Thursday, December 26, 2013

Generator reactances


SCR (short circuit ratio) = Excitation current at rated voltage in OC condition/ Excitation current at rated                                                      voltage in sc condition
                                     = 1/ direct axis

Generator reactants (per unit)
 


Typical Range
Effective time
Condition
X’’d
0.09-0.22
0 to 6 cycles
Maximum instantaneous current & current at time  circuit breaker usually open
X’d
0.13-0.20
6 cycles to 2 sec
Current at short time delay of CBs
Xd
1.0-2.5
After 2 sec
Steady state current without excitation support
X0
0.06-0.15

L-G fault current
X2
0.10-0.22

L-L fault current