Sub-Transient, Transient and Steady State Reactances of a Generator?

A generator's fault current is limited by 3 factors:

1. Sub- Transient Reactance Xd" 0 - 6 cycles

2. Transient Reactance Xd' 6 cycles - 5 seconds

3. Synchronous Reactance Xd - greater than 5 seconds

At the moment the fault occurs, substantial high current flows through the generator for first few cycles and the fault current rapidly decays. The generator reactance experienced by the fault current during this period is called subtransient reactance. After the subtransient period is over, the fault current value decreases slowly for another comparatively longer period of time. The generator reactance experienced by the fault current during this period is called transient reactance. Finally the short circuit current becomes steady and generator reactance experienced by the steady fault current is called steady state reactance.

Go to these links they have good information about the topic:

http://www.ee.lamar.edu/gleb/power/Lecture%2012%20-%20Symmetrical%20faults.ppt

http://www.cumminspower.com/www/literature/technicalpapers/PT-6008-GeneratorReactances-en.pdf

http://www.sayedsaad.com/Protection/files/short_circuit/1_short_circuit.htm

https://milsoft.com/smart-grid/images/uc2009/presentations/ea/Distributed%20Generation%20-%20Dan%20Harms.pdf

- MS