×
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

Power Flow study- Short Ckt calculation

Power Flow study- Short Ckt calculation

Power Flow study- Short Ckt calculation

(OP)
Have a question on short circuit study on our system.
Shown in a 115 kV system model. The model is missing a generator synch breaker at Point A and we are interested in the maximum available fault as seen by this breaker. The model calculates the fault on 115 kV Bus (say point B). Is this fault going to be same as the fault seen by the breaker at point A?
 

RE: Power Flow study- Short Ckt calculation

There will be a difference due to any impedance in the line between point A and point B.

If this impedance is small, and it probably is relative to transformer impedances, then the fault levels will be similar.

RE: Power Flow study- Short Ckt calculation

Agree with redfurry.

It also depends where the fault is. A fault on bus side of the breaker will only see contribution from its own generator. A fault on line side of the breaker will see fault currents from rest of the system (all other sources).

Rafiq Bulsara
http://www.srengineersct.com

RE: Power Flow study- Short Ckt calculation

Since you're using OneLiner (so it appears), there is a breaker rating module you could use to evaluate this.  You really need a breaker rating program or a thorough understanding of the breaker rating standards because it isn't enough just to compare breaker interrupting rating against available fault current magnitude.  Those breaker ratings are based on the fault X/R being less than some value.  If the X/R is greater, then you have to multiply the fault current by some factor (or divide the breaker rating by the same factor) and then make your comparison.  Out on the system well away from generators the X/R rarely enters into consideration, but close to generators you can find very high X/R and very difficult to interrupt fault currents.  On location on our system the maximum fault current through a generator breaker is about 44kA, but because of high X/R the breaker rating needs to be over 65kA.

RE: Power Flow study- Short Ckt calculation

(OP)
Appreciate the input
Redfurry/Rbulsara
I agree with your point. Other than the impedance of the line between A & B, the maximum fault seen by the breaker should be same as far as I can see. Rbulsara, I didn't mention the point about the fault since I am interested in the maximum available fault that the breaker needs to interrupt, whichever side it is from.

Davidbeach
Thanks and you mention an interesting point. The X/R that our system uses is around 38-39 which is high.  However, are you referring to the X/R multiplier used to convert symmetrical current to asymmetrical? It sounds like the fault of 44 kA for you is symmetrical and your asymmetrical rating is 65 kA. Other than this I am not familiar with any other multiplier ratio. Any article on this?
 

RE: Power Flow study- Short Ckt calculation

(OP)
I just realized you might be referring to the multiplier from ANSI C37.10 (1.12 for X/R=38.4). Yes, we have considered that already.

RE: Power Flow study- Short Ckt calculation

Pdshah
This is only a curiosity:
Are connections Y-Y  grounded-grounded,ungrounded-grounded or ungrounded-ungrounded?
 

RE: Power Flow study- Short Ckt calculation

(OP)
grounded grounded for the most part

RE: Power Flow study- Short Ckt calculation

Thank you. I guess this kind of connection does not exist in my country.(Brazil)

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