CR4 - The Engineer's Place for News and Discussion ®
Login | Register for Engineering Community (CR4)

 Previous in Forum: Heating Elements on Different Voltage and Hz Next in Forum: TDR
Commentator

Join Date: Jul 2010
Posts: 90

# Generator Decrement Curve

01/24/2011 5:37 PM

Hi all, I'm trying to calculate how SKM Powertools (power system modelling software) comes up with this initial value for the generator decrement curve:

Paramaters im using: Xd"=.15pu, Xd'=.21pu, Xd=.5523pu; Td"=.025s, Td'=.42s, Ta=.04s, Sbase = 800kVA, Vbase=415V, generator is running at 1.06*415V supply

My calculations are as follows:

Xd(base) = Vbase^2/Sbase = 415^2/800k = .21ohms

Xd"(ohms) = .15 * .21 = .0315 I(symmetrical) at t~=0s: (1.06*415)/(1.73*.0315) = 8.148kA

Why does SKM list the current value at .01s as 6.7kA (symmetrical)? I think that is too low...

__________________
Electrical Engineer
Interested in this topic? By joining CR4 you can "subscribe" to
Guru

Join Date: Dec 2010
Posts: 933
#1

### Re: Generator Decrement Curve

01/26/2011 8:00 AM

If Td" is 0.025 sec, this implies exponential decay to 1/e = 1/2.7183 = 37% initial after 0.025 sec. Since exp(-0.01/0.025) = exp(-.4) = 0.67 and 0.67 x 8.15 = 5.5 kA, then 6.7 kA does not seem low.

OK??