This guide covers BTEC Level 3 Unit 18 Electrical Power Transmission and Distribution, listed in the Pearson BTEC Level 3 Nationals in Engineering (2016) as Unit 18: Electrical Power Distribution and Transmission. It explains what the assignments usually ask, how to plan each one, and gives model paragraphs plus two fully worked calculations on transmission losses and distribution cable design.
What the Unit 18 Electrical Power Transmission and Distribution assignment asks you to do
Unit 18 follows electricity from the power station to the customer. Centres usually split it into three assignments that match the three stages of the system:
- Generation: investigate how thermal and nuclear power stations produce electricity that is fed into the national grid.
- Transmission: examine the design of a typical transmission network, including voltage levels, transformers, overhead lines, cables and protection.
- Distribution: design a distribution system that meets a customer’s requirements for a new electrical installation, such as a small industrial unit.
A common scenario puts you in the role of a trainee engineer at a distribution network operator or an electrical contractor, producing a technical report, annotated diagrams and a design with calculations. Your brief may word the tasks differently, so map each task to the stage it covers before you start.
Learning aims for Unit 18
The table summarises the three areas the unit is built around. Use the exact learning aim wording printed on your own brief.
| Learning aim | What it covers |
|---|---|
| A: Power generation for the national grid | Thermal (coal, gas, combined-cycle) and nuclear stations; energy conversions; the steam cycle; reactor types; efficiency, output and environmental issues |
| B: The transmission network | Why power is transmitted at high voltage; UK voltage levels; step-up and step-down transformers; overhead lines versus underground cables; HVDC links; substations, switchgear and protection |
| C: Distribution to the customer | Primary and secondary distribution; radial, ring and interconnected networks; earthing arrangements; selecting cables and protective devices; meeting the customer’s load and supply requirements |
Pass, Merit and Distinction criteria explained
We have not reproduced criterion codes here because they should be taken from your own brief. In general terms, the grades for this unit separate like this:
| Grade | What it asks | How to evidence it |
|---|---|---|
| Pass | Explain how thermal and nuclear stations generate electricity; describe the transmission network and its main components; produce a distribution design that meets the stated load | Labelled block diagrams of each station; a network diagram showing voltage levels; a design with correct current and cable calculations |
| Merit | Analyse or compare: for example compare thermal and nuclear generation, or analyse why particular transmission voltages and line types are used; justify design choices | Comparison tables with efficiency, start-up time, emissions and cost; calculations of losses at different voltages; reasons for every component chosen |
| Distinction | Evaluate: judge the suitability of generation methods and network designs, and evaluate your distribution design against alternatives and regulations | A reasoned judgement backed by numbers; consideration of reliability, safety, cost and future demand; reference to BS 7671 and the Electricity Safety, Quality and Continuity Regulations 2002 |
How to answer Learning aim A: thermal and nuclear generation
Show that you understand the energy chain, not just the names of the parts.
- Draw a block diagram for a thermal station: fuel, boiler, steam turbine, generator, condenser, cooling, generator transformer.
- Explain the energy conversions: chemical to heat, heat to kinetic (steam), kinetic to rotational, rotational to electrical.
- Explain combined-cycle gas turbine (CCGT) plant and why it is more efficient than a simple steam station.
- For nuclear, explain fission, the role of the moderator, control rods and coolant, and how the heat raises steam. Name the UK reactor types: advanced gas-cooled reactors (AGR) and the pressurised water reactor (PWR) at Sizewell B.
- For Merit and Distinction, compare the plants on efficiency, flexibility, fuel security, carbon emissions and waste.
Example paragraph: A nuclear station and a coal-fired station share the same back end: steam drives a turbine coupled to a three-phase synchronous generator, and the exhaust steam is condensed and pumped back as feed water. The difference is the heat source. In a pressurised water reactor, fission in the fuel heats water that is kept under high pressure so it does not boil; this primary circuit passes through a steam generator, where heat is transferred to a separate secondary circuit that drives the turbine. Keeping the two circuits apart means the turbine side does not carry radioactive coolant, which simplifies maintenance.
How to answer Learning aim B: the transmission network
- Show the voltage path: generation at around 11 to 25 kV, stepped up to 275 kV or 400 kV for the transmission system, then stepped down at grid supply points to 132 kV and lower distribution voltages.
- Explain why high voltage reduces losses: for the same power, a higher voltage means a lower current, and line loss is proportional to the square of the current (I²R).
- Compare overhead lines and underground cables: cost, visual impact, fault finding, cooling and capacitance.
- Describe HVDC links, such as interconnectors to other countries, and why DC suits long subsea routes.
- Describe substation equipment: transformers, circuit breakers, isolators, busbars, current and voltage transformers, and protection relays.
Example paragraph: Most of the transmission network is built as overhead lines because they are far cheaper per kilometre than buried cables at 400 kV. Air provides both insulation and cooling, and faults can be located and repaired quickly. Underground cables are used where overhead lines are not acceptable, such as in cities or protected landscapes. Their drawback on long AC routes is high capacitance, which draws charging current and limits the useful length, which is one reason subsea links are often built as HVDC.
How to answer Learning aim C: designing a distribution system
- Start with the customer’s requirements: total load, power factor, single or three-phase, any motor starting currents, and the distance from the supply.
- Choose a network arrangement (radial for a small site; ring for higher reliability) and an earthing system (for example TN-C-S), giving reasons.
- Calculate the design current, select a protective device and a cable, then check voltage drop.
- Present a single-line diagram with ratings and a component schedule.
- For Distinction, evaluate your design against an alternative, consider future expansion, and check it against BS 7671.
Example paragraph: A radial supply from the site’s main distribution board was chosen because the unit has one building and a modest load, so the added cost of a ring arrangement is not justified. The main risk of a radial design is that a cable fault interrupts the whole supply; this is reduced by using armoured cable on a protected route and by sizing it with spare capacity of about 25% so the planned second production line can be added without replacing the cable.
Worked example
Part 1: transmission losses at 132 kV and 400 kV
A three-phase line carries 100 MW at unity power factor. Each conductor has a resistance of 5 Ω. Compare the losses at 132 kV and 400 kV.
- Line current: I = P ÷ (√3 × V × cos φ).
- At 132 kV: I = 100,000,000 ÷ (1.732 × 132,000 × 1) = 437.4 A.
- Loss = 3 × I² × R = 3 × 437.4² × 5 = 2,870,000 W, about 2.87 MW (2.87% of the power sent).
- At 400 kV: I = 100,000,000 ÷ (1.732 × 400,000 × 1) = 144.3 A.
- Loss = 3 × 144.3² × 5 = 313,000 W, about 0.31 MW (0.31%).
- Check: the ratio of losses should equal (400 ÷ 132)² = 9.18. And 2.87 ÷ 0.313 = 9.17, which agrees within rounding.
Raising the voltage roughly three times cuts the losses about nine times. This is the core argument for high-voltage transmission.
Part 2: sizing a distribution circuit
A new workshop needs a 40 kW three-phase supply at 400 V with a power factor of 0.9. The cable run is 60 m.
- Design current: Ib = 40,000 ÷ (1.732 × 400 × 0.9) = 64.2 A.
- Select a protective device with In ≥ Ib: an 80 A device is chosen, which leaves room for future load.
- Select a cable whose rated current, after correction factors for grouping and temperature, is at least 80 A. Assume your BS 7671 table gives a suitable cable with a voltage drop of 2.5 mV/A/m.
- Voltage drop = (mV/A/m × Ib × length) ÷ 1000 = (2.5 × 64.2 × 60) ÷ 1000 = 9.6 V.
- As a percentage: 9.6 ÷ 400 × 100 = 2.4%. This is within the 5% guidance for non-lighting circuits supplied from a public low-voltage network, so the cable is acceptable.
The cable data value here is an assumption for illustration; use the actual table for your cable type and installation method.
Common mistakes that cost marks
- Describing power stations in general terms without a labelled diagram or the energy conversions.
- Stating that high voltage “reduces losses” without showing the I²R calculation.
- Mixing up transmission (275 kV and 400 kV in England and Wales) with distribution voltages.
- Using single-phase formulae for a three-phase load, so the current is wrong by a factor of √3.
- Choosing a cable without checking voltage drop or applying correction factors.
- Missing units, or rounding too early and getting inconsistent answers.
- Naming regulations wrongly; use their full titles, such as the Electricity at Work Regulations 1989.
How to move from Merit to Distinction
Distinction work in this unit evaluates. For generation, judge which plant best suits a modern grid, weighing low-carbon output against flexibility: nuclear provides steady baseload but cannot follow demand quickly, while CCGT can. For transmission, use your loss calculations to justify the voltage and line type rather than simply stating them. For distribution, compare your design with at least one alternative (for example a larger cable to reduce voltage drop, or a ring arrangement for reliability), show the cost or performance trade-off, and confirm compliance with BS 7671 and the statutory supply limits. Finish each section with a clear, justified conclusion.
FAQs
What voltages does the UK grid use?
The transmission system in England and Wales runs at 400 kV and 275 kV, with 132 kV also part of the transmission network in Scotland. Distribution networks then use 132 kV, 33 kV, 11 kV and finally 400 V three-phase or 230 V single-phase for customers.
Why is power transmitted at high voltage?
For a given power, raising the voltage lowers the current. Losses in the conductors depend on I²R, so a lower current means much smaller losses and allows thinner, lighter conductors.
Which regulations should I mention?
The Electricity Safety, Quality and Continuity Regulations 2002 for network operators, the Electricity at Work Regulations 1989 for safe working, and BS 7671 (the IET Wiring Regulations) for installations on the customer’s side.
What is the difference between a radial and a ring distribution system?
A radial system feeds loads from one end, so it is simple and cheap but a fault cuts off everything downstream. A ring system can be fed from either end, so a faulted section can be isolated while the rest stays live.
Is Unit 18 called Transmission and Distribution or Distribution and Transmission?
Pearson’s 2016 specification lists it as Electrical Power Distribution and Transmission. Many centres and students use the reversed wording. The content is the same.
Need one-to-one support from a subject specialist? Our BTEC Level 3 Engineering assignment help service covers planning, feedback on drafts and referencing.
For related engineering units, see our Engineering Principles answer guide, which covers the electrical theory this unit builds on. If you need help with your calculations or design, you can get expert help with your BTEC assignment.
