Switchgear Short Circuit Fault Level

[UtilityGy]

Hello All,

I am buying a 4.16 KV swgr fed by two incoming XFRM's rated 5 MVA, 27.6 kV/4.16 kV. Both will operate in parallel with no tie.

Voltage= 4.16 kV
TR1= 5 MVA %Z1= 5
TR2= 5 MVA %Z2=5

Short Circuit Contribution MVA(TR1)= 20 P.U.=13895 A
Short Circuit Contribution MVA(TR2)= 20 P.U.=13895 A

Total Contribution= 40 P.U.

S.C Contribution(Total from TR1 & TR2)= 27790 A

My switchgear is rated at 31.5 kVA rms symettrical. I just wanted a second opinion if I am missing anything in calculating maximum short circuit contribution towards 4.16 kV switchgear.
....

Appreciate your input !

 

[jghrist]

You might have to consider the X/R ratio since you are close to the rating. The rating is based on an X/R ratio of 17. See thread238-254443

 

[pwrtran]

your math is correct.

 

[dpc]

If the switchgear is rated at 31.5 kVA, you definitely have a problem.

If it's actually 31.5 kA, refer to jghrist's response.

Also, you need to consider any motor contribution that may be coming from feeders connected to the switchgear. In a large facility, these cannot be ignored.

David Castor

http://www.cvoes.com