Unsaturated and saturated subtransient reactance

[tricard]

Hi Guys,

I am working on a short circuit study with four generators (range in size from 5MW to 30MW). I have been provided the subtransient reactance, Xd'', however it turns out that all the values I have been given are unsaturated values. There isn't a way to relate unsaturated values to saturated values is there? I was just going to default to IEEE std. 141 if I can't find any type of relation (and my hopes are pretty low on there being a boiler plate relation, given the physical differences from one generator to the next)...

None the less, if anyone knows a place i can look it would be greatly appreciated.

Tim

 

[dpc]

I would use the data in the Red Book. You could look at Analysis of Faulted Power Systems by Paul Anderson, or "A Practical Guide to Short Circuit Calculations" by Conrad St. Pierre. But I'm not sure if there is anything in those that would be any better.


David Castor

http://www.cvoes.com

 

[ScottyUK]

Check whether the Red Book values relate to the basic type of machine you have - 'typical' values for a turbo machine will be quite different to those for a salient pole type. I don't have the Red Book, so can't comment further.


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If we learn from our mistakes I'm getting a great education!

 

[tricard]

Thanks for the info ScottyUK and dpc. The specs I received for the unsaturated values are greater than the Red Book values as expected. For each generator, the Red Book Xd'' is about 30% lower than the tested values. I agree dpc: I am going to stick with the Red Book; this will at the least give me higher fault currents.

Out of curiosity, does anyone have any published information with both unsaturated and saturated reactances? I definitely won't make any conclusion based on your published info, but it is nice to see some actual values for generators. Specifically, salient pole hydro gens.

Thanks for the input

Tim

 

[ScottyUK]

Have some at home, but mainly for bigger turbo machines. Not sure if that's going to be any use for you. I'll have a look when I get back if you think it will help.



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If we learn from our mistakes I'm getting a great education!

 

[EddyWirbelstrom]

Australian Standard AS3851 Section 7.8.1 states "where only unsaturated values of X"d are available, assume the saturated values to be 85% of the unsaturated values".

AS3851 is based on IEC60909, however section 3.6.1 of IEC60909 does not mention any ratio of X"d sat / X"d unsat to use when only X"d unsat is available.

 

[burnt2x]

Try IEEE Std 115. If my memory serves me right, Xdu may be used in place of Xds if the test is performed at around 0.4 p.u. of normal voltage. Don't have my test procedure reference right now.

 

[wolf39]

tricard:

Eddy is right. With an X"d sat = 0.85 * X"d unsat one is on the safe side.

For determining X"d unsat you have to perform a sudden three-phase short test with a terminal voltage as low as possible (10 to 15%). For X"d sat determination a terminal voltage of 100% is required. This is very risky and is therefore done not very often. A test with 40% rated voltage gives you neither a correct "sat" nor a correct "unsat" X"d value.

Regards

Wolf

http://www.hydropower-consult.com

 

[VTer]

Hi, I only have numbers for a 2MW/2.5MVA diesel engine gen and I get about X”dsat/X”dunsat = 0.87, X’dsat/X’dunsat = 0.9, and Xdsat/Xdunsat = 0.7.

The actual pu values are

X”dsat = 0.200
X”dunsat = 0.230
X’dsat = 0.270
X’dunsat = 0.300
Xdsat = 1.860
Xdunsat = 2.660

These number vary from one set to the next, but X”d ratio is close to what Eddy has stated.


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