X/R ratio of Generator

[power2020]

Dear Folks,
I have started power system modelling very recently. For generator modelling, in X"/R -ratio, which resistance we have to consider? between phases or phase to neutral (for star winding generator)
I tried with both resistances for calculaing Transient and sub-transient fault currents but not matching with Generator supplier's fault current data which is attached.
The generator manufacturer has given max. 3ph. Symm.S.C. current:8314A (Is it steady state fault current or Transient fault current).
Your reply will be highly appreciated.
and thanking you.

 

[power2020]

Generator Decrement curves:

 

[tin2779]

Are you conducting fault at the line terminals of your generators ?

 

[power2020]

Thank you tin2779 for clarification regarding resistance. What is current given in decrement curve - Max. 3 ph. Symm. S.C. current. Is it sub-transient/transient or steady state fault current?
Advanced thanks..

 

[QBplanner]

8314 A I believe is a subtransient Sys. fault current within Td" period (Start from 0.01s-0.1s). ASys.value would be the peak value as mentioned in the drawing 1.732 X 8314 A half cycle after fault.
Don't know why the LN fault or LL fault current increases after fault ? - Lack of damping?

 

[power2020]

Thank you QBplanner,
Fault current should be reduced generally from sub-transient state to Steady state.
But in the attached decrement curves, as you said, L-L & L-G currents started increasing.
What are the possibilities?

 

[QBplanner]

Hi power2020:
A typical curve should be like the one I attached.
Don't know how and what they did to get the curve you may want to go back ask them the detailed modeling of both machine, exciter and fault.
Regards,

 

[QBplanner]

Sorry I missed th attachment

 

[power2020]

Thank you QBplanner,
I will ask manufacturer, the reason for increasing trend of L-L and L-G fault over a period of time.
W.r.t. typical attached curve, what is the reason for increasing fault current after approx.0.1 sec.

 

[rcw retired EE]

Could the increase be from voltage regulator action increasing the field current in response to the fault? 100mSec seems a little quick for a voltage regulator response when the time constants of the field windings and exciter windings are considered.

 

[QBplanner]

I would believe it is because of the AVR.The fast of AVR can be acted within 0.1 Sec.
Regards,