×
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

Generator short circuit

Generator short circuit

Generator short circuit

(OP)
Hi,

I want to calculate the short circuit for 3 generators in parallel. The purpose is to specify the circuit breakers. Gensets are 2500 kVA each.

By hand:
1. Current per phase @ 230V I = 3623 A.
2. Worst case x"d = 0.11.
3. Max short circuit current (symmetrical) = I/x"d = 33000 (more or less).
4. (3) gensets in parallel=> Isc max = 33 kA x 3 = 100 kA

I think too high.

By software (DOC Win from ABB):
1. For one generator, Isc  = 10 kVA (a bit less actually).
2. For 3 gensets, Isc = 24 kA.

Which one is correct you think?
 

RE: Generator short circuit

When you short a genset you are limited by what the AVR can whack out - this can drop the output and result in a reduced shortcircuit current leading to increased time to clear the fault from the system. protection is always an issue with this sort of set up as you spec for mains then you have to spec for generator.....

RE: Generator short circuit

(OP)
Ok, i understand that a lower Isc will take longer time to trip. Terasaki have "specially calibrated I2t curves for generators" in the electronic relay on the air circuit breakers (AGR-31B Type "S").

So what will be the sc on the synchronized busbar?  

RE: Generator short circuit

Your hand calculation looks about right for the generators themselves.
The software solution looks way too low, but what have you got in between the generators and the bus?

RE: Generator short circuit

A quick modelling of your generators (using SKM power tools) produces 23kA per gen and a maximum of 63.7kA on the busbar with all three connected.

RE: Generator short circuit

I agree with genhead, the hand calculation looks correct. Has glendale perhaps taken the 230V as the phase-phase voltage, and included the "c-factor" of 1.1 in the calculations?

Auslee, is it possible to have a look at your DocWin-file (the *.ile-file)?

ijl at http://pp.kpnet.fi/ijl

RE: Generator short circuit

AusLee, when modeling your (?) setup by DOC using default generator: turbogenerator 2500 kVA, 400 V phase-to-phase voltage, xd"=12.5 %,  I receive 35 kA/generator, what is close to your hand calculations.
 

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
 

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