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6.2.1Generating and Distributing Electricity and Household Use

The National Grid

5 min read 5 parts Lesson 8 of 22 Unit quiz

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Section 1 of 5Structure of the Grid

6.2.1 The National Grid
1
Step-up transformers at power stations raise the voltage to a very high value, typically 400 kV, for transmission.
2
Electricity flows along overhead transmission cables across the country at this high voltage and with a low current.
3
Step-down transformers at substations reduce the voltage to around 11 kV for local distribution.
4
Further step-down transformers reduce the voltage to the 230 V supply used in homes and businesses.

Section 2 of 5Why Transmit at High Voltage?

Power loss
  • Current flowing through the resistance of the cables transfers energy to the surroundings as thermal energy, which is wasted.
Power formula
  • The power lost in a cable is P = I² × R, so halving the current reduces the power lost to one quarter.
Transmission principle
  • For a fixed power, raising the voltage lowers the current, which greatly reduces the power lost in the cables.
Safe supply
  • Distributing electricity at lower voltages makes it safe for homes and businesses to use.

Section 3 of 5Grid Demand

Grid Demand
Peak demand
  • Demand is highest at around 6 pm in the UK, when many households use lighting, cooking and heating together.
Seasonal demand
  • Winter demand is higher than summer demand because people use more lighting and heating.
Monitoring
  • The grid monitors power usage and responds to changes in demand as they happen.
Balancing supply
  • Power stations can be started or stopped as needed so that the supply matches the demand at all times.

Section 4 of 5Worked Example

Question

  • A transmission cable has a resistance of 10 Ω and carries 1.0 MW of power.
  • Calculate the power lost in the cable at 10 kV and at 100 kV.

  1. At 10 kV: I = P ÷ V = 1 000 000 W ÷ 10 000 V = 100 A.
  2. Power lost = I² × R = 100² × 10 = 100 000 W (100 kW).
  3. At 100 kV: I = 1 000 000 W ÷ 100 000 V = 10 A.
  4. Power lost = 10² × 10 = 1000 W (1 kW).

Answer Raising the voltage tenfold cuts the power lost in the cable from 100 kW to 1 kW, a hundredfold reduction.

Section 5 of 5Exam Checklist & Tips

6.2.1 – Key Points for the Exam

  • National Grid The system of cables, transformers and substations that transmits and distributes electricity across the country.
  • Transmission The transfer of electricity at high voltage from power stations over long distances.
  • Step-up transformer A transformer that increases voltage and decreases current.
  • Step-down transformer A transformer that decreases voltage and increases current.
  • Distribution The delivery of electricity at lower voltage from substations to consumers.

Common Mistakes

  • Stating that high voltage increases the current in transmission cables; it decreases the current.
  • Using P = V × R to find the power lost in a cable; the correct formula is P = I² × R.
  • Thinking the grid transmits at low voltage to save energy; high voltage is used to reduce losses.
  • Saying the grid stores electricity; it transmits and distributes it as it is generated.
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