The history of electricity is a very interesting one indeed. In the early 1900s, Thomas Edison is said to have hated AC, simply because he didn't understand it. Edison made efforts to squash AC distribution, but ultimately failed due to the economics of power generation and distribution: he just couldn't send DC very far. Edisons rival, Nikola Tesla developed and patented much of AC power generation and distribution technology used today. George Westinghouse purchased Teslas patents and profited from them. Even with these patents, the company Edison founded, General Electric, is many times the size of Westinghouse. Telsa fell into relative obscurity, he is rarely mentioned in the history books. Nikola Tesla does not get the kind of recognition he truly deserves, even though he is the creator of polyphase transformers and machinery. Nikola Tesla is the real reason why we use 3-phase distribution.
- Why is alternating current (AC) used for power distrubution over direct current (DC)?
- Why is three-phase used rather than single phase?
To answer the first part, AC generators cost less and can produce more power than an equivalent size DC generator. AC in general is easier to distribute because very efficient transformers can be used to step up and step down distribution voltage. The step up helps reduce distribution losses by substantially reducing the current carried by the long distance lines. The voltage on high tension transmission lines may range anywhere between 230kV to as much as 500kV with currents up to 450 Amperes. Transmitting that amount of power at low voltage (240 volts) would mean extremely high currents resulting in unacceptably high ohmic losses.
In some cases, very high voltage lines (upwards of 1MV) actually use DC! It turns out that very long AC runs (hundreds of miles or so) can actually introduce significant radiation losses. The wavelength of 60 Hertz (used in North America) is 5000 Kilometers or about 3000 miles. Even an antenna of 1/10 of a wavelength (300 miles) will radiate quite a bit. Some of the long haul lines on the US West coast between large metropolitan areas run DC.
There are several reasons why electricity is distributed in three phase:
- Polyphase generators can generate more power and cost less to maintain than single phase generators
- 3-phase distribution is more efficient than single phase AC
- Many industries require 3-phase power
Lets say that we have a generator with polyphase windings. For each phase winding there are two ends. For a 3-phase system there are six wire ends. At first, one would think that all six wire ends must be distributed, but it turns out that only three wires are needed. The reason is that at any point in time, the total current on all three conductors is exactly zero! This assumes, of course, that all three phases are exactly balanced and that current is equally distributed. In reality, this isn't always the case. Varying loads, broken segments, and system failures imbalance the system, but engineers still do a fairly good job of current balancing. The result is that a given amount of power can be distributed using much less wire with 3-phase than single phase.
A 450 Amp aluminum transmission wire is about an 1.5 inches thick. Over hundreds of miles, the cost of wire really adds up! Individual homes are usually wired for single phase 120/240 . Each neighboring house is fed from different phases in sequence to balance the load. Clearly, if one house uses significantly more power than another, the phases will be imbalanced. In reality, it more or less evens out when hundreds of houses are involved.
