Base Voltage Throught Transformer

[TNPE]

@rg1

That is the only approach I've ever used when working with pu.  Not that you're approach is "wrong," but it is convoluted and more difficult than the pu approach.  You can do everything necessary with a given power base and a given voltage base.  From this, move from the generator down line to the motor, changing bases as required across XFMRS (i.e. this could be a voltage and/or power base, depending on the problem statement and the given initial bases and ratings).   

I have never seen your approach in academia (or texts) when working with pu analysis.  Also, it'd be wise to be careful with your approach as you may have to do multiple change of bases.

 

[kduff70]

Thank you all but I have finally figure it out . I agree with TNPE the base method is the best approach. I attach my hand written calculation to show how I came up with  the answer . When finding the Vbase of the motor  I took the Vbase of T1 over The ratio of T2 and I get the Vbase of the motor and from there I get the Zpu of the motor .Thank you all for the help

View attachment NewDoc2017-07-29.pdf

 

[rg1]

Thank you all but I have finally figure it out . I agree with TNPE the base method is the best approach. I attach my hand written calculation to show how I came up with  the answer . When finding the Vbase of the motor  I took the Vbase of T1 over The ratio of T2 and I get the Vbase of the motor and from there I get the Zpu of the motor .Thank you all for the help

View attachment 9903

Yes either you take horse to water or water to horse, the result is same. When you travel through transformers, it will result into transformation of the quantity(V,I,Z not VA) and you can not avoid it in this question. It was a learning for me as I do not remember I did ever take bases across transformers. This too, seems interesting. For this a big thank you to all of you. 

Having said that just for the benefit of the forum transferring impedance across a transformer (the one I used in the solution) is also an important concept, need to be learned. I saw a few questions on that concept in few practice exams and may be in NCEES paper.

Thanks a lot again.

 

[cos90]

Combining everything together like that confuses me. What helps is remembering the transformer ratio is the relationship between phase voltages.

To answer your original question this way:

T1 ratio is 220/ (1.73*25)=5.086

T2 ratio is 130/13=10

22*5.086 = 111kV ph-n

111/10 = 11.1kV 

That said I missed it first workthrough. This question makes me want to get the Kaplan test. I need to practice with questions harder than the test I am taking to succeed.

 

[rg1]

Combining everything together like that confuses me. What helps is remembering the transformer ratio is the relationship between phase voltages. There is nothing wrong in that provided you can keep track of the ghost sqrt3 and the ratios given but generally in three phase we take Line values. I just want to understand how you do and see if there is any gap or chance of error by doing that way.

To answer your original question this way:

T1 ratio is 220/ (1.73*25)=5.086 ( Should phase Voltage not be 25/sqrt3 )

T2 ratio is 130/13=10 

22*5.086 = 111kV ph-n

111/10 = 11.1kV 

That said I missed it first workthrough. This question makes me want to get the Kaplan test. I need to practice with questions harder than the test I am taking to succeed.

 

[TNPE]

Yes either you take horse to water or water to horse, the result is same. When you travel through transformers, it will result into transformation of the quantity(V,I,Z not VA) and you can not avoid it in this question. It was a learning for me as I do not remember I did ever take bases across transformers. This too, seems interesting. For this a big thank you to all of you. 

Having said that just for the benefit of the forum transferring impedance across a transformer (the one I used in the solution) is also an important concept, need to be learned. I saw a few questions on that concept in few practice exams and may be in NCEES paper.

Thanks a lot again.

You're not necessarily taking bases "across" a XFMR, per se. Think of it more along the lines of a zone.  A generator zone up to a XFMR, say.  From the secondary "base" of the XFMR, traveling down the transmission line to the next XFMR... Secondary base of this XFMR down line to the load.  You're merely relating each zone to a starting system-wide base, and adjusting/relating each successive zone as necessary.  But the genius in pu analysis is, everything in the system is related by a scalar and it's easy to spot mistakes.  If you're working with fractional values (ideally, 1pu or less), but you end up with 12.862pu, pretty good indication you've made an error.