MV bus short circuit I

[AMBMI]

We have a 11 kV bus bar with about 14 MVA of loads (90% motors).
We got (customer data) the short circuit current values: 35 kA as Ikss (subtrans I)
I feel such value a bit too high. Also considering the motor contribution I was expecting 12-13 kA.
(assuming a 15 MVA 9% trafo feeding the system we get 8.75 kA at 11 kA + about 3600A as motor contribution (sum of their starting currents))
What's your experience?
I have the impression that we could get 35 KA only if directly connected to a bus with the generators (we don't know how "our" 11Kv bus is connected)
Thanks in advance for any feedback

 

[Alex68]

I think that 35kA (for 1s or 3s) is the rated short-time withstand current of the switchgear

Alex68

 

[AMBMI]

Thanks for the answer but the bus rated current is 40 kA for 3sec.
The value we got is 35.2 kA, it's coming from a calculation.
35.2 looks like the Ip (peak value)....

 

[7anoter4]

Hi AMBMI
If your 11 kV distribution bus should be supplied from a 40 MVA 7.5%uk 33/12 kV Transformer from an Utility Substation the total Ikss would be 35 kA as per your customer data. The shortcircuit current depend on grid impedance more than on your facility motors.
I don't think there is a utility transformer dedicated for your supply only.
If we take:
All motor Ist/In=7.5
14 MW 100% motors
Substation transf.40 MVA 7.5% uk
50 Hz only
Medium Voltage [Substation]=33 kV
Inom33kV= 50 KA [33 kV Utility Grid ]
No feeder [12 kv supply to your Facility] impedance [Distance =0 ]
we will get Ikss=35 kA
Best regards