Section 1 of 9Layout of the National Grid
1Power station
- Power station Generators produce electricity at about 25 000 V.
2Step-up transformer
- Step-up transformer Raises the p.d. to as much as 400 000 V and lowers the current.
3Transmission lines
- Transmission lines Overhead cables on pylons, or buried cables, carry electricity across the country.
4Step-down transformers
- Step-down transformers Reduce the p.d. in stages to 230 V before it reaches homes and businesses.
Section 2 of 9Why high voltage is used
1
For a given power, P = I × V, raising the potential difference means the current can be much smaller.
2
Power wasted as heat in the cables is P = I2 × R, so a smaller current reduces losses dramatically.
3
Halving the current cuts the heat loss to a quarter; raising the voltage ten times cuts the loss by a factor of one hundred.
4
Therefore, less energy is wasted and the grid is more efficient.
Section 3 of 9Worked example: effect of voltage
| Transmission p.d. | Current for 100 kW | Power lost in cable of resistance 10 Ω |
|---|---|---|
| 1000 V | I = P ÷ V = 100 000 ÷ 1000 = 100 A | P = I2R = 1002 × 10 = 100 000 W (all of it) |
| 10 000 V | I = 100 000 ÷ 10 000 = 10 A | P = 102 × 10 = 1000 W (1%) |
| 100 000 V | I = 100 000 ÷ 100 000 = 1 A | P = 12 × 10 = 10 W (0.01%) |
Section 4 of 9Pylons and cables
Overhead lines
- Overhead lines Cheaper to build and easier to repair, but spoil the landscape and can be damaged by weather.
Underground cables
- Underground cables Hidden and protected from storms, but far more expensive to install and fix.
Low-resistance conductors
- Low-resistance conductors Cables are made of aluminium with a steel core, which is a good conductor that is also light and strong.
Section 5 of 9Transformer roles
| Transformer | Where | Effect on p.d. | Effect on current |
|---|---|---|---|
| Step-up | At the power station | Increases | Decreases |
| Step-down | Near homes, factories and towns | Decreases | Increases |
Section 6 of 9How a transformer works
Transformer
Primary coil
- Primary coil An alternating current in the primary coil produces a changing magnetic field in the iron core.
Iron core
- Iron core The soft iron core links the changing magnetic field to the secondary coil.
Secondary coil
- Secondary coil The changing field induces an alternating p.d. in the secondary coil.
- This only works with a.c., not d.c.
Turns ratio
- Turns ratio Vp ÷ Vs = Np ÷ Ns.
- More turns on the secondary makes a step-up transformer; fewer makes a step-down.
- For an ideal transformer Vp × Ip = Vs × Is.
Section 7 of 9Costs and benefits of the Grid
Reliability
- Reliability Linking many power stations means demand can be met by sharing the supply, and a fault in one station does not cut power to everyone.
Matching demand
- Matching demand Operators bring extra stations online at peak times, such as winter evenings, and reduce output overnight when demand is low.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| National Grid | The network of cables and transformers that carries electricity from power stations to consumers. |
| Transformer | A device that changes the p.d. of an alternating supply. |
| Step-up transformer | A transformer that increases the potential difference. |
| Step-down transformer | A transformer that decreases the potential difference. |
| Efficiency | The proportion of energy supplied that is usefully transferred. |
| Alternating current | Current that repeatedly reverses direction, needed for transformers to work. |
Section 9 of 9Exam technique
1
Saying high voltage is used because it makes the electricity travel faster; the real reason is the lower current and reduced heating.
2
Mixing up step-up and step-down — the step-up transformer is next to the power station.
3
Claiming the current is increased in the transmission lines; it is reduced.
4
Using P = I × V to explain the losses in the wires instead of P = I2 × R.
5
Suggesting transformers work with direct current.