Short Circuit Calculation

[XFMRGirl]

Hi This is my first post and am working as an EIT. I am working on a 4.16 kV swgr spec which is supplied by two transformers in parallel. Both transformers are old and will be replaced in the near future. I want to take a conseravtive approach and make sure that the new switchgear will be able to hanlde short circuit.

My swgr specs. calls for swgr rated at 31.5 kA RMS Symmetrical at 4.16 kV.

This is what I have considered;
TR1- 27.6/4.16 Kv 5%z 5 MVA

Substation TR1 Unit
Size 5 MVA
Vol. 4.16 kV
%Z(One in Service) .05 P.U
%Z(Two in Service) 0.025 p.u
I Fault(P.U. one in service) 20 P.U.
%Z(Two XFRM in Parallel Impedance) .025 p.u
I(Fault P.U.) two in Service 40
I Fault(1) One in Service only 13895.06 Amps
I Fault(2) Both in service 27790.13 Amps

Based on this , Is a 31.5 kA rating would work. Just wondering if I missed anyting.

 

[magoo2]

Calcs look OK to me.

 

[davidbeach]

That looks like it might work, if there are no sources, such as motors or generators, on the 4.16kV system.

 

[ScottyUK]

It assumes an infinite bus upstream, which might not be a bad assumption if it's tapped direct off the HV transmission grid. If it is fed from an intermediate voltage then the fault level should be a bit lower than your worst-case calcs suggest.


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If we learn from our mistakes I'm getting a great education!

 

[bacon4life]

On medium voltage breakers is there a need to consider the X/R ratio? With higher voltage breakers if the L-G fault is greater than 70% of the rating or the three phase fault is greater than 80% of the rating, you have to check the curves in C37 as described on page 22 of this history of the C37 standards.

http://ewh.ieee.org/soc/pes/switchgear/TRVTutorial/C37InterruptingCurrentRatingsforHVCB-Wagner.pdf

 

[magoo2]

Looks like the standard is abased on an X/R ratio of 15 (or 17). If your situation is less that that, then you don't have to look any further. If it is greater than that value, then you need to look at the slower decay of the asymmetrical current.

 

[gdeep]

A couple of things to take into account during fault calculation

1) Fault contribution from the Utility( Ultimate vs present day)
2) Any synchronous motors attached on 4.16KV bus
3) 31.5KA is the short circuit interrupting rating, and not the bracing rating of the equipment.

Please correct if I am wrong. Thanks

 

[magoo2]

gdeep - Looks like you missed davidbeach's comments on the first 2 points.

on utility fault contribution - this amounts to adding a source impedance so the calculated results would be lower than what was already shown.

motor contribution - this would add some current to what was already calculated.

switchgear rating - short circuits have both thermal effects and forces that result from the current level. In order to meet a particular interrupting rating, I would expect that the bracing is already taken care of by the manufacturer.