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- Thread starter lost4ever
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Your KV pu is not correct.

12.47 System - Vbase=12.47 V=1 pu, MVAbase=1MVA MVA= 40 pu Z= (1)^2/40=.025 pu

12.47 side Transformer = Vbase= 12.47V V=1pu, MVAbase=1MVA MVA=1pu, Z= .04pu

480 side Transformer = Vbase=480V V=1pu, MVAbase=1MVA MVA = 1pu, Z= .04pu

I=V/Z Z=Ztransformer + Z system = .04pu + .025pu V=1pu V/Z=1/(.04+.025)=15.4

12.47 System - Vbase=12.47 V=1 pu, MVAbase=1MVA MVA= 40 pu Z= (1)^2/40=.025 pu

12.47 side Transformer = Vbase= 12.47V V=1pu, MVAbase=1MVA MVA=1pu, Z= .04pu

480 side Transformer = Vbase=480V V=1pu, MVAbase=1MVA MVA = 1pu, Z= .04pu

I=V/Z Z=Ztransformer + Z system = .04pu + .025pu V=1pu V/Z=1/(.04+.025)=15.4

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It's given in the problem "transformer with 4% impedence"

For transformers the PU impendence quantity is always given for the rated voltage and MVA.

If you're operating the transformer at a different voltage then you have to change the impendence but if you're operating it at the rated voltage and MVA and you've chosen those rated values as your base then you can just use the given PU impendence.

For transformers the PU impendence quantity is always given for the rated voltage and MVA.

If you're operating the transformer at a different voltage then you have to change the impendence but if you're operating it at the rated voltage and MVA and you've chosen those rated values as your base then you can just use the given PU impendence.

Last edited:

@RedRaider2020, thanks now its clear.

Hi @RedRaider2020, I'm just wondering how did you get the PU values for the 12.47 system.

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I'm assuming we are using the 1 MVA as a base since the Transformer Base is 1000KVA=1 MVA. You did this to make the math easier if I understand that correctly.

Also for the 40 MVA, is that considered the actual MVA value because I know that S_pu = S_actual/S_base so in that case would S_actual = 40 MVA and S_base=1MVA therefore S_pu is 40pu as you stated?

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That's correct. You have to pick one base for the whole system so if you can pick one where all the values are 1 the math will be a lot easier. Yes 40MVA is the actual and divided by 1 becomes 40 pu. The values with units like MVA, KVA, KW, V, A will always be the actual value and the base is whatever base you choose it to be if they don't tell you to use a specific one in the question.I'm assuming we are using the 1 MVA as a base since the Transformer Base is 1000KVA=1 MVA. You did this to make the math easier if I understand that correctly.

Also for the 40 MVA, is that considered the actual MVA value because I know that S_pu = S_actual/S_base so in that case would S_actual = 40 MVA and S_base=1MVA therefore S_pu is 40pu as you stated?

Thank you for the clarification!That's correct. You have to pick one base for the whole system so if you can pick one where all the values are 1 the math will be a lot easier. Yes 40MVA is the actual and divided by 1 becomes 40 pu. The values with units like MVA, KVA, KW, V, A will always be the actual value and the base is whatever base you choose it to be if they don't tell you to use a specific one in the question.

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