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jamesjithin
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Join Date: Jul 2014
Posts: 9

Transformer Sizing Calculation

01/22/2015 4:12 AM

Dear all,

I have a doubt in the attached transformer sizing calculation sheet.In the sheet, we have calculated the total KW , KVAR & KVA loads are.

Then we have assumed to take a 2500 KVA tfr for loading these loads.

The power factor correction is also done afterwards.

However, in Row 20, some calculations are very confusing. Maybe my consultant had copied the formula from somewhere.

can somebody explain how the internal reactance(or that is what I guess) of this 2500 KVA is taken , as shown in cell H20?

Does that mean, the formula; reactive KVAR trafo= %Impedance * KVA, is not enough?

A small brief of the formula used in the cell 'H20' would be extremely helpful, since i cannot in anyway find a link between this formula and the one I wrote in the previous sentence,.

Regards

Jithin James

Attachment: http://1drv.ms/1JkdhHD
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Pathfinder Tags: calculation size Sizing transformer
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IdeaSmith
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#1

Re: Transformer Sizing Calculation

01/22/2015 8:06 PM

Trouble-shouting Excel spread sheets is a pain. If in doubt follow your instincts and use the formula that is rfrom the books and not the one that is embedded in Excel.

So chances are your consultant has it wrong, but why not talk to him???

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jamesjithin
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#2
In reply to #1

Re: Transformer Sizing Calculation

01/26/2015 11:57 PM

The consultant resigned! moreover, its just one cell, that you need to look into !
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IdeaSmith
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#4
In reply to #2

Re: Transformer Sizing Calculation

01/27/2015 8:44 PM

I beg to disagree. You will have to check the whole sheet, the whole file unless you trust that consultant in her/his work.

What does you theory book tell you then? Is the formula correct?

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7anoter4
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#3

Re: Transformer Sizing Calculation

01/27/2015 9:36 AM

If the reactive power of the transformer itself is Q=3*X*I^2 then if I=Sload/sqrt(3)/V and X=[approx.]=uk%/100*V^2/Srat

Q=3* uk%/100*V^2/Srat *( Sload/sqrt(3)/V ) ^2=uk%/100*Sload^2/Srat=uk%*Sload^2/(Srat*100) Q.E.D.

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7anoter4
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#5

Re: Transformer Sizing Calculation

01/28/2015 1:13 AM

You are right. By definition: uk%/100=sqrt(3)*Zk*Irat/Vrat.Then:

uk%/100=SQRT(3)*Zk*Irat^2/(Vrat*Irat)=SQRT(3)*Zk*Irat^2/(Srat/SQRT(3))

Or: uk%/100*Srat=3*Zk*Irat^2=Qrat.

If Xk=[approx.]=Zk then indeed Qrat=uk%/100*Srat but only for rated load.

For any other load Q=3*Xk*Iload^2.

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7anoter4
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#6
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Re: Transformer Sizing Calculation

01/28/2015 8:04 AM

I am sorry! It is clear that Qload=uk%*Sload-NOT ONLY FOR RATED LOAD , as you said in O.P.- but it is the same with Qload=3*Xk*Iload^2.

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