The grid moves electricity at very high voltage on purpose
- Power stations generate at around 25 kV25\ \text{kV}25 kV, and the National Grid carries the electricity across the country at up to 400 kV400\ \text{kV}400 kV.
- A step-up transformer at the power station raises the voltage before the electricity enters the transmission lines.
- A chain of step-down transformers at substations lowers it again in stages, ending at 230 V230\ \text{V}230 V for houses.
- The pylons carry bare aluminium cables, held clear of the ground and of each other by long strings of insulators, because at these voltages nothing may be allowed to touch.
- Only alternating current can be transformed like this, which is one of the reasons the grid runs on a.c.
High voltage means small current, and small current means small losses
- Power is delivered at P=IVP = IVP=IV, so the same power can be sent as a large current at a low voltage or a small current at a high voltage.
- The cables have their own resistance RRR, and the power wasted heating them is P=I2RP = I^{2}RP=I2R.
- That loss depends on the square of the current, so halving the current cuts the wasted power to a quarter.
- Raising the transmission voltage by a factor of 100100100 cuts the current by 100100100, and so cuts the heating loss by 1002=10 000100^{2} = 10\,0001002=10000.
- Almost all the generated energy therefore reaches the far end, instead of quietly warming the countryside on the way.
- A smaller current also allows thinner, lighter cables, which are cheaper to make and need less substantial pylons to hold them up.
What high-voltage transmission costs in return
- High-voltage lines are far more dangerous, so they need tall pylons, long insulator strings and wide safety clearances beneath them.
- Transformers are large and expensive, and each one wastes a little energy of its own, so a substation is needed at every change of voltage.
- Those costs are worth paying, because the energy saved over hundreds of kilometres of cable is far greater than the energy the transformers waste.
- Transmitting at 230 V230\ \text{V}230 V instead would need enormously thick cables and would still lose most of the energy as heat before it arrived.
Common Mistake
- The saving comes from the smaller current, so make the link through P=IVP = IVP=IV before quoting P=I2RP = I^{2}RP=I2R.
- Do not write that high voltage means less energy is transmitted; the same power is sent, but less of it is wasted on the way.
- In P=I2RP = I^{2}RP=I2R the RRR is the resistance of the cables, not of the appliance at the far end.
- Transformers only work on a.c., so the whole arrangement depends on the supply alternating.
Exam technique
- An "explain the advantage" answer needs three links: high voltage, small current, and less power wasted as heat in the cables.
- Quote P=I2RP = I^{2}RP=I2R by name and point out the squared term, since that is what makes the saving so large.
- In a calculation, work out the current from I=PVI = \dfrac{P}{V}I=VP first and show that line, because it usually carries its own mark.
- Watch the prefixes, converting kV\text{kV}kV to V\text{V}V and MW\text{MW}MW to W\text{W}W before substituting anything.
Self review
- Where are step-up and step-down transformers used in the transmission of electricity?
- State two advantages of transmitting electricity at high voltage.
- Explain, using two equations, why a higher transmission voltage reduces the power wasted in the cables.
- A line of resistance 5.0 Ω5.0\ \Omega5.0 Ω carries a current of 40 A40\ \text{A}40 A. Calculate the power wasted.
- Why must the grid carry alternating current rather than direct current?
