (Hz) and is delivered to houses at 230 Volts (V).

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 burning fossil fuels to heat water. The steam then drives turbines that will then turn a generator or by other methods using water and wind turbines.

Transformers

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 an iron core. The coil ratio between the primary and secondary 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.

These very high voltages are reduced in stages to a safer level of 230 V for the consumer using step-down transformers.

Transmission lines

As an electric current flows through the steel reinforced aluminium 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 made as thick as possible 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 cables will not heat up as much. As a result, most of the power is delivered to the consumer, and not dissipated as heat in the cables.