Core Physics Topic 8 - The National Grid

How do we get electricity to our homes?

Electrical energy cannot be stored as electricity. In a battery it is converted to chemical energy. We are constantly using electricity, so it has to be constantly generated.

Electricity is generated in large quantities in power stations. We have looked at power stations in Topic 7.

Electricity is passed from the power station to our homes through a complex network of wires called the National Grid. When you see pylons, these are part of the National Grid. The Grid is constructed in such a way that if one line of wires (a transmission line) fails, other lines can take over so that the electricity supply to our homes does not fail.

Generally the system is very reliable, although overhead power lines are vulnerable to:

• lightning strikes;

• high winds;

• heavy snowfall.

Underground cables are less vulnerable, but are much more expensive to put in.

Mains electricity comes in to our homes at a voltage of 230 Volts (V). However it is not generated at 230 V because the current needed would be huge. Big currents need heavy cables that get hot. In the power station the electricity is generated at 25 000 V (at a current of 100 000 A).

Outside the power station there are huge step-up transformers that take the voltage from 25 000 V to 275 000 V. The voltage can be as high as 415 000 V in the super grid.

Power = current (A) × Voltage (V)

In Physics Code:

P = IV

As the voltage goes up ten times, the current comes down ten times.

The picture on the left shows a step up transformer while on the right are the huge wires that feed it.

Obviously 275 000 V is far too high a voltage to use, so step-down transformers are used to reduce the voltage as follows:

 Application Voltage Local distribution 33 000 V Railways 25 000 V Heavy industry 11 000 V Light Industry 415 V Homes 230 V

The drawing shows a very simplified diagram of the National Grid.

The reason for voltage being so high is that the current is lower. Therefore the wires can be thinner, and don't get so hot. Therefore less energy is wasted. Even so there is a fair amount of energy lost in heating up the wires which heats up the countryside.

 Question 1 Fill in the spaces in the interactive question.