# How to model a medium transmission line for both nominal pi and t sections

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Salah Hammouda on 10 Dec 2021
Answered: Joel Van Sickel on 22 Dec 2021
I've been trying to model examples from mehta book but i don't know what is zero sequence inductance, capacitance and resistance and how to calculate them from mehta book examples.. also i get an error that the 3 phase power supply cannot be connected in parallel with the pi section model, there must be a resistance.. but why?

Joel Van Sickel on 22 Dec 2021
Hello Salah,
to start, I would just set them to your positive sequence values (this is incorrect, but will get you started with a simulation). As you are probably starting with balanced systems and not doing faults, the zero sequence current should be zero and therfore irrelevant at this point. The best way to get these values is usually by contacting vendors, using data sheets, or using a tool to calculate them based on the geometry of the transmission lines. This is typically not trivial especially when just getting started.
As to the inducatance. Specialized power systems implements inductances as controlled current sources under the hood. You cannot have 2 controlled current sources in series with each other unless there is another path added (the resistor in this case) so that any difference in current can flow through this alternate path. If this is causing problems, instead of using speicalized power systems, you can use Simscape (the blue circuit components) to do the same type of model.
There is official free onramps for simscape https://www.mathworks.com/learn/tutorials/simscape-onramp.html
If you want to learn how to use specialized power systems instead, you will need to search on youtube for non-official tutorial videos.

R2018a

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