Engineer Boards

# Transient Study Question

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I'm strugguling to understand why the current in this problem is I = 1pu. The problem is as follows:

An electric generator is rated for 20MVA, 13.8KV. It is a diesel engine driven, three phase, four-wire system. The generator is sailent pole with dampers. The manufacturer's data for reactances is:

X"d = 0.20, X'd = 0.30, Xd = 1.25, Xq = 0.90

Assume a terminal voltage of 1pu for a transient study conducted at rated power and voltage. The power factor is 1.0. What magnitude of generated voltage should be used for this study?

I know the following:

E'm = Vt - iX'dmI , where E'm = generated transient voltage

Vt = 1pu Xdm = 0.3 These where stated in the problem.

Not really sure why I = 1pu. I guess it must have come from Vt being 1pu and the study being conducted at rated voltage and power.

Some explanation behind this would be appreciated.

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If the base values are the same as the rating of the machine (20MVA and 13.8kV), then the pu current at rated output will be 1.0 by definition. The only way that changes is if you are doing the study on an MVA base other than that of the machine (100MVA or other).

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Flyer_PE,

Thanks for the response. I now realize the stupidity of my question.

If the tests are run at the machine ratings; the ratings are the machines base values. Since the per unit value is:

Actual value/Base value

Then the I, V, and S will all be 1.

Thanks again.