What you'll learn
The National Grid is the network of cables and transformers that distributes electricity from power stations to consumers across the UK. You need to understand why high voltages are used for transmission, how transformers work, and how to calculate power losses in transmission lines. This topic combines your knowledge of electricity, power calculations and energy efficiency.
Key terms and definitions
National Grid — the network of cables and transformers that distributes electricity from power stations to homes, schools and businesses across the country
Transformer — a device that changes the voltage of an alternating current supply; contains two coils of wire wrapped around an iron core
Step-up transformer — a transformer that increases voltage (and decreases current) for efficient transmission over long distances
Step-down transformer — a transformer that decreases voltage (and increases current) to safe levels for domestic and commercial use
Power loss — energy wasted as heat in transmission cables due to the resistance of the wires, calculated using P = I²R
Transmission cables — thick cables (usually overhead power lines) that carry electrical energy at high voltage across long distances
Alternating current (AC) — electric current that continuously changes direction; essential for transformers to function
Potential difference — the voltage between two points; measured in volts (V)
Core concepts
Structure of the National Grid
The National Grid connects power stations to consumers through a series of transformers and cables:
Power station → step-up transformer → transmission cables → step-down transformer → consumers
At each stage, the voltage is changed to optimise efficiency and safety:
- Power stations generate electricity at 25,000 V (25 kV)
- Step-up transformers increase voltage to 400,000 V (400 kV) or 275,000 V for transmission
- Transmission cables carry electricity across the country at very high voltage
- Step-down transformers at substations reduce voltage to 33,000 V
- Further step-down transformers reduce voltage to 230 V for homes and businesses
The National Grid allows electricity to be distributed from any power station to any location, providing flexibility and reliability. If one power station fails, others can compensate.
Why high voltage transmission is used
Transmitting electricity at high voltage significantly reduces energy losses during transmission. This is the most important concept in this topic and frequently appears in exam questions.
The physics behind high voltage transmission:
When electricity flows through cables, some energy is wasted as heat due to the resistance of the wires. The power lost is calculated using:
P = I²R
Where:
- P = power lost (W)
- I = current (A)
- R = resistance (Ω)
For the same amount of power transmitted (P = VI), increasing the voltage allows the current to be reduced proportionally. Since power loss depends on I² (current squared), even a small reduction in current produces a large reduction in wasted energy.
Example calculation:
If voltage is doubled, current is halved (to transmit the same power). Since power loss = I²R, halving the current reduces power loss to one quarter (½)² = ¼.
If voltage is increased by 10 times, current decreases to 1/10, and power loss is reduced to (1/10)² = 1/100 of the original value.
This is why the National Grid uses 400 kV for transmission rather than the 230 V used in homes — it dramatically improves efficiency.
How transformers work
Transformers only work with alternating current (AC), not direct current (DC). This is because they rely on a changing magnetic field.
Basic structure:
- Primary coil (input) — connected to the power source
- Secondary coil (output) — connected to the load or next part of the circuit
- Iron core — links both coils and concentrates the magnetic field
Operating principle:
- AC flows through the primary coil
- This creates a changing magnetic field in the iron core
- The changing magnetic field induces an alternating voltage in the secondary coil
- The size of the induced voltage depends on the number of turns in each coil
The transformer equation:
$$\frac{V_p}{V_s} = \frac{n_p}{n_s}$$
Where:
- V_p = potential difference across primary coil (V)
- V_s = potential difference across secondary coil (V)
- n_p = number of turns on primary coil
- n_s = number of turns on secondary coil
For 100% efficient transformers (assumed in GCSE calculations):
Input power = output power
$$V_p \times I_p = V_s \times I_s$$
Where I_p and I_s are the currents in the primary and secondary coils respectively.
Step-up transformers:
- More turns on secondary coil than primary (n_s > n_p)
- Voltage increases, current decreases
- Used at power stations
Step-down transformers:
- Fewer turns on secondary coil than primary (n_s < n_p)
- Voltage decreases, current increases
- Used before electricity reaches consumers
Energy efficiency in transmission
Real transformers and transmission systems are not 100% efficient. Energy is wasted as:
In transformers:
- Heat in the coils due to resistance
- Heat in the iron core due to eddy currents
- Sound energy (transformers often hum)
Typical large transformers are 98-99% efficient.
In transmission cables:
- Heat due to resistance (I²R losses)
- This is minimised by using thick cables with low resistance and high voltage (low current)
The overall efficiency of the National Grid is approximately 92%, meaning about 8% of generated electricity is lost during transmission and distribution.
Safety and practical considerations
Why domestic supply is 230 V:
While high voltages are efficient for transmission, they are extremely dangerous for domestic use:
- 400 kV would be lethal and require extensive insulation
- 230 V provides enough power for household appliances while being relatively safer
- Underground cables and proper insulation make 230 V manageable in homes
Environmental and practical factors:
- Transmission cables are usually overhead to reduce costs and make maintenance easier
- Overhead cables are supported by pylons, which have visual impact on landscapes
- Underground cables are used in urban areas and areas of natural beauty, but cost 10-20 times more
- Cable thickness is a compromise between low resistance (thick cables) and cost/weight
Worked examples
Example 1: Transformer calculations
Question: A step-up transformer at a power station has 200 turns on its primary coil and 8000 turns on its secondary coil. The input voltage is 25,000 V. Calculate the output voltage. [3 marks]
Solution:
Step 1: Write down the transformer equation $$\frac{V_p}{V_s} = \frac{n_p}{n_s}$$
Step 2: Substitute values $$\frac{25000}{V_s} = \frac{200}{8000}$$
Step 3: Rearrange and solve $$V_s = \frac{25000 \times 8000}{200}$$ $$V_s = 1,000,000 \text{ V} = 1000 \text{ kV}$$
Mark scheme:
- Correct equation [1 mark]
- Correct substitution [1 mark]
- Correct answer with unit [1 mark]
Example 2: Power and current calculations
Question: A power station transmits 50 MW of electrical power at 400 kV.
(a) Calculate the current in the transmission cables. [3 marks]
(b) Explain why transmitting at 400 kV rather than 25 kV reduces energy losses. [3 marks]
Solution:
(a)
Step 1: Write power equation P = VI
Step 2: Rearrange for current I = P ÷ V
Step 3: Convert units and substitute P = 50 MW = 50,000,000 W V = 400 kV = 400,000 V $$I = \frac{50,000,000}{400,000} = 125 \text{ A}$$
Mark scheme:
- Correct equation or rearrangement [1 mark]
- Correct unit conversion [1 mark]
- Correct answer with unit [1 mark]
(b)
Power loss in cables = I²R [1 mark]
Higher voltage means lower current for the same power transmitted [1 mark]
Since power loss depends on current squared, reducing current significantly reduces energy wasted as heat / using 400 kV instead of 25 kV reduces current by a factor of 16, reducing power loss by a factor of 256 [1 mark]
Example 3: Comparing power losses
Question: Transmission cables have a total resistance of 10 Ω. Compare the power lost when 10 MW is transmitted at:
- (a) 25 kV [3 marks]
- (b) 400 kV [2 marks]
Solution:
(a) At 25 kV:
Step 1: Calculate current I = P ÷ V = 10,000,000 ÷ 25,000 = 400 A [1 mark]
Step 2: Calculate power loss P = I²R = 400² × 10 = 1,600,000 W [1 mark]
Power loss = 1.6 MW [1 mark]
(b) At 400 kV:
Current = 10,000,000 ÷ 400,000 = 25 A Power loss = 25² × 10 = 6,250 W = 6.25 kW [2 marks]
Conclusion: Transmitting at 400 kV reduces power loss from 1.6 MW to 6.25 kW — a reduction by a factor of 256.
Common mistakes and how to avoid them
Confusing step-up and step-down transformers — Remember: step-UP increases voltage for transmission, step-DOWN decreases voltage for consumers. Use the word "transmission" as your trigger for "step-up"
Forgetting to convert units — Power stations generate MW (megawatts), transmission uses kV (kilovolts). Always convert to base units (W, V, A) before calculating: 1 MW = 1,000,000 W, 1 kV = 1,000 V
Using P = V²/R instead of P = I²R for transmission losses — The resistance R in transmission cables is constant. Current determines the power lost, so always use P = I²R for calculating energy wasted in cables
Thinking transformers work with DC — Transformers only function with alternating current because they require a changing magnetic field to induce voltage in the secondary coil
Not explaining WHY high voltage reduces losses — Don't just state "high voltage is more efficient." Explain the physics: high voltage → low current → I²R losses are reduced because power loss depends on current squared
Mixing up primary and secondary coils in calculations — Label your variables clearly: subscript p for primary, s for secondary. Check whether you're dealing with a step-up (n_s > n_p) or step-down (n_s < n_p) transformer
Exam technique for "The National Grid and electricity transmission"
"Explain" questions (3-4 marks) — Must include physical reasoning, not just statements. For high voltage transmission: state the equation P = I²R, explain that higher voltage means lower current for the same power, then conclude that lower current means less power wasted
Calculation questions — Show all working clearly. Write the equation first, substitute values on the next line, then calculate. Always include units in your final answer. Check if the question uses kV, MW or kW and convert appropriately
Command word "compare" — You must make a direct comparison with comparative language ("higher," "lower," "more efficient"). Stating two separate facts without linking them loses marks
6-mark extended response questions — Structure your answer logically: describe the path from power station to consumer, explain the role of each transformer, justify why high voltage is used (with P = I²R), and mention efficiency or safety considerations
Quick revision summary
The National Grid distributes electricity from power stations to consumers using transformers and transmission cables. Step-up transformers increase voltage to 400 kV for transmission, dramatically reducing current and therefore power losses (P = I²R) in cables. Step-down transformers then reduce voltage to safe levels (230 V) for homes. Transformers work using electromagnetic induction and only function with AC. The transformer equation V_p/V_s = n_p/n_s links voltage to the number of coil turns. High voltage transmission is essential for energy efficiency across long distances.