Electricity Transmission

Before we can discuss electricity transmission, we will have to talk a bit more about electricity itself.

There are two ways of generating electricity: alternating current (AC) and direct current (DC). Alternating current means that the electric current flows first one way, then the other. The voltage therefore alternates between positive and negative. DC is simpler. As its name would suggest, direct current means the current flows directly around the circuit in one direction. It will be transmitted down two wires, one positive and one negative. The voltage is constant. Nice and simple.

However, there is a major problem with DC. It is very difficult to change the voltage.

Wait a minute, what is voltage? If you think about an electric wire as a hosepipe carrying water, the voltage, measured in Volts (V) would be the pressure of the water. The current, measured in Amps (A) would be the actual volume of water that is flowing.

So why bother changing the voltage? Couldn't we just transmit the electricity all the way from the power station to our homes at one voltage?

Unfortunately not. Household appliances in Europe use a voltage of 230V. The problem with transmitting electricity over long distances at 230V is resistance. This is the tendency of a wire to resist the flow of electrical current, losing power in the form of heat.

Power is measured in Watts (W). There is a simple equation for working out the power in an electrical circuit, Power = Voltage x Current. Engineers write this as P=VI, where I means current. (The letter C was already taken!) We can use the equation to work out that if something uses 2300 Watts of power (about right for an electric kettle) at 230 Volts, it will use 10 Amps of current.  If something uses 2.3W of power (e.g. a mobile phone charger), at 230V it will need 0.1A of current.

Say a small town uses 23 million Watts of power. This would be 10,000 kettles boiling at the same time, or more likely, a lot of kettles, radiators, cookers and so forth. (A million Watts is a Megawatt, MW.) If the power was transmitted to the town at 230V, it would take 100,000 Amps of current to supply 23MW of power.

This is where resistance comes in. Remember, resistance is the tendency of an electrical wire to lose power as heat. Resistance is measured in Ohms.

There is another equation to work out how much power is lost as heat in electrical wires. This equation is: Power = Current squared x Resistance. (To an engineer, P=I2R.) What this means is, if you double the current in a wire, you will lose four times as much power as heat. If you increase the current ten times, you will lose a hundred times more power as heat.

So if we tried to transmit 23MW to the town for all those kettles, and had to send 100,000A of current at 230V down our wires, which we will say have a resistance of 1 Ohm, the power loss would be 10,000MW!!! That's just ridiculous, we would have to send 10,023MW down the wire just to get 23MW out the other end!

Next we will show how to transmit power without such enormous heat losses in the wires.