Resistance of transmission lines ?

Thread Starter

DarthVolta

Joined Jan 27, 2015
477
I'm doing a practice test and for some HV trans. line it lists only 0.05 ohm /km (0.08ohm/mile). Is that anywhere near right ? I'm used to little tiny wires with maybe a DCR of 0.018ohm/meter (0.006ohm/ft) for AWG18 for example.
 

dl324

Joined Mar 30, 2015
10,998
It would be helpful if you told us the resolution/accuracy of your meter and whether you subtracted the lead resistance from your calculations.
 

SteveSh

Joined Nov 5, 2019
105
I'm doing a practice test and for some HV trans. line it lists only 0.05 ohm /km (0.08ohm/mile). Is that anywhere near right ? I'm used to little tiny wires with maybe a DCR of 0.018ohm/meter (0.006ohm/ft) for AWG18 for example.
Need to know the diameter and material of the HV transmission line. Keep in mind that at 60 Hz, the skin depth of copper is 8.43 mm, or about 1/3 inch. This means that 67% of the current is carried in the outermost 1/3 inch of the cable. So just measuring, or through analysis coming up with the DCR may not mean much.
 
Hi, are you interested in knowing the impedance or purely the resistance?
Adding to Steve's answer, it would also be helpful to know the length of the line and frequency (is it DC or AC? 50 or 60Hz?)
There are three models which can be used depending on the line length, you can use the models to compare with your measured values
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
477
No no it's just a basic question on a practice test, I'm just wondering if thats a ballpark DC resistance for trans. lines, in the question its for 500kVrms, but just DC res.


I guess it must be pretty low, what ever size they are
 

KL7AJ

Joined Nov 4, 2008
2,225
I'm doing a practice test and for some HV trans. line it lists only 0.05 ohm /km (0.08ohm/mile). Is that anywhere near right ? I'm used to little tiny wires with maybe a DCR of 0.018ohm/meter (0.006ohm/ft) for AWG18 for example.
Yes, that is a good approximation of reality. It's rather deceptive when you look way up at high tension lines, just how thick those conductors are!
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
477
Yes, that is a good approximation of reality. It's rather deceptive when you look way up at high tension lines, just how thick those conductors are!
That's super low indeed.

Wait a minute so a 200km stretch of wire is 10 Ohm, and if its 500A(rms), it really does loose 2.5MW, I just fooled myself into thinking is was only 2.5kW, but it really is 2.5MW
 
Last edited:

SteveSh

Joined Nov 5, 2019
105
That's super low indeed.

Wait a minute so a 200km stretch of wire is 10 Ohm, and if its 500A(rms), it really does loose 2.5MW, I just fooled myself into thinking is was only 2.5kW, but it really is 2.5MW
I think if you do the math, you'll find that the reactive losses - those due to the inductance of the cables - is highers than the I^2*R losses. That's one of reasons some new transmission lines are DC. Even with the losses of converting from AC to DC, then back to AC, the overall efficiencies are higher than if the line was AC.
 
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