A/C Current in Power Lines 6

[mikego34]

Hello all,

I currently work for a electric utility and I am trying to delve a little more into the transmission process. I recently was trying to reason through how alternating current works and it makes sensse in to me within a circuit, but I am at a loss for how alternating current behaves in distribution lines. A/C current reverses dicrection as it is goes into negative region on a current vs time graph. How does this apply in a conductive element such as a distribution line.

Thanks in advance

 

[stevenal]

I'm not sure why you are distinguishing lines from circuits in this respect. Lines are simply a component of the circuit they are part of. Picture electrons wiggling fore and aft without ever really going anywhere.

 

[mikego34]

So what your saying is that if you were to track a single electron within the distribution line it does not actually travel a distance?

 

[jraef]

Eventually they move a little, but that's not really important to what we do with them. The WORK we get out of the action is based on the energy transmitted by the "wiggling" if you will.

Another analogy is to think of a line of that old "clacking balls" desk toy (aka Newtons Cradle) you used to see all over, where a row of steel balls hung from a frame on strings. If you pull back the end ball and release it, the energy of your pull gets expressed on the ball at the other end, but all the balls in between don't really move much. Then the energy in that opposite end ball swings back into the lineup and transfers back, making the first ball move out.

The transmission line, or really ANY conductor, is essentially that row of balls. It doesn't create or use the energy, it just transmits it.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[mikego34]

thanks to everyone for responding jreaf that was a great explanation.

 

[VadimKadyrov]

To cut it short, that is why speed of current in any conductor is the speed of light. Electrons themselves move at several millimeters per second, but this impulse jumps from one particle to another at the speed of light _ see the picture of jraef.

English - Russian technical translator and DTP specialist. One of top EN-RUS technical translators in Russia and Ukraine according to proz.com. Full compliance with DIN 2345 standard.

 

[itsmoked]

Actually not the speed of light but only 1/3 the speed of light.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VadimKadyrov]

I mean, the speed of spread of instant electromagnetic influence over neighboring electrons is the speed of light.

English - Russian technical translator and DTP specialist. Full compliance with DIN 2345 standard.

http://engrutra.com

 

[davidbeach]

Not quite the speed of light, but far closer to the speed than 1/3. In a typical transmission line the velocity is above 97% of the speed of light.

 

[Skogsgurra]

My rule of thumb, when calculating distance to a fault in a Cable and using TDR, is 2/3 of speed of light, or 67 %. That makes the round trip from cable end to fault and back again equal to 100 m/microsecond, which is a convenient number and facilitates mental calculation from time measured with the scope.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

The main differences between the conductors to your kitchen light fixture and any two conductors in a transmission line are higher voltage and greater current in the transmission line.
Thanks for that rule of thumb Gunnar.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

The speed is a function of 1/sqrt(LC). In cables, Skog's 2/3 is a reasonable number. In an open conductor, air insulated, high voltage transmission line it will be much higher. For one of our 230kV lines the propagation velocity is 97.78% the speed of light; this value confirmed through traveling wave fault location.

 

[Skogsgurra]

Yes, David. The capacitance between conductors in a Cable is very high in comparison with an overhead transmission line. But I never do TDR on transmission lines. Only signal Cables and 400 V Power Cables. The 100 m/microsecond seems to be rather constant etween those Cables.

(My puter has started to put capital letters in the beginning of certain Words. How does one change that?)

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Thank you David and Gunnar for the value added.
A question Gunnar. I understand the TDR measures the time to the fault and back. Would Keith's figure of 1/3 be the one way time and your figure of 2/3 be the two way time?
Thanks
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

mikego34
Many of the simplified explanations for electricity come with build-in misconceptions. This website has several good analogies and points out some of the more common misconceptions:

http://amasci.com/miscon/eleca.html#light

By circuit vs distribution line, do you mean a source and resistance in a single loop versus 3 wires on a utility pole? In the simple single phase circuit, all electrons synchronize their wiggles, meaning that energy stops flowing when the electrons momentarily pause to change direction. In a balanced 3 phase distribution system, electron wiggles are offset by 1/3 of a cycle. This allows energy to continue flowing, even when the electrons in any one wire stop to change directions.

 

[Skogsgurra]

Yes Bill. That's what methinktoo.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Will anyone believe that I meant "1/3 less than c"?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[prc]

As usual, great contributions from David,Gunnar and jraef and thank you for the same.
Gunnar what is TDR? Can you explain how you arrived at 100 metre figure? In transformer impulse testing we use 1.2/50 microsecond surge.When a failure occurs in test, there will be distortion in voltage and neutral current oscillogram wave shape.To locate fault in winding, we take a surge speed of 150 metres per micro second and the time from start to distortion in oscillogram.All these years, I never knew from where this 150 metres came. David now cleared my doubt.

 

[itsmoked]

prc; TDR = Time Domain Reflectometry

You inject a pulse into a wire, transmission line, free-space, fluid-filled pipeline, even a closed space. You then accurately time how long it takes to reach the "end" and return. Divide it by 2 and you have the one way distance.

The same thing works whenever there is an anomaly in a transmission system of any kind. Say you have a coax cable that has a squirrel gnawed shield somewhere. A piece of the 'pulse' will unexpectedly come back (reflect) from even that. This allows one to pinpoint where an issue is.

I used it to find tiny breaks in petrochemical pipelines.

Keith Cress
kcress -

http://www.flaminsystems.com