Question About Short-Circuit Current On A Synchronous Generator

[constantlylearning]

When we model short circuit capacity of a synchronous generator, we usually model it based on the subtransient reactance for the "worst-case" scenario, which would also include the increase related to the DC offset based on the X/R ratio. For our purposes, let's just talk about the symmetrical fault value, leaving the DC offset out of it.

I've always taken it for granted that the subtransient reactance would give you the worst-case value, without thinking about it too much. One thing that's been nagging the back of my brain for years is this: If the generator is running at something less than maximum capacity (say 50% of MVA capacity), it SEEMS like the energy available for a fault would be less than the theoretical maximum and therefore the short circuit current would be less than the maximum theoretical. OR... Is the subtransient symmetrical fault current strictly a function of the subtransient reactance and is it not affected by the generator operating capacity?

I feel like I should know this, but I'm more of a transformer guy than a generator guy.

 

[cranky108]

Are you assuming the energy of the inerita would be something less if you were running at a lower MVA output?
How do you reach that?

If you put a fault on a generator, the generator would go through two stages before it reaches the stedy-state with the fault remaining. At which point would you want to model?
The stedy-state would be affected by the MVA output of the prime mover. But the inerita should not be.

 

[dpc]

Generator loading prior to a fault is not a significant factor in generator SC output. Use X"dv value for maximum symmetrical fault current at the generator terminals.