Power Flow study- Short Ckt calculation

[KnicksJets]

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?

 

[redfurry]

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.

 

[rbulsara]

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

 

[davidbeach]

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.

 

[KnicksJets]

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?

 

[KnicksJets]

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.

 

[odlanor]

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

 

[KnicksJets]

grounded grounded for the most part

 

[odlanor]

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