# Per Unit and #527



## Kuku (Oct 3, 2008)

I feel dumb on this one, but all my reference material explains pu as being actual / base. So why is the solution found by taking the base / actual?


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## Dark Knight (Oct 3, 2008)

Kuku said:


> I feel dumb on this one, but all my reference material explains pu as being actual / base. So why is the solution found by taking the base / actual?


I am not sure of what your question is but will give it a try:

The wording of the problem tells you the Xmer information on a MVA. Take that as the actual or "Old". They are just asking what would the impedance be if calculated at another MVA base. That would be my explanantion.

Think of this as a pu transfer. Sometimes we transfer the pu from the primary to the secondary, others we just tranfser from an old base(actual in this case) to a new one.

Hope this helps


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## Kuku (Oct 3, 2008)

Dark Knight said:


> I am not sure of what your question is but will give it a try:


My texts show solving for per unit values by dividing the actual by the base value. In this problem it says that the transformer is 25MVA and that we are on a 100MVA base. I don't understand why it is not solved by:

8% * (25MVA/100MVA)


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## Dark Knight (Oct 3, 2008)

Kuku said:


> My texts show solving for per unit values by dividing the actual by the base value. In this problem it says that the transformer is 25MVA and that we are on a 100MVA base. I don't understand why it is not solved by:
> 8% * (25MVA/100MVA)


Sorry man. I might be shooting blanks here but from what I remember (was tangled in that same game of words), when they gave you the ratings of the transformer they said it was rated at 25MVA. That makes 25MVA your actual MVA base. Now you have to convert to the new one, that is 100MVA.

If it makes it easier for you, think in terms of old and new base. The old base is 25MVA and the new is 100MVA. If you have a SCHAUM's Power Systems book, they explain that really good.


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## MDElec (Oct 3, 2008)

Think about it this way:

0.08/ZB = 25MVA/100MVA

1/ZB = 25MVA/(100MVA*0.08)

ZB = (100MVA*0.08)/25MVA

or

25MVA is 25% of the base MVA

Therefore, 0.08 is 25% of the base impedance. Therefore, base impedance equals 0.32


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