Generator Reactances Xd", Xd'

I want to know the difference of

1- Saturated transient Xd"
2- Unsaturated transient Xd"
3- Saturated subtransient Xd"
4- Unsaturated subtransient Xd"

and which one is(are) to be used for short circuit calculations?

 

[Etrix]

Hello E_Power,

I think in the first part of your question, both transient reactances should be written like so - xd'- and not xd". Probably a typo.

Answering the second part of the question, for SC contribution from the generator, you need to use the saturated direct-axis subtransient reactance xd" (as per IEC 60909) and apply its full negative tolerance to get a degree of pessimism. The xd" is the effective reactance at the moment of short circuit.

I quote the IEC which is European, because I use that and since you haven't supplied me with any info of location or standards, that's all I can give you.

I am still a novice myself in power systems analysis, but I thought I'd get you going. The definitions you need would probably be best off found in a textbook (which I haven't got handy with me at the moment). Hope this little bit helps.

Etrix

 

Thnx Etrix,
Sorry for typographical error. I m following American standards so I need IEEE or ANSI reference.

 

[redtrumpet]

IEEE Buff Book has procedure.

 

[McTaggart]

The reactance of a generator changes during a fault. There are three stages, subtransient, transient and synchronous. For most faults, subtransient reactance is used because it produces the highest fault current, i.e worst case scenario.

With regards to saturated and unsaturated, I believe this depends on whether it is the voltage or current that is held constant during the initial generator testing. There is some debate over whether it is better to use the saturated or unsaturated values (at least where I work). But if I am setting a relay to operate against a highly damaging fault condition I would probably use saturated subtransient reactance.