What the National Grid does
Definition
National Grid
The National Grid is the nationwide network of cables, transformers and substations that transfers electrical power from power stations to consumers.
- The National Grid links power stations to homes, schools, shops, hospitals and factories through a nationwide network of cables, transformers and substations.
- The grid carries alternating current, which allows transformers to change the potential difference at each stage of the journey.
- Because every power station feeds the same network, electricity generated in one part of the country can supply demand in another part.
- A shared network means the supply continues when one power station is shut down for maintenance or fails unexpectedly, because the others take up its share.
- Demand changes through the day, so the grid allows extra power stations to be brought on line at peak times such as the early evening.
- The grid does not store energy, so the total power fed in has to match the total power taken out at every moment.
Transformers in the grid
Definition
Step-up transformer
A step-up transformer is a transformer with more turns on its secondary coil than on its primary coil, so the output potential difference is larger than the input potential difference.
Definition
Step-down transformer
A step-down transformer is a transformer with fewer turns on its secondary coil than on its primary coil, so the output potential difference is smaller than the input potential difference.
- A step-up transformer sits between the power station and the transmission cables, raising the potential difference to as much as 400 kV400\ \text{kV}400 kV.
- Step-down transformers at substations lower the potential difference again in stages, each stage suited to a different type of user.
- Heavy industry such as a steelworks may take its supply at 33 kV33\ \text{kV}33 kV or 11 kV11\ \text{kV}11 kV directly from a regional substation.
- A local substation on a housing estate holds the final step-down transformer, which supplies homes at 230 V230\ \text{V}230 V.
- The stages exist because 400 kV400\ \text{kV}400 kV would be far too dangerous inside a house, while 230 V230\ \text{V}230 V would be unusable for long-distance transmission.
- Transformers only work on a changing supply, so the whole grid has to run on alternating current for this arrangement to be possible.
The route from power station to socket
- Generation:
- A turbine turns the coil of an alternator, producing an alternating potential difference of roughly 25 kV25\ \text{kV}25 kV.
- Stepping up:
- A step-up transformer at the power station raises the potential difference to 275 kV275\ \text{kV}275 kV or 400 kV400\ \text{kV}400 kV for transmission.
- Transmission:
- Thick aluminium cables slung between pylons, or buried cables in towns and cities, carry the power across the country.
- Regional distribution:
- Grid substations step the potential difference down to 132 kV132\ \text{kV}132 kV and then to 33 kV33\ \text{kV}33 kV or 11 kV11\ \text{kV}11 kV for towns and industry.
- Local supply:
- A final step-down transformer in a local substation supplies homes at 230 V230\ \text{V}230 V through underground cables and a fuse box.
Overhead and underground cables
- Overhead cables are far cheaper to install, because the cable is supported in air rather than buried in a trench.
- Overhead cables are easier to inspect and repair, since a damaged section can be seen and reached from the ground.
- Overhead cables are held on ceramic or glass insulators and are surrounded by air, which is a good insulator and keeps the very high potential difference away from the earthed pylon.
- Overhead cables are exposed to wind, ice and lightning, and many people object to the appearance of pylons across open countryside.
- Underground cables are hidden and are protected from the weather, but they cost much more to lay and need thick insulation because they are not surrounded by air.
- A fault in an underground cable is slow and expensive to find and repair, so underground routes are usually kept for built-up areas.
Example
Following one route across the country
- A gas-fired power station in Yorkshire generates at 25 kV25\ \text{kV}25 kV using alternators driven by steam turbines.
- A step-up transformer on site raises the potential difference to 400 kV400\ \text{kV}400 kV before the power reaches the transmission lines.
- Overhead cables on pylons carry the power south to a grid substation, where a step-down transformer lowers it to 132 kV132\ \text{kV}132 kV.
- Further step-down transformers reduce it to 33 kV33\ \text{kV}33 kV for a nearby industrial estate and to 11 kV11\ \text{kV}11 kV for the local distribution network.
- A final step-down transformer in a housing estate substation supplies each home at 230 V230\ \text{V}230 V.
- Every stage of the journey uses alternating current, because a transformer produces no output from a steady supply.
Exam technique
Describing the grid
- Give the stages in order and name the type of transformer at each one, rather than writing that transformers are used along the way.
- Quote at least one real potential difference, such as 400 kV400\ \text{kV}400 kV for transmission or 230 V230\ \text{V}230 V for a home, to show the scale of each stage.
- State that the grid uses alternating current when a question asks why transformers can be used at all.
- For a comparison of overhead and underground cables, give a cost point and a maintenance point rather than two versions of the same idea.
- Use the term substation for the place where the potential difference is changed, since it is the word the mark schemes use.
Common Mistake
- Do not write that the National Grid stores electricity, because the power fed in must match the power taken out at all times.
- Do not swap the two transformers, since the step-up transformer is at the power station and the step-down transformers are near the users.
- Do not describe homes as being supplied at 400 kV400\ \text{kV}400 kV, because that is the transmission potential difference and not the domestic one.
- Do not say that pylons are insulated, since it is the ceramic or glass insulators and the surrounding air that keep the cables from the earthed metal.
- Do not claim that underground cables waste less power, because the potential difference and the current, not the position of the cable, decide the loss.
Self review
- State what the National Grid is made of and what it does.
- Describe the position and purpose of the step-up transformer and the step-down transformers in the grid.
- Give a typical potential difference used for transmission and a typical potential difference supplied to homes.
- Explain why the National Grid has to carry alternating current.
- Give one advantage and one disadvantage of overhead cables compared with underground cables.
