What the national grid is
Definition
National grid
The network of cables and transformers that transfers electrical power from power stations to consumers across the country.
Definition
Power station
A site where an energy resource is used to drive generators that produce electricity on a large scale.
- The national grid links the power stations and wind farms of Great Britain to every home, school, hospital and factory that uses electricity.
- Power stations are often built far from the towns they supply, for example on the coast for cooling water or out at sea where the wind is strongest.
- The grid carries electrical power across these distances, so electricity generated in the north of Scotland can be used in London a fraction of a second later.
- Because every generator feeds into one shared network, a breakdown at one power station does not cut off the homes nearby, since other stations can make up the difference.
- The grid is made of overhead cables carried on steel pylons, underground cables in towns and cities, and substations where the potential difference is changed.
- Potential difference is measured in volts, V\text{V}V, and is often called voltage, especially when describing the grid.
High voltage from power stations
Definition
Substation
A site in the national grid where transformers change the potential difference of the supply.
- The generators in a typical large power station produce a potential difference of about 25 kV25\ \text{kV}25 kV, where 1 kV=1000 V1\ \text{kV} = 1000\ \text{V}1 kV=1000 V.
- Before the power leaves the site, a substation beside the power station raises the potential difference to a very high voltage.
- The main cross-country cables in Great Britain run at 275 kV275\ \text{kV}275 kV or 400 kV400\ \text{kV}400 kV, which is more than 100010001000 times the potential difference of a plug socket.
- This part of the grid is called the transmission network, and it is the part you see as lines of tall pylons striding across the countryside.
- The cables hang high above the ground and are held away from the steel pylons by long insulators, because such large potential differences are extremely dangerous.
Lower voltage in each locality
- When the power reaches a region, substations reduce the potential difference in stages to 132 kV132\ \text{kV}132 kV, then 33 kV33\ \text{kV}33 kV and then 11 kV11\ \text{kV}11 kV.
- These lower-voltage cables form the distribution network, which carries power along smaller poles or underground into each locality.
- A small substation in each street finally reduces the potential difference to about 230 V230\ \text{V}230 V for domestic use in homes.
- Large users such as factories and railways may take their supply at 11 kV11\ \text{kV}11 kV or 33 kV33\ \text{kV}33 kV and reduce it on site.
- Homes use a low potential difference because it is far safer for people to handle and allows appliances to be built with ordinary plastic insulation.
Example
Following power from a wind farm to a home
- An offshore wind farm off the Yorkshire coast generates electricity, and a substation raises its potential difference for the journey to shore.
- On land, the power joins the transmission network and travels west on pylon lines at 400 kV400\ \text{kV}400 kV.
- A substation outside Leeds lowers the potential difference to 132 kV132\ \text{kV}132 kV and then to 33 kV33\ \text{kV}33 kV for the city's distribution network.
- Further substations in the suburbs lower it to 11 kV11\ \text{kV}11 kV, and a street substation lowers it again to about 230 V230\ \text{V}230 V.
- The power then enters a house through its electricity meter and reaches the sockets at the same 230 V230\ \text{V}230 V.
Note
How this links to other grid topics
- Other parts of the national grid you should know include the step-up and step-down transformers that change the potential difference, which are covered in detail in 8.2.4.
- The reason the grid uses such high potential differences for transmission, and why this makes it efficient, is covered in 8.2.5.
Common Mistake
- Do not write that homes receive electricity at the same high voltage used in the pylon lines, because the potential difference is reduced in each locality first.
- Do not say that "voltage travels through the cables", because it is electrical power that is transferred while the potential difference describes the supply.
- Do not mix up the two networks, since high-voltage transmission links power stations across the country and lower-voltage distribution supplies each locality.
Exam technique
Recalling the grid in two stages
- A full description of the grid needs two stages: power is transferred at high voltage from power stations, then at lower voltage in each locality for domestic use.
- Name where each change happens, using the words power station, locality and domestic or homes.
- Write "potential difference" or "voltage" with the words high and low, rather than saying electricity is "strong" or "weak".
- Quote 230 V230\ \text{V}230 V as the domestic value when a question asks for a figure.
Self review
- What is the national grid?
- Is electrical power transferred from power stations at a high or a low potential difference?
- State the potential differences used on the main transmission cables in Great Britain.
- What happens to the potential difference when the power reaches each locality?
- Give one reason why homes are supplied at a low potential difference.
