Distribution voltage drop over distance 2

[eengK]

Hello,

I am trying to figure out a rough calculation of the voltage drop of a 25kv, 336 kcmil ACSR conductor over given lengths (40km or more) from 2 8MW feeders.

It has been a while since I have touched this stuff and can't remember the equations I should consider for a rough estimate or any additional data that I might need to make an informed answer.

Any help would be greatly appreciated

 

[davidbeach]

V=I*Z, where all three values are complex quantities. Z depends on line geometry, so 336 ACSR is not sufficient to know Z.

 

[eengK]

davidbeach

Normally I think you would need to know the code name for the conductor and for a 336 kcmil I beleive there are a couple choices :


Merlin
Oriole
Linnet
...

Ok so stay with me here,


if we select Merlin, we know
R(AC@75C) = 0.0625 ohm/1000ft
519 allowable amps

then I would think I would need to consider the current flow for my case, as I believe that is the biggest factor in determine losses. Given that I have 2 8mw feeders @ 25kv this yeilds 8mw/25kv = 320amps

then I could use Vdrop = Ir*R + Ix*X however I don't know the power factor... or the reactance. I suppose I could just assume unity PF...

So am I on the right track here? Or is it possible to get more or less a rule of thumb over distance for given voltage drops without knowing much else?

 

[dpc]

The spacing between phases has an effect on the inductance.

The exact stranding of the 336 ACSR doesn't have as much effect.

A standard reference book such as EE Handbook or Elements of Power System Analysis will have information on calculating the line impedance per unit length.

Then you use whatever current loading you want to determine the voltage drop at that current.

 

[desrod]

Single or 3 phase load?

Power factor?

 

[eengK]

It will be a 3 phase, but no knowledge yet on PF.

Thanks for the input dpc. I will give those books a look over.

I guess I was looking for a rule of thumb or a basline to use for approximate voltage drop over given lengths. Stictly for observation purposes.

 

[desrod]

eengk

Ran a quick simulation with what I have in CYME's equip database for 336 kcmil. Ideal source, 40 km line and 3 phase spot load.

That should give you an idea.

We use to load our line at 350 A with margin for cold load pick-up up to 1.8 to 2.0 times normal load without problem.

Load type might cause problem (e.g. large motor) for such a distance.

 

[desrod]

Forget one step
Here's the output

Desrod

 

[eengK]

desrod,

Thank you very much for that!

 

[eengK]

Turns out the line is to be Linnet ACSR, in a typical T pole construction with 88" spacing between the furthest conductor and the middle one in the center @ approx 5" height.


After looking up the specs on the conductor here:

http://www.cable.alcan.com/NR/rdonlyres/BC388B10-A9C3-45B6-AA90-F5A9BA9D203D/0/ACSRweb.pdf

and calculating the equvalent R= 0.2076 ohm/km X = 0.39567 ohm/km

I think I have enough data to display various lengths using a simple short line model.

 

[cherry2000]

eengK..do you have taps from this o/h line for various loads? Then the voltage drop calculations get a bit tricky..the last section near the source carries current i1+i2+i3+..+in. For the next section the line carries i2+i3+..+in..if the first tap withdraws i1 current. I had a spreadsheet which I used for this type of distribution from o/h line.