EMF Calculation Question

[edevans]

I need to calculate transmission line induced EMF for a project I am working on. There are two transmission lines in the same corridor where power flows in opposite directions.
The question is: Do I have to represent one current as negative since they are flowing in opposite directions?

Thanks in advance!

 

[rterickson]

That's correct, each phase current on your southbound line will be 180-degrees from its corresponding phase on the northbound line.

 

[bacon4life]

If you are doing this in PLS-CADD, be sure that you check where each each electrical phase is, which may be different than the structural phase designations. For example, two identical structures on the right of way may be numbered 123 123, but really have electrical phases ABC CBA.

 

[edevans]

Thanks for the help, I have a similar follow up question.

I need to calculate the neutral return current, for example A, B, and C phase at 0, 240, and 120 Degrees respectively, have 255.4, 257.5, and 250.3 amps, respectively. When you add the phasors you will have 6.4 amps of neutral current at 283.9 degrees.

My question, if you're still awake: Do I need to subtract 180 degrees from this neutral current, since it will flow the opposite direction of the phase current?

 

[bacon4life]

Yes.

 

[magoo2]

If you draw out your circuit, with the 3 phase currents summing to 6.4 A @ 284 degrees, that represents the current into the node. The outgoing current is what flows through the neutral back to the source. For your EMF calcs, since the arrows for the phase currents are in opposite directions to the neutral current, you need to shift the neutral current by 180 degrees if you are basing the calc on them all flowing in the same direction.

You can see this better if you just deal with a single phase case.

BTW, my transmission lines don't have neutrals. This would be distribution line.

 

[edevans]

Right, I thought I needed a 180 degree shift. I am not aware of any transmission lines that are wye-connected with a neutral. I was referring to a distribution line.

 

[7anoter4]

If you will invert the star of the phasors you'll get
180, 60 and -60 degrees with respect to the old A.
Then the neutral will be 6.413 A at +76.47 degrees.
If they are the same currents in opposite sense !