Short circuit current on the switchboard

[shkim2000]

I received short circuit calculation on the switchboard however, the short circuit level needs to be lowered in order to select Circuit Breaker with less fault current in aspect of cost.
I therefore suggest to adjust constants values such as subtransient reactance on the four generators. If constants are increased I know that voltage drop should be taken consideration. Except voltage drop, is there any other adverse effects I should consider in adjusting constants ?
The constants are as below ;
Xd : 240%, T'd : 0.1478 sec
X'd : 25% T''d:0.0055 sec
X"d : 12.7% Ta : 0.04sec
RA : 0.8%

 

[davidbeach]

And how do you propose to adjust them? They are physical properties of the machine, not something you can adjust just because you don't like the fault currents.

 

[waross]

Is this the main breaker or a small breaker feeding a panel for local station power?
yours

 

[nightfox1925]

Hi, I am new to the group. I suggest reducing the short circuit level with respect to the overall system impedance like employing the use of series reactors, bus reactors, load re-arrangments or increasing cable or transformer impedance to limit the S.C. level. Of course you have to check voltage drops and check for transient stability if these generators are on-line. The generator constants are related to the dynamic properties of the generator as per designed duty and you need to consult the manufacturer of any limitations they may be having before making specified S.C. constants.

 

[nightfox1925]

In addition to the suggestion above. in one of the refineries we had upgraded wherein new sets of generators were added, the recommended generator impedances were specified by the client and the manufacturer designed the generator to meet the requirement. However, before this was done, short circuit, load flow and transient stability were conducted thoroughly to ensure that the system will operate as designed. Recommending the generator reactances requires careful and detailed system studies and close coordination with the generator manufacturer.