Overhead transmission lines 1

[GadreP]

Hi All,
I am working on overhead transmission lines & as I am new to this field I want your help.
I have 2 Steam generators & a 100KVA DG feeding to a 415V bus. There is no transformer at incoming line. The bus will be charged by 2 Steam generators & DG will work only in emergency condition.
Then there 8 O/G feeders going to 8 different panels. The distance between main 415V bus & these 8 panels is about 2 Km & power will be transmitted by overhead lines.
Can anybody pls help me out to find the voltage drop in transmission & what do I need to do to compensate this Voltage drop?
I guess the Vd will be 180V & is capacitor bank a good solution to this problem? If yes, what will be its size?
Pls explain with the formula..

 

[magoo2]

I'm afraid you don't just guess at a voltage drop. What current, power factor and impedance did you base it on?

The same issues concern the placement of the capacitor bank. What voltage rise are you shooting for?

 

[GadreP]

Hi Magoo..
I used Vd=I*1.732*(R*cos(phi)+X*sin(phi))*length/(1000*No. of runs).
Is this formula correct?
The maximum current demand is 150 Amp,P.F at generating point is 0.8 lag & load side KW rating is 80 KW. As the design is at primary stage I don't have motor details like at what PF the motors will be running.
I considered 70 sqmm copper cable's data sheet (for R & X values) as i did not have ACSR conductor current carrying rating & other details.
My basic concern is voltage drop compensation..

 

[GadreP]

So, based on above formula voltage drop comes around 160V. So on 415V bus, I will be getting 255V considering 2KM distance. I need to maintain voltage at 415V level.

 

[davidbeach]

You need a higher voltage.

 

[alehman]

I agree with davidbeach. Medium voltage would probably cost less also.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[GadreP]

Guys, in the above case, it is absolutely not possible to go to the higher voltage. Power will be generated at 415V only.
Is the method of calculating voltage drop is correct? (I will use ACSR's data sheet once I get it)
The primary use of Capacitor bank is to improve PF but can it be used for voltage level improvement? KVAr=1.732*V*I*Sin(phi)? And here I will consider V=160V(As system's Vd is 160V) and based on this I will use a CapBank.
Is it a right solution?

 

[davidbeach]

Ever heard of a transformer? You need to do your distribution at a high enough voltage to have minimal voltage drop.

 

[alehman]

Your formula is basically correct. "number of runs" in the denominator is only accurate if the circuits are relatively distant from one another as compared to the phase-to-phase spacing.

Capacitance can improve voltage drop if your load is inductive, but not enough to fix this problem.


Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[GadreP]

Ok.. so capacitor bank KVAr rating (25 KVAr) calculated from formula KVAr=1.732*V*I*Sin(phi)will be correct? I will assume PF as 0.85 or 0.9 at receiving end & 0.8 at generating end. I hope by using capacitor bank of 25 KVAr at receiving end panel will compensate my 160V voltage drop.
To Davidbeach: I don't have transformer here thats my main problem.
Please suggest on using 25KVAr capacitor bank on receiving end considering PF at receiving end 0.85 or 0.9.

 

[waross]

Doomed to failure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Your capacitor bank can only compensate the inductive volt-drop in the line. It can't compensate resistive drops, and until you address that you will not get an acceptable volt-drop.

Your choices are:
1) Transmit at a higher voltage, with appropriate transformers at either end
2) Install outrageously large conductors to keep the resistive volt-drop within acceptable limits.

Option 1 is used by the utilities, not because it is complex but because it is economically viable. The alternative is to build something akin to the Brooklyn Bridge out of copper. If you choose that option please post some photos; it will be interesting. Let me know in advance - I'll be buying shares in Rio Tinto.




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If we learn from our mistakes I'm getting a great education!

 

[GadreP]

I guess Booster transformer at receiving end panel seems to be better option..

 

[ScottyUK]

You will have to put nearly double the load power into the cable at the supply end to meet the I²R losses. Economically that doesn't make sense unless you have a very cheap power source. A step-up / step down transformer pair is not especially expensive, and payback will be fairly short. At this rating you are looking at a couple of little dry types. Honestly, if what you propose made sense why wouldn't utilities the world over be doing it?


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If we learn from our mistakes I'm getting a great education!

 

[ArcFlashTraining]

Definitely need higher voltage.

http://www.arcflashtraining.net

http://www.arc-flash-training.com

 

[rockman7892]

Just a question maybe a little off target but in some cases isn't it possible to have the recieveing end voltage larger than the sending end voltage by adding a bunch of capacitance at the recieving end? I know this is not the case here in the OP but was wondering in what scenario this might exist?

 

[ScottyUK]

Google 'Ferranti effect' - tends to affect long, lightly loaded transmission lines. The voltage rise can be surprisingly large under the right (wrong?) conditions.


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If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

Isn't it possible to have the recieveing end voltage greater than the sending voltage if you have a leading current dropped across a highly inductive circuit?

Maybe this is what the Ferrani Effect is. I will check it out.

 

[rockman7892]

After looking up the Ferrani Effect it is exactly what I suspected with a capacitive current dropping across an inductive circuit. This occurs at light loads so that the charging current it the predominant current and is much greater than the load current to keep the current leading by a large amount.

What about for a loaded system that did have inductive loads and therefore a lagging current however a large amount of capacitance was added so that the capacitive current is greater than the inductive current and therfore forced the overall load current to be largely leading. This is of course assuming tha the transmission line itself exhibits a predominately inductive characteristic. Or is what I am suggesting here similar to the solution that the OP is suggesting and is simply not feasable in nature?

I know that the transformer is the correct solution but just for theoretical discussion I'm curious to the results of capacitor addition for such a case.

 

[GadreP]

For U/G cables we consider Vd= 1.732*I*(R cos (phi)+ X sin (phi))/1000* No. of runs.
I just wanted to know what formula do we consider while estimating voltage drop for overhead transmission line. Do we calculate by same formula or it will change considering 1 conductor?