Engproguides question 54 exam

Is Ibase incorrect here?  Shouldn't it be using 13.8kV instead of 1kV?

a4u2fear said:

Is Ibase incorrect here?  Shouldn't it be using 13.8kV instead of 1kV?

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S_base is the only term that is maintained across all transformer changes in secondaries and primaries.  

Ibase, Vbase, and Zbase must be computed per zone.

They would have to be, otherwise (for instance) you would calculate a fault current several magnitudes smaller / larger then what would actually be present.  

I believe the problem incorrectly states to use 13.8 KV as the base.  It should say 1 KV base voltage (since we are in zone 2).

Edit....

They are skipping a step which may be part of confusion.  the problem states to use the 13.8 as the base originally becuase if the gen base was different then then the xfne primary it would add additional steps to the problem.  that is what they are trying to say there.

The secondary of the transformer still needs to use 1 kv as its vbase though.  

i think Zgen needs to be converted to 1kV base, for the rest of the solution to be valid, as in the author's calculation of Ibase on 1kV. 

after that the Ia1 value will change (because of change in Zgen) and finally we will have a different Ia1 value

kshitijs13 said:

i think Zgen needs to be converted to 1kV base, for the rest of the solution to be valid, as in the author's calculation of Ibase on 1kV. 

after that the Ia1 value will change (because of change in Zgen) and finally we will have a different Ia1 value

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The purpose of per unit calculations is to avoid doing what you are suggesting.

The Z PU, 15% or .15, is already calculated for sbase and rated voltage.   it would only need to change if either the rated power output differed from sbase or the transformer primary voltage was other then the terminal voltage of 13.8 kv.

The procedure is to determine sbase, determine the zones, pick one zone and work your through the system using the transformer ratios to determine vbase for ewch zone, calculate ibase and zbase for each zone, and finally adjust z pu of machines if different then sbase or vbase for that zone.