The Physics of... Power Transmission Lines

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Thread Starter

Phil Corso, PE

Robin (College Student),

I have taken the liberty to post this new thread because your post is too far removed from the intent of the MVARS thread! However, your questions are excellent ones. The answers follow:

A1) Yes, power does not "flow" in the conductor, but instead outside! The paragraph you quoted about transmission lines is on target. Electrons do not directly transmit power, and their "speed" certainly not that of the speed-of-light. In fact it's only about mm per sec. If you want additional information than can be found in college physics texts I suggest you perform a Web Search for, "The Poynting Vector!" (Those offended by the V-word need not participate!)

A2) Yes, a 1,000-mile separation of the phase and neutral wires would prevent you from starting your computer, but not for the reason you posited. Instead, it has to do with inductance associated with a two-wire transmission line. Its magnitude is proportional to the ratio of S, their separation and, D their diameter! Of course I've ignored their resistances, their beginning and end-constructs, and the not so trivial case when D = S, i.e., 1,000 miles! Note: This problem appeared early in the history of AC power; e.g., for series street lighting when a single conductor ran down one street and its return along the next street!

Regards, Phil Corso ([email protected])
 
Please pardon me for posting a very basic question. My question is too basic.

I suddenly felt as if I have forgotten my college day physics when I was reading about the word wavelength. I have always read that a Wavelength is the distance between two peaks (or for that matter any two points where the wave repeats itself). Typically a sine wave comes to mind.

Suddenly, I felt that I am unable to understand the physical significance of wavelength for a power cycle. For a typical 50 Hz signal used in my country, the wavelength will be equal to 6000 Kilometers (SoL/Freq).....!!!!! Is that right or I am making some mistake. I googled it and found that it IS indeed the case.

Then what does it mean to have a wavelength of 6000Kms in real physical sense.

A cable is attached to a Generator that generates the AC Current which has a wavelength of 6000Kms. So assuming that the cable that carries the AC to a distant motor, say, 600 meters from the generator will NEVER be able to see the peak of the wave cos the Peak of the wave will occur at a quarter of the distance of the wavelength from the source (generator) i.e., 1500 Kms in this case!!! What does it mean, then, to have a wavelength of 6000 Kms for a Powerline Waveform.

Sorry again for asking a basic question but I googled and couldn't really find a satisfying explanation to my misunderstanding.

Thanks in advance and regards
Ritika
 
College Student Ritika... yours is a very intuitive thread, and NO, you did not misunderstand!

In fact, had you had gone a little further you could have answered the question I presented in my 23-Feb-11 (09:53) response to ??????'s Thread No.:

http://control.com/thread/1298383238 "Power Distribution"

Fortunately, the answer to my question will provide you with additional insight to solve your dilemma. You (and anyone else) can contact me at:

Phil Corso (cepsicon[at]AOL[dot]com)
 
Think of this more as a wave traveling down the transmission line from the generator to the motor. The generator terminal voltage is rising and falling to produce the 50 Hz wave, and the voltage at the motor terminal 600 meters away would be following this delayed about 0.1 millisecond IF the motor was directly connected to the generator terminals. In the real world, the generator is likely connected to a step-up transformer to the transmission line, and there is going to be at least 1 step-down transformer between the transmission line and the motor. These transformers may introduce phase shifts so the delay may appear to be significantly greater than 0.1 millisecond. Regardless, if you were to connect an oscilloscope to the motor terminals, you would see a 50 Hz sine wave displayed. (If you really want to do this, connect the oscilloscope to the low voltage side of a potential transformer that has the high side connected to the motor feeder. Otherwise, you and the oscilloscope may not survive.)
 
C

curt wuollet

It means, for the most part, we just don't worry about wavelength at power line frequencies.

Regards
cww
 
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