Reducing the Amount of Power Lost as Heat in Wires
On the previous page we saw that transmitting serious amounts of power down an electrical wire at 230 Volts would result in incredible amounts of heat being lost in the wire. What can be done to help?
The power lost in an electrical wire is governed by the equation Power = Current squared x resistance. So to reduce the power loss, we just have to reduce either the current or the resistance. To reduce the resistance of a wire, we need to make it larger. There will be more metal to carry the current, so the resistance will be lower. Unfortunately, we would soon end up with an enormous wire, which would be hideously expensive.
The better solution is to reduce the current. This has the advantage that if we can reduce the current by ten times, we will reduce the power loss by a hundred times! (Remember, power loss in wires is equal to current squared times resistance.)
How do we reduce the current and still transmit the same power?Simply increase the voltage. Remember, Power = Voltage x Current. If we increase the voltage by 10 times, we reduce the current by 10 times, and this reduces the power lost in the wire by 100 times.
So let's go back to our small town using 23MW of power. If we tried to transmit that power to the town at 230 volts down wires with a resistance of 1 Ohm, it would have needed 100,000 Amps of current, resulting in 10,000MW of power loss. However, what if we used a High Voltage power line running at 132,000 volts? (132kV) This would reduce the current required to just 174 Amps. Transmitting this down a wire with a resistance of 1 Ohm would result in just 0.03MW of power lost as heat. To provide 23MW to our town, we would have to send 23.03MW down the power line.
The longer a wire is, the higher its resistance will be. If a piece of wire has a resistance of 1 Ohm, then a piece of wire twice as long will have twice the resistance, i.e. 2 Ohms. This means that longer wires lose more power due to resistance, so the further you transmit power, the more you lose as heat.
Clearly, you would be mad to try and send electricity a long distance at low voltage. You would use a high enough voltage to reduce power losses to an acceptable level, and that's exactly what happens in real life.But how do you change the voltage? As I said earlier, the two ways of generating electricity are direct current or DC, and alternating current or AC. The reason why we use AC in our homes is that it is easy to change the voltage, whereas with DC it would be very difficult. The reason for this comes back to the link between electricity and magnetism.
Next we will look at how to use magnetism to change the voltage of an alternating current.