Resistance of transmission lines ?

DarthVolta

Joined Jan 27, 2015
521
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
16,924
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
109
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.

InterestingStuff

Joined Nov 6, 2019
14
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

DarthVolta

Joined Jan 27, 2015
521
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
2,229
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!

Alec_t

Joined Sep 17, 2013
14,318
You can see how thick the conductors are in the linked video in post #2 of this thread.

DarthVolta

Joined Jan 27, 2015
521
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

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SteveSh

Joined Nov 5, 2019
109
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.