What a transformer does
Definition
Transformer
A device made of two coils of wire wound on an iron core that changes the size of an alternating potential difference.
- A transformer is the device the national grid uses to change the size of the potential difference as power is transferred from power stations to consumers.
- It is built from two separate coils of insulated copper wire wound around a shared iron core.
- The input coil is connected to the supply, and the output coil delivers power at a different potential difference to the next part of the grid.
- The two coils are not joined by any wire, so the transfer of energy from one coil to the other happens through the iron core.
- A transformer has no moving parts, and large grid transformers waste only a very small fraction of the energy passing through them.
- Transformers work only with an alternating supply, which is one of the main reasons the national grid uses alternating current.
- Each transformer is either step-up or step-down, depending on whether its output potential difference is larger or smaller than its input.
Step-up transformers
Definition
Step-up transformer
A transformer that increases the potential difference, so its output potential difference is greater than its input potential difference.
- A step-up transformer gives out a higher potential difference than the one it receives.
- In the grid, step-up transformers sit in the substation beside each power station.
- A typical one raises the 25 kV25\ \text{kV}25 kV produced by the generators to 400 kV400\ \text{kV}400 kV for the transmission cables.
- Wind farms and solar farms also use step-up transformers to connect their output to the high-voltage network.
- There is normally only one step-up stage between a power station and the transmission network, because the potential difference is raised in one large jump.
- A step-up transformer does not create extra energy, because the energy it transfers out each second cannot be greater than the energy supplied to it.
Step-down transformers
Definition
Step-down transformer
A transformer that decreases the potential difference, so its output potential difference is smaller than its input potential difference.
- A step-down transformer gives out a lower potential difference than the one it receives.
- Step-down transformers are used where the transmission network meets each region and again in substations throughout towns and cities.
- The potential difference is lowered in several stages, typically from 400 kV400\ \text{kV}400 kV to 132 kV132\ \text{kV}132 kV, then to 33 kV33\ \text{kV}33 kV, then to 11 kV11\ \text{kV}11 kV and finally to about 230 V230\ \text{V}230 V.
- The last step-down transformer is often housed in a fenced green cabinet or small brick building at the end of a street.
- Step-down transformers are also found in everyday devices, such as the chargers and adaptors that lower the 230 V230\ \text{V}230 V mains supply to the few volts needed by a phone or laptop.
Example
Identifying transformers along a route
- A power station generates at 25 kV25\ \text{kV}25 kV, and its transformer gives out 400 kV400\ \text{kV}400 kV, so the output is larger than the input and it is a step-up transformer.
- A regional substation receives 400 kV400\ \text{kV}400 kV and gives out 132 kV132\ \text{kV}132 kV, so it uses a step-down transformer.
- A town substation receives 33 kV33\ \text{kV}33 kV and gives out 11 kV11\ \text{kV}11 kV, so it also uses a step-down transformer.
- A street substation receives 11 kV11\ \text{kV}11 kV and gives out 230 V230\ \text{V}230 V, which is the final step-down stage before homes.
- Along the whole route there is one step-up transformer and several step-down transformers.
Common Mistake
- Do not say that a step-up transformer increases the power or the energy, because "step-up" describes the change in potential difference only.
- Do not say that a transformer generates electricity, because it only changes the potential difference of a supply that already exists.
- Do not suggest connecting a transformer to a cell or battery, because transformers do not work with a steady direct supply.
Exam technique
Describing transformers in the grid
- For each transformer, state its type, the change it makes to the potential difference, and where in the grid it is found.
- Write "a step-up transformer increases the potential difference at the power station" rather than "a step-up transformer is at the start".
- When a question gives input and output values, compare them before naming the transformer, since a larger output always means step-up.
Self review
- What does a transformer change?
- Describe the basic structure of a transformer.
- Where in the national grid is a step-up transformer used, and what does it do?
- Where are step-down transformers used, and why are several needed?
- Name the type of transformer that has an input of 33 kV33\ \text{kV}33 kV and an output of 11 kV11\ \text{kV}11 kV.
