The National Grid

The National Grid is a nationwide system of cables and transformers that connects power stations to homes, schools and businesses. It carries electrical power from wherever it is generated to wherever it is needed, often across hundreds of kilometres.

The grid has two jobs to balance. It must deliver a large amount of power to consumers, and it must do so without wasting too much energy in the cables along the way. The device that makes this possible is the transformer, a component that changes the size of the potential difference in the circuit.

The National Grid is a system of cables and transformers linking power stations to consumers.

Electricity leaves a power station, its potential difference is raised for transmission, it travels across the grid, and its potential difference is lowered again before it reaches your home. The next slides explain why this is done and why it saves energy.

Two types of transformer sit at opposite ends of the transmission journey.

A step-up transformer is placed between the power station and the transmission cables. It increases the potential difference to a very high value, sometimes hundreds of thousands of volts. Raising the potential difference lowers the current, and the low current is the key to efficient transmission.

A step-down transformer sits near the consumer. It decreases the potential difference back down to a much lower, safer value suitable for use, ending at about 230 V for domestic mains.

Transformer	Where it is	Effect on pd	Effect on current
Step-up	After the power station	Increases	Decreases
Step-down	Before homes and businesses	Decreases	Increases

A step-up transformer raises the pd for transmission; a step-down transformer lowers it for domestic use.

The very high transmission voltage would be lethal in the home, so the step-down transformer is essential for safety as well as for supplying appliances at a usable pd.

A transformer changes the potential difference without changing the total power carried (transformers are treated as very efficient). Electrical power is $P=VI$, so if the power stays the same, raising the potential difference must lower the current in the same proportion.

$P=VI$

• $P$ = power, in watts (W)
• $V$ = potential difference, in volts (V)
• $I$ = current, in amps (A)

$P=VI$ is a recall-and-apply equation; it is not given on the Physics equation sheet.

Worked example. A power station delivers 400 MW of power. Compare the current needed at 25 000 V and at 400 000 V.

Rearrange $P=VI$ to $I=P/V$ (with $P=400000000\text{ W}$):

At 25 000 V: $I=\frac{400000000}{25000}=16000\text{ A}$.

At 400 000 V: $I=\frac{400000000}{400000}=1000\text{ A}$.

Raising the transmission voltage 16 times cuts the current 16 times, from 16 000 A down to 1000 A, while still delivering the same 400 MW.

The cables have resistance, so some energy is always wasted heating them up. The power wasted this way depends on the square of the current, given by $P=I^{2}R$.

$P=I^{2}R$

• $P$ = power wasted heating the cable, in watts (W)
• $I$ = current in the cable, in amps (A)
• $R$ = resistance of the cable, in ohms (Ω)

$P=I^{2}R$ is a recall-and-apply equation; it is not given on the Physics equation sheet.

Because the current is squared, halving the current cuts the wasted power to a quarter. This is the whole reason the grid transmits at high voltage: high voltage allows a low current, and a low current wastes far less energy in the cables.

Worked example. A cable has a resistance of 5 Ω. Compare the power wasted at 16 000 A and at 1000 A (the two currents from the previous slide).

At 16 000 A: $P=I^{2}R=16000^2\times 5=2.56\times 10^8\times 5=1.28\times 10^9\text{ W}$.

At 1000 A: $P=I^{2}R=1000^2\times 5=1000000\times 5=5\times 10^6\text{ W}$.

Cutting the current 16 times cuts the wasted power 256 times ($16^2=256$). The high-voltage route wastes a tiny fraction of the power that the low-voltage route would.

Putting the pieces together explains why the National Grid is an efficient way to transfer energy across a country.

The reasoning runs in a chain:

1. Step-up transformers raise the potential difference to a very high value.
2. Because power is delivered at high pd, only a small current is needed ($P=VI$).
3. A small current wastes very little energy heating the cables ($P=I^{2}R$, current squared).
4. Step-down transformers then lower the pd to a safe value for consumers.

Transformers themselves are very efficient, so the energy they lose is small compared with the huge saving from transmitting at a low current. The diagram below shows the split for a simplified 400 MW supply based on the earlier figures.

Transmitting the same power at a low voltage would waste far more, so the grid design keeps the losses small.

1. Saying transformers increase the power

Transformers change the potential difference, not the power. They are treated as efficient, so the power in roughly equals the power out. What a step-up transformer does is raise the pd and lower the current together.

2. Getting step-up and step-down the wrong way round

Step-up comes first, near the power station, to raise the pd for transmission. Step-down comes last, near the consumer, to lower it for safe use. Link each one to its position and its purpose.

3. Not explaining why low current matters

Stating that the grid uses high voltage is not enough for full marks. Explain that high voltage means a low current, and that a low current wastes less energy because the power lost in the cables depends on the current squared ($P=I^{2}R$).

4. Forgetting the domestic pd is about 230 V

The step-down transformer brings the pd down to the mains value of about 230 V for use in homes, not to zero and not to some tiny voltage. High transmission voltages would be dangerous indoors.

5. Confusing wasted heating with a current being "used up"

Current is not used up in the cables. The energy wasted comes from the cable's resistance heating up as charge flows through it, and reducing the current reduces that heating.