2.11 - National Grid
The National Grid
- The National Grid is a nationwide system of cables and transformers that connects power stations to consumers.
- It transfers electrical power from power stations to wherever it's needed, such as homes and industry.
Step-up and step-down transformers raise the potential difference for transmission, then lower it.
Matching supply to demand
- Electricity demand changes throughout the day: it rises in the morning, when people get home from work or school, and during cold or dark weather.
- The minimum demand over a period is called the base load.
How the grid copes with changes in demand
- Power stations often run below their maximum output, leaving spare capacity for sudden surges in demand.
- Stations with short start-up times are kept on standby to respond quickly.
- Electricity can also be imported or exported through links with other countries.
Transmitting at high potential difference
- Transmitting electricity at a high current causes the cables to heat up, wasting energy as heat to the surroundings (P = I2R).
- Electrical power is given by P = IV, so for a given power, increasing the potential difference lowers the current.
- Transmitting at a very high potential difference (around 400,000 V) keeps the current low, minimising thermal energy loss and making the grid much more efficient.
Step-up and step-down transformers
- Transformers change the potential difference of the alternating current on the National Grid.
- Step-up transformers increase the potential difference to around 400,000 V as electricity leaves the power station. This decreases the current, reducing energy wasted as heat in the transmission cables.
- Step-down transformers decrease the potential difference to safe, usable levels (230 V in UK homes) before electricity reaches consumers.
TermDefinition
National GridA nationwide system of cables and transformers that connects power stations to consumers.
Base loadThe minimum demand for electricity over a given period.
TransformerA device that changes the size of an alternating potential difference (voltage).
Step-up transformerA transformer that increases the potential difference (voltage) and decreases the current. It has more turns on its secondary coil than on its primary coil.
Step-down transformerA transformer that decreases the potential difference (voltage) and increases the current. It has fewer turns on its secondary coil than on its primary coil.