The National Grid

In a nutshell

The National Grid is the infrastructure for the electricity distribution from the power stations to consumers across the country. In order to achieve optimal efficiency, the National Grid uses a series of transformers to transport the electricity over vast distances. Transformers are devices which can step-up or step-down voltage.

Equations

Word equation	symbol equation
$power=potential\,difference\times current$	$P=VI$
$power=(current)^2 \times resistance$	$P=I^2R$

Variable definitions

Quantity name	symbol	unit name	unit
$power$	$P$	$Watt$	$W$
$potential \, difference$	$V$	$Volt$	$V$
$current$	$I$	$Amp$	$A$
$resistance$	$R$	$Ohm$	$\Omega$

What is the National Grid?

The National Grid covers all of the electricity distribution in the United Kingdom. This includes the power stations, substations, transformers and all of the cables that connect everything together. The national grid is responsible for the delivery of the electricity from the power station, called the supply, to the consumers around the country, called the demand.

The National Grid has to make sure that the supply meets the demand. Electricity demand isn't always constant, there are surges in demand at various times of the day and week.

Example

In the morning between $06:00$ and $08:00$ a lot of people are waking up, having showers and putting the kettle on for a cup of tea. This means there is a higher demand than normal for electricity.

Power stations are often run at a low power output, which is enough to cover the background demand level. At times of peak demand, the power station can create more electricity to put into the national grid to cope with the extra demand. There are also backup power stations which can be brought online in times of very heavy demand.

Transformers

Transformers are devices which change the values of current and potential difference flowing in a wire. A transformer is a soft iron core which has two coils of wire on opposite sides.

The primary coil is the coil is the side of the input power. The secondary coil is the side of the output power. Transformers do not change power, the power input of a transformer is the same as the power output, it is just the current and voltage which changes.

$power=potential \,difference \times current \\ \ \\ P=VI$

As $P=VI$ , if the current is reduced by $\frac 15$ then the voltage must increase by a factor of five to keep power the same.

The number of wires determine the function of the transformer. Step-up transformers, step the voltage up and decrease the current. Step-down transformers step the voltage down and increase the current.

The number of coils on each side determines whether the transformer is a step-up or step-down transformer. Step-up transformers, step the voltage up and decrease the current. Step-down transformers step the voltage down and increase the current.

If the secondary coil has more coils than the primary coil, then it is a step-up transformer. If the primary coil has more coils than the secondary coil, then it is a step-down transformer.

A	Primary Coil
B	Soft Iron Core
C	Secondary Coil

Physics; Electricity; KS4 Year 10; The National Grid

The diagram shows a step-up transformer as there are more coils on the primary coil than there are on the secondary coil.

Efficiency

As electricity is transmitted across the National Grid, some of the electrical power is transferred to heat and will be lost to the surroundings.

The equation for power lost during transmission is given as:

$power=(current)^2 \times resistance \\ \ \\ P=I^2R$

Transformers are used to reduce the current being transmitted in the wires and thick wires are used to reduce the resistance.

Electricity leaving the power station is stepped up from approximately $25 000\ V$ to $400 000\ V$ . This increase in voltage by a factor eight decreases the current by $\frac 18$ . As $P\propto I^2$ it means that power loss is reduced by $(\frac18)^2=\frac{1}{64}$ . In other words, power loss is 64 times smaller by stepping up the voltage.

Before electricity can be distributed to consumers, it needs to be stepped down from its extremely high voltage to a much safer voltage ready for consumers to use.

Physics; Electricity; KS4 Year 10; The National Grid

1	Power station
2	Step-up transformer
3	National Grid distribution network
4	Step-down transformer
5	Consumers

Diagram shows all of the stages of how electricity is distributed across the country.