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 Post subject: Subtransient Reactance
PostPosted: Wed Sep 08, 2010 10:37 am 
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Joined: Tue May 18, 2010 7:03 am
Posts: 53
When provided with the "typical" data sheet for an alternator, is this the data (subtransient reactance) used in the study? Or is the "typical" data used to calculate the subtransient reactance of the genset?

Just want to make sure I'm interpreting this correctly.


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PostPosted: Thu Sep 09, 2010 6:01 am 
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Location: Spartanburg, South Carolina
You need to be more specific in your question. What software? What input? If the data sheet gives the subtransient reactance, what do you mean by using the data to calculate the subtransient reactance?


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PostPosted: Thu Sep 09, 2010 8:00 am 
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I'll try to explain it better. The generator mfg provided a alternator data sheet that is called "typical submittal data". The test data is for a specific alternator on a 480 volt, 1300kw generator.

This alternator was actually provided on a 480v, 1000kw generator. Cummins has a technical paper that states "generator reactances are published in pu values on a specified base alternator rating. Where the gen set rating differs from the alternator base rating it will be necessary to convert the pu values from the alternator base rating to the gen set rating."

The alternator Xd" provided is .119pu. After conversion from a 1300kw unit to a 1000kw unit, Xd" is .092pu.

The same alternator can be used on a variety of sized generators but it looks like the test data uses one size as a base. So must the "typical data" reactances be converted before being used? Just wondering what most people doing studies use - the typical data or converted data.


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PostPosted: Mon Sep 13, 2010 5:38 am 
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The subtransient reactance is based on the per unit (p.u.) system. Converting from 1300 kW to 1000 kW is a matter of scaling the value.

0.119 x (1000/1300) = 0.092 p.u.

I scaled using 1000 and 1300 as the base values except normally the base is kVA not kW (or MVA not MW). Using kW still works in this case since the power factors would cancel.

However, the actual impedances in ohms requires using kVA or MVA. So assuming both sizes have a 0.8 power factor, then

1000 kW = 1000/0.8 = 1250 kVA
1300 kW – 1300/0.8 = 1625 kVA

The actual impedance of the unit in Ohms is the same regardless of which set of p.u. impedance and kVA values you use.

As an example:

Zactual = Zp.u. X Zbase

Zbase = (kVbase)^2 / MVAbase

kVbase = 0.48 kV
MVAbase = rating of the unit

1300kW / 1625 kVA

Zbase = 0.48^2 / 1.625 MVA = 0.1418 Ohms
Zactual = 0.119 p.u. X 0.1418 = 0.0169 Ohms

1000 kW / 1250 kVA

Zbase = 0.48^2 / 1.250 MVA = 0.1843 Ohms
Zactual = 0.092 p.u. X 0.1843 = 0.0169 Ohms

Zactual 1625 kVA = Zactual 1250 kVA = 0.0169 Ohms

I believe most software just uses the data including for your specific application kVA, Xd’’ etc. and the software takes care of the rest of the calculations. This assumes it takes the kVA rating as the unit’s base value.

_________________
Jim Phillips, P.E.
Brainfiller.com


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