# Kaplan Problem sample 21



## kduff70 (Nov 20, 2014)

Hi I was working this kaplan problem and the only thing I dont understand is how they are calculating the Voltage base thru the transformer winding can anybody explain this better to me ? thank you

klapan.pdf


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## bripgilb (Mar 26, 2018)

Rallying the troops here to bring back an old thread and a much loathed practice exam.

Kaplan... Practice Problems 20 &amp; 21

@knight1fox3 @supra33202 @rg1 &amp; @Zach Stone, P.E....

I've attached my solution to #21 below. 

I've also attached the Q&amp;A to #20...  The original post contains a .pdf link to the #21 Q&amp;A.

My concern is the same as the original threader.  Is the reflected Vbase1 across the transformers in the Kaplan solution correct?

Thank you in advance.

brip


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## rg1 (Mar 27, 2018)

bripgilb said:


> Rallying the troops here to bring back an old thread and a much loathed practice exam.
> 
> Kaplan... Practice Problems 20 &amp; 21
> 
> ...


I could not see complete question #21. How to deal with pu system when you encounter a transformer? the answer is the the base power remains the same on both sides of a transformer. The base voltage and currents change according to transformation ratio. The pu power, voltage, current and pu Z remains the same on both sides of the transformer. I have not checked the calculations of question #20 but the procedure is okay.


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## bripgilb (Mar 28, 2018)

@rg1Explaining that the base is the same for both sides of the transformer makes sense.

But, if you are transferring voltage across the transformer why do they use the "a (turns ratio)" and not "a^1 (inverse turns ratio)".

One explanation I read online is to use intuitive reasoning to determine if the voltage is going from high potential to low potential, or visa versa.

I just don't like how vague that concept sounds even though I do understand the intent.

Best regards,

Brip


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## rg1 (Mar 28, 2018)

bripgilb said:


> @rg1Explaining that the base is the same for both sides of the transformer makes sense.  Base power or say Base VA remains same throughout!!! Not the base Voltage, currents and impedance.
> 
> But, if you are transferring voltage across the transformer why do they use the "a (turns ratio)" and not "a^1 (inverse turns ratio)". Once a  ratio is there, it is there. For example if a transformer transforms 1 Volt on one side (side A) to say 10 Volts on another side ( side B), it always does it with same ratio. So if you apply 2 Volts on side A, you get 20Volts on side B for 5 Volts you get 50Volts............. The BASIC formula for transformation are - Volts on side1/Volts on side2=Number of turns on side1/Number of turns on side 2 and for currents it is I1*N1=I2*N2. You call it anything, turns ratio or inverse turns ratio is your choice. I generally pickup things from basics to avoid confusion of nomenclature. Did it make some sense!!! Transformer is a both way device, so primary and secondary changes depending on how it is used. The side where power enters is Primary and so is the ratio.
> 
> ...


Brip please see my comments in red. I hope it will help you. If not, let me understand the specific latch in your understanding, I will try to help it out. The concept it easy, once you get it. 

Thanks


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## rg1 (Mar 28, 2018)

rg1 said:


> Brip please see my comments in red. I hope it will help you. If not, let me understand the specific latch in your understanding, I will try to help it out. The concept it easy, once you get it.
> 
> Thanks


The question 21 is wrong. You simply transform the current across transformers to get Generator current. There is no role of base voltage and base power there. Rather they could have specified actual Generator Voltage to ask for Generator current. You can  do it directly by Power formula Power= V1*I1=V2*I2 =V3*I3. You have been given Load current (Region 3) so you can directly jump to region1---- The power supplied by generator is equal to consumed by load.


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