Quick short circuit calculations 2

[anna8]

I have being calculating the short circuit currents for generators connected to a local busbar.
When the information was put into a power system program the results were lower than expected.

Are there any quick short circuit hand calculations to check the results?

In the example I had a 538kVA Generator connected to 440v busbar.

Xd = 0.464 pu
Xd’ = 0.0224 pu
Xd” = 0.154 pu
Base = 100 KVA

The three phase symmetrical rms current calculated was 6.2kA

Any suggestions would be much apprenticed

Thanks

 

[GreyGoose]

what is the rated voltage of the generator? Also, the value you have listed for transient reactance looks wrong (=0.224 maybe?). The values of reactance listed, are those the saturated values?

The calculation of short circuit contribution from a generator is very involved. However, a quick and dirty way of calculating approximate short circuit contribution is to divide the generator base current by the subtransient reactance. so if your generator has a base rating of 538 kVA & a rated voltage of 440V, the generators base current is: 538000/440*sqrt(3)=706A. divide that value by the subtransient reactance (x"d): 706/0.154 = 4.6kA rms sym. this value can be expected from the generator 1/2 cycle after fault. check the input values into the program again.

The method above also works for motor sc contribution.

If you want to do an involved hand calculation of
generator short circuit contribution check out IEEE std C37.013, the appendices have an extended explanation.

 

[jghrist]

Further to what GreyGoose wrote:

You give a base of 100 kVA. If the reactances you gave are pu at a 100 kVA base instead of at the machine rating base, then you have to use 100 kVA instead of 538 kVA in the calculations. Chances are, the reactances are at the machine rating and the 100 kVA base is irrelevant.

 

[jbartos]

Suggestion to the original posting: The data appear to be inconsistent. Where has 100kVA base come from? Are the reactances on 100kVA base or 538kVA base?
There are certain conversions to be made if bases are changed, namely,
Znew = Zold x (kVAbase,new/kVAbase,old) x (kVbase,old/kVbase,new)**2
e.g. if kVA=100 is old, then
Xd"new=.154 pu x (538kVA/100kVA)=.828 pu, which appears to be too high