The National Grid
The national grid system is the network of cables which transport electricity from the power stations in Britain to homes, factories and other places that require it.
Power stations produce electricity at high currents. Electricity transmission at high currents would encounter a large resistance in the transmission wire and therefore lose a lot of its energy as heat. To prevent this, the current generated is passed through a step up transformer. Here the voltage is increased to as much as 400,000V and the current decreased (remember the relationship P = VxI an increase in voltage and decrease in current by the same ratio will give the same power).
The high voltage (400,000 Volts) electricity is carried along overhead lines and underground cables referred to as the supergrid. The voltage is the reduced in several stages making before reaching the end user. The voltage reduction is made in step down transformers.
The diagram below gives a simplified representation of the National Grid system: