×
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

Available Fault MVA from separate source

Available Fault MVA from separate source

Available Fault MVA from separate source

(OP)
Consider: Two separate 69kV lines feeding two 12.47kV lineups through 20MVA transformers.  The lineups are connected through a N.O. 1200A tie breaker.  The fault duty at one lineup is 100MVA and 105MVA at the other lineup.  If conditions on one line are inadequate the load will be transferred to the other line in a closed transition process.

What is the fault duty at the interconnecting breaker during the closed transition process when the tie breaker is closed?

RE: Available Fault MVA from separate source

If the two 69-kV lines are essentially decoupled from each other (which they probably are not), then the fault MVA with the bus tie closed will be approximately the sum of the individual contributions.  If the 69-kV lines cannot be considered decoupled, then the total fault MVA will be less than the sum and you need to know more about the line impedances and the equivalent impedance ahead of the lines.  In any case, the fault MVA with the bus tie closed will never exceed the sum of the individual contributions.

RE: Available Fault MVA from separate source

Agreed, but note that the sum of the two fault levels is NOT the current that the tie breaker will see.  The feeder breakers will be subject to the summated fault level, but the tie breaker will only ever see the larger of the two contributions.

RE: Available Fault MVA from separate source

True that the tie breaker will only see the larger of the two fault contributions as a maximum.  However, any feeder from that bus will see the sum (or nearly so, as described above).  This is a common problem and is usually ignored if the closed transition is for a very short duration.  I prefer to either make the transition automatic so that it will be limited to .25 seconds or less or, if manual, only give the operator 10 or 15 seconds to complete the operation before automatically tripping the tie breaker again.  The reasoning is that the chance of a downstream fault during this switching operation is quite remote.

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