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Continious Current Carring Capacity Of Oh Line

02/05/2011 7:30 AM

what is the procedure of converting DC resistance to AC resistance of an OH line and what are the factors involved during the conversion.

i reguare a details calculation procedure.

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Guru
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#1

Re: Continuous current carrying capacity of OH line

02/05/2011 8:45 AM

The procedure is to first get the terms correct. The resistances will actually be the same for AC and DC but not the impedances. In DC circuitry the resistance is the impedance. In AC circuitry the added linear components that change the impedance are the two energy storing devices of an inductor (L) and a capacitor (C). For AC circuitry you must also know the frequency of the AC voltage for unlike a resistor that has the same impedance for every frequency the impedance of these components change with frequency. The equation for the impedance for each component is as follows:

  • Xr=R
  • Xl=sL=jωL
  • Xc=1/(sC)=1/(jωC)

Now j=√(-1), ω=2∏f and s is the Laplace variable.

To properly use this information takes a lot more time and explanation that always includes the student doing homework. If you wish to examine part of this knowledge base on your own I recommend you search the web for Kirchoff's, Norton's, and Thevenin's work on network theory. To properly learn this information I implore to you that you take some organized classes in the Physics of electricity and Calculus.

Also, I know that an OH line could not possibly be a hydronium ion line or a cable to Ohio. What do you mean by an OH line? If you want details in an answer you should provide details in your question. (Punctuation and capitalization would also be polite.)

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#2
In reply to #1

Re: Continuous current carrying capacity of OH line

02/05/2011 9:36 AM

I suspect OH ≡ overhead.

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Guru

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#3

Re: Continious Current Carring Capacity Of Oh Line

02/05/2011 9:41 AM

You can begin with published conductor resistance for the type and construction of the conductor used for your OH (overhead) line. In the US the DC resistance at 20°C is typically shown in ohms/mile with AC resistance in the same units at 25, 50 and 75°C. This information is published in many places, including catalogs and handbooks. Once you have those values, system voltage and line design dictate the remaining characteristics.

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#4

Re: Continious Current Carring Capacity Of Oh Line

02/05/2011 11:29 AM

When the line is glowing red and sagging it's overloaded

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Guru
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#5

Re: Continious Current Carring Capacity Of Oh Line

02/05/2011 9:15 PM

amitabha0107 may be referring to the increase in resistance due to skin effect in AC as against no such problem in DC? If so, please check out this link :

http://en.wikipedia.org/wiki/Skin_effect#Resistance

At 50 or 60Hz, the conductor needs to be ~ 17 mm in dia to see any difference.

In any case, overhead lines are usually steel-cored with aluminium strands on the outside for this purpose.

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#6
In reply to #5

Re: Continuous Current Carrying Capacity Of Oh Line

02/05/2011 11:21 PM

That's one valid possibility of what the OP meant. But it scares me to think that somebody that works on any distribution grid voltage would come to us with this kind of a question. I hope that this is all theoretical.

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#7
In reply to #6

Re: Continuous Current Carrying Capacity Of Oh Line

02/06/2011 12:37 AM

Yes, doesn't it ? Well, hopefully it is just theoretical. However, many young engineers are expected to be productive from day 1, without much of training. Scary.

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