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I am studying electric power distribution. I've read in Kirtley's Electric Power Principles (2010) that a transmission line can be modeled by the 'pi' model. Which is a two port network with series conductance and parallel capacitance. But a transmission line (single phase) is just one cable, with current going through it. How can this be modeled by a two port network?

Thanks.

JMcB
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  • Return channel is through the ground? – zeta-band Feb 08 '18 at 22:21
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    Would [electronics.se] be a better home for this question? – Emilio Pisanty Feb 09 '18 at 00:02
  • @Emili Pisanty - Also physicists, perhaps with exception of unworldly theorists, should understand transmission lines and two-port networks. And for experimentalists, it's quite often a necessity! – freecharly Feb 09 '18 at 02:35
  • Now, @freecharly, we have all seen people taking data with little understanding of the electronics! (Sad, but true). – Jon Custer Feb 09 '18 at 14:13
  • As for the question, how many ports do you think are needed to describe the transmission line as a lumped element? I see in and out, which means two. – Jon Custer Feb 09 '18 at 14:15

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A power transmission line is, indeed, one cable, but the current that flows into the cable has a return path via the ground. Thus you have two terminals (first port) at the power station and two terminals (second port) where the load is located. Therefore you can consider the power transmission line as a two-port network.

freecharly
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  • Since there is a current, I assume the line is in a circuit. On both ends is a transformer. One terminal of each transformer is connected to the transmission line and the other terminal of each is connect to the ground at each respective location. That is what I gather from the answer above. If I understood that correctly, I don't see how the earth closes the circuit to allow the current to flow in the transmission line. – JMcB Feb 09 '18 at 18:50
  • @JMcB - The ground serves as the second conductor of the transmission line. You can easily check that at you local power outlet. If you stand with bare (wet) feet on the not insulating ground (like stone or tile), and you touch only one terminal with your finger (you have to try which is the not grounded one), you will feel a shock. This is because the ground serves as the conductor back to the grid voltage supply. (Please, don't really try to do this, it is dangerous!) – freecharly Feb 10 '18 at 03:26