The National Grid

Electricity is generated in power stations and transported across the UK via the National Grid.

To move power around the National Grid:

  • before electrical power leaves a power station it is transferred at high voltages by using 'step-up' transformers to increase the voltage to 275,000 V or 400,000 V
  • before electrical power enters homes and factories the voltages are decreased by 'step-down' transformers to 33,000 V (large factories), 11,000 V (medium factories), 230 V (homes, shops, small factories and offices)

The transfer of electrical energy via the grid is very efficient. When currents in a cable are higher, more energy is dissipated to the surroundings through heating. As high currents waste more energy than low currents, electrical power is transported around the grid at a high voltage and a low current.

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

Step-up transformers

A transformer is a device that can change the voltage of an alternating current (ac). A basic transformer is made from two coils of wire ‐ a primary coil from the ac input and a secondary coil leading to the ac output. The coils are not electrically connected. Instead, they are wound around an iron core. This is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil.

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

Step-up transformers are used to increase or 'step-up' voltages. These are used when electrical power output at the power station is stepped up from 25,000 V to 275,000 V or 400,000 V for transportation around the UK. A step-up transformer has more turns of wire on its secondary coil than it does on its primary coil. Transformers will only work with an alternating current (ac) input. This transformer steps up the voltage by reducing the current.

Graphic showing a square shaped iron core with a hole in the middle. Two coils are on the core, one is wrapped on the left side of the core 5 times; the other on the right side is wrapped 20 times.

Step-down transformers

Step-down transformers are used to decrease or 'step-down' voltages. These are used when voltages need to be lowered for use in homes and factories. A step-down transformer has fewer turns of wire on the secondary coil than on the primary coil. This transformer steps down the voltage by increasing the current.

Graphic showing a square shaped iron core with a hole in the middle. Two coils are on the core, one is wrapped on the left side of the core 50 times; the other on the right side is wrapped 10 times.