The demand for electricity can alter greatly from hour to hour, day to day, and from month to month. It also varies from one region to another. A single power station could not function effectively, or fulfill the unpredictable requirements of its customers all by itself. For this reason, each power station is part of a network or grid, which are all interconnected and therefore deliver their electricity into a common pool. This means that extra needs for electricity in one area can be met by superfluous power generated in another.
Power is distributed around the national grid at very high voltages (400,000 volts in the UK) carried on steel wires supported on tall structures called pylons. Cables are kept apart and insulated from the metal pylons by ceramic insulators. Once the grid reaches an industrial or urban area it feeds into a substation where the high voltage is `stepped down' to varying degrees for different customers, for example industry takes its electricity at a much higher voltage than domestic customers.
In many countries, the network system can operate nationwide. Britain has the largest grid system in the world. It consists of over 140 power stations which are capable of supplying 55,000 megawatts of electricity simultaneously, in both England and Wales.
The British grid has been joined to the French electricity network, by way of a submarine cable. This benefits both countries by allowing excess electricity to be sold in either direction.
