Synchronous generator short-circuit current 2

[Cubrilo]

It's well known that the typical value of induction motor short-circuit current is about six times rated current. My question is: "Is there a typical value for synchronous generator steady-state symmetrical short-circuit current, like 1.5, 3 or 6 times rated current?" I made next calculations: xs=1.8 (a typical value, let's say) and cosphi=0.8 (lag) then maximum excitation voltage is e0=sqrt(e^2+xs^2*i^2+2*e*xs*i*sinphi)=2.53 yields isc=e0/xd=1.4. For xs=3 (diesel-driven generators) by the same way isc=1.23. If I'm not making some mistake, that means that synchronous generator steady-state short-circuit current has value between, let's say, 1.2 and 1.5. If that is true then short-circuit state on synchronous generator wouldn't be dangerous and unwanted occurrence as I thought. Or the danger of short-circuit laying in dc component of short-circuit current? Also I would like to know where can I find data on manufactured synchronous machines which include parameters like Xd, Xq, Rs etc.
My question is primary addressed to people who are designing or testing synchronous generators(hydrogenerators, steam-turbine generators and diesel-driven generators), because they know actual data of real, existing generators. Many thanks to all for their time!

 

[peterb]

You are correct in that the SUSTAINED short circuit current from a synchronous generator may be a relatively low value. This is because the effective reactance in this steady state time frame would be the machine synchronous reactance, Xd, which has a high value - typically around 2.0 per unit or higher.

The value of reactance that is considered when evaluating short circuit conditions is the subtransient reactance, X"d, which typically will be in the region of 0.09 pu for a 2-pole generator. This determines the maximum value of short circuit current at the instant that a fault occurs. For a machine operating with 1.0 pu terminal voltage and X"d=0.09, the maximum Isc = (1.0/0.09) = 11.1 pu RMS symmetrical.

Incidentally, the value of X"d for induction motors is typically around 0.17 pu, which gives the 6x typical value for motor short circuit contribution. There is no steady state contribution from induction motors.

 

[Audun]

Since I had a datasheet beside me, here you are...
Typical syncronous generator data:
Voltage: 480 V threephase
PF = 0.8
Power = 1150 kVA
Freq. = 60Hz

X'd = 0.157 p.u
X''d = 0.122 p.u
Xq = 1.210 p.u
T'd = 0.132 sec.
T''d = 0.010 sec.

Audun