Home Page | About Me | Home Entertainment | Home Entertainment Blog | Politics | Australian Libertarian Society Blog | Disclosures

Know It All: Electrical Power Distribution -- Waves of power

Published in Geare magazine, Issue #41, Jan/Feb 2007

Thomas Edison is justly famous for his inventions, but there is one battle that he lost big-time.

In 1882, having developed the first economical light bulb, he switched on the first ever electrical distribution system. This provided 110 volts to 59 customers in Manhattan. To this day the standard voltage used in the United States remains 110 volts, but little else is the same.

That's because Edison chose direct current (DC) for his system. His electrical distribution rival, George Westinghouse, went for alternating current (AC), and it was this that became the standard across the world.

DC is the kind of power supplied by batteries, and used internally in all electronic equipment. AC is really only useful for power applications, such as lighting, motors, heating and so on. But DC could be used equally well for those purposes. DC has a constant positive, and a constant negative, so the electrons all travel in the one direction. AC alternates the positive and the negative between the two 'poles', so the electrons shuffle backwards and forwards along the wire.

So why did AC come to dominate? For all its complexity, AC elegantly solves several problems in power distribution.

There is no easy way to change the voltage of a DC source. At least there wasn't back in those days. But running low voltage current down long wire distances is a good way to waste power, and it costs an awful lot in wire.

The power of an electrical system is the voltage times the current. Exactly, in the case of DC. Approximately in the case of AC. In fact, to work out power precisely with AC you have to know the reactance of the load, and then do some arithmetic with complex numbers. By complex, I don't mean complicated. I mean numbers in which the square root of -1 is a component (that's called, I kid you not, the 'imaginary' part of the complex number). But for our purposes, it's close enough.

Now let's say you have to run the output of a 500 megawatt generating plant down fifty kilometres of wire to a city. We might as well at this point use Australian values, so the end voltage required is 240 volts. That would mean that the power cables would have to carry some two million amps of current. The power lines would have to be of titanic thickness to handle that kind of current, and would be so expensive that electrical power could not be economic.

But with AC, you stick a transformer at one end, convert the voltage up to 330,000 volts, and at the other end, back down to 240 volts. The current in the middle section falls to 1,500 amps. That is much more manageable. Now all you need is tall towers to carry the cables to keep them well away from everything. That kind of voltage can spark quite a distance.

There are other tricks you can do, and are done, with AC. Look around a suburb with overhead power lines. You will find some of poles carry four cables, and some carry three. That's because the 330,000 volts isn't stepped down immediately to 240 volts. Instead, at a power substation that may serve several suburbs, it is stepped down to 11,000 volts, so again relatively economical cabling can be used to get the power to close to each section of housing. The transformers that do the final conversion from 11,000 volts to 240 volts are on power poles.

The 330,000 and 11,000 volt sections use three cables while the 240 volt sections use four. Why? That's because AC power is carried in three phases, each 120 degrees out of phase with the others. So when one is at peak voltage, another is still going up while the third is going down.

If you have heavy machinery in your home, you may have two or three phase power. One advantage of this is that the voltage between any two of the three 'active' lines is 415 volts. But for most of us, 240 volts is enough. So we get one of the active lines from the power pole to our home, and the fourth 'neutral' line. The three active lines are divided pretty evenly among the houses on the circuit, but everyone gets the same neutral line. Because the three active lines are out of phase, the 'return' of the current to the neutral line pretty much balances out, so the system gets away with just one of these.

To add to this theme of 'something for nothing', a so-called 'delta-star' transformer is used for converting between 11,000 and 240 volts. By the time the neutral wire gets to the transformer, it has been so much balanced out that it can be dispensed with completely, so instead of four cables, the long runs can get by with just three.

If Edison lost the power distribution battle, how come his original 110 volts still prevails in the US? Simple. Edison established the standard through the first widely available product that used electrical power: the light bulb. It ran best at around 100 volts, so the AC systems in the US were designed to deliver a nominal 110 volts (around 100 volts after losses) at the light socket.

© 2002-2009, Stephen Dawson