# The transmission of electricity

The transformer is a vital part of the National Grid that distributes electrical energy around the country.
Electrical energy is generated in power stations by generators at a potential of 25 kV. It is first stepped up to 400 kV by a transformer and then transmitted across the country in aluminium cables roughly 2 cm in diameter.
High voltages are used because the power loss per kilometre (I2R) for a given power output will be much less at high voltage and low current than at low voltage and high current. Despite this, even after the current has been reduced many transmission lines carry up to 2500 A! (What must the current output from the generators be in these cases?)

In Britain the grid system can meet a simultaneous demand of 56 000 MW supplied through some 8000 km of high-voltage transmission line. Alternating current is used in the National Grid, although this has not always been the case, because it may be transformed to high voltage. However, the underground cross-Channel link between Britain and France uses d.c. because of the large losses in the dielectric with a.c.

A simplified diagram of part of the grid system is shown in the following diagram.

Teacher demonstration

Construct a model power line using a 12 V power supply as the 'power station' and two 1 m lengths of constantan wire as the power lines. Investigate the power losses with 12 V d.c. and then with 12 V d.c and finally with two 20:1 transformers.

A lamp should be placed at the power station end and another at the house end.

DANGER: AVOID CONTACT WITH THE HIGH VOLTAGE SECTION OF THE LINES. THEY SHOULD BE PROTECTED BEHIND A PLASTIC SCREEN.

A simulated version of the experiment can be found at the following animation:

### schoolphysics: Power lines animation

To see an animation of the power lines simulation click on the animation link.