MadSci Network: Engineering Query:

Re: Why engineers in electricity transport the electricity with 3-phase cables

Date: Sat Mar 6 16:03:34 1999
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Engineering
ID: 919196125.Eg
Message:

Hi Cantin,

First, there are really two parts to this question:

1. Why is alternating current (AC) used for power distrubution over direct current (DC)?
2. 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 in North America. Street level distribution is about 13.8 kilovolts 3-phase at 200 amps or so. 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. Next time you are driving through a neighborhood with above ground power distribution, study the phases. You will see three thick wires (the hot phases) and one thin wire (neutral). The neutral only carries current if the hot phases are imbalanced.

Industries are some of the largest consumers of electricity. The primary supplier of electricity in the Washington, DC metropolitan area is Potomac Electric Power Company (PEPCO). There is no large industry in the DC area, so it turns out that the largest commercial user of electricity is PEPCO itself! They have all kinds of machinery to operate: pumps, grinders, conveyers etc. It takes electricity to make electricity. Homes and business don't take a lot of power in the grand scheme of things. Industries often run large motors. 3-phase motors, like generators, are also cheaper and cost less to operate for a given power rating.

Most cars and motorcycles also use 3-phase alternators. This is because they are cheaper and need less mainenence than DC generators. Also, 3-phase can be easily rectified to DC. Single phase AC needs considerably more filtering when compared to 3-phase.

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.

I hope I have answered your question fully. If not, please drop me a line at madhu@madhu.com.

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