6 · IGCSE Physics
Transformers and the national grid
Change the transmission voltage and see how the current in the cables and the power wasted as heat (I²R) change for the same power sent.
I = P ÷ V
P lost = I² R
Vs ÷ Vp = Ns ÷ Np
Generator: 25 kV · Homes: 230 V
P lost = I² R
Vs ÷ Vp = Ns ÷ Np
Generator: 25 kV · Homes: 230 V
- Cable current I
- Power lost I²R
- Percentage lost
- Power delivered
- Generator current
- Step-up Np : Ns
- Step-down Np : Ns
Try this
- With P = 100 MW and R = 1.0 Ω, note the power lost at 50 kV, then at 100 kV. By what factor does the loss fall when V doubles?
- Set P = 200 MW, V = 25 kV and R = 2.0 Ω. Now find the lowest transmission voltage that keeps the loss below 1 %.
- Compare the step-up turns ratio needed for 132 kV and for 400 kV, then explain why the voltage must be stepped down again before it reaches homes.
Common error: using V²/R with the transmission voltage to find the cable loss; the voltage across the cables is only IR, so find I = P ÷ V first and then use P = I²R.