Electric City

Background

Why use a high voltage?

Electricity is transmitted at a high voltage to reduce power losses in the transmission lines. A typical house in the UK might have a power requirement of 2.3 kW. This could be supplied at a voltage of 230 V, which would require a current of 10 A in the transmission lines: power delivered = voltage x current

However, this current will heat up the transmission wires. The power loss from heating is proportional to the square of the current in the wires: power lost = (current)² x resistance

If the resistance of the wires is 10 ohms, then the power lost is 1,000 W. This means that, in order to deliver to the house a power of 2,300 W, the power company has to generate 3,300 W - because 1,000 W are wasted.

However, if the transmission voltage is raised to 2,300 V, then the current only needs to be 1 A. This will still deliver a power of 2,300 W to the house. But now the power lost in the wires is only 10 W: I²R = 1² x 10 = 10

The amount wasted is 100 times smaller than before.

Using ten times the voltage allows a tenth of the current to be drawn, while still delivering the same power. And reducing the current reduces the heating in the wires: one-tenth of the current produces one-hundredth (one-tenth squared) of the waste.

A bicycle chain is rather analogous: in order to drive the back wheel, the bicycle can be in a low gear, which requires a small push (like a low voltage). But the chain will have to move quickly, and energy will be wasted through friction in the chain. However, in a high gear, although the cyclist has to push harder (like a high voltage), the chain doesn't move so fast (smaller current) and there is less wastage through friction in the links.

Transformers

Having transmitted the power at a high voltage in order to reduce waste, the voltage has to be changed down to a lower value that is safe to use in the home. Only alternating current (AC) can have its voltage changed in a transformer. Hence, transmission systems use high voltage AC with transformers close to the consumer. Alternating current can be converted into direct current using a rectifier circuit.

A little history

The first-generation system was built by Thomas Edison in New York's Pearl Street in 1882. It used a direct current (DC) generator. However, the range over which the system could efficiently deliver power was limited because the voltage had to be kept low for safety reasons. In 1888 Nikola Tesla developed a system of generating and using alternating currents, and showed that the voltage could be changed using transformers. This meant that the electricity could be carried at a high voltage, keeping the system efficient, and then transformed down at the customer's home to keep it safe. Edison never accepted that AC was the way forward and there was a commercial battle between his Electric Light Company and the Westinghouse Corporation, which had bought Tesla's patents.