×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Short Circuit Calculation

Short Circuit Calculation

Short Circuit Calculation

(OP)
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.

RE: Short Circuit Calculation

Calcs look OK to me.

RE: Short Circuit Calculation

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

RE: Short Circuit Calculation

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.
  

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

RE: Short Circuit Calculation

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.

RE: Short Circuit Calculation

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

RE: Short Circuit Calculation

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources