×
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

CT calculation
16

CT calculation

CT calculation

2
(OP)
can anyone show me with an example how to calculate
the over dimensioning required for a CT  in order to keep
it from saturating due to transient effects?

RE: CT calculation

Joan 271273 please forward me the CT info you have it would be greatly appricated.

RE: CT calculation

Suggestion: Reference:
1. Sutherland P. E. "Applying CTs with Digital Ground Relays," IEEE Industry Applications Magazine, Vol. 7, No. 2, pp 71-79, March/April 2001
Reference 1 indicates that the best solution is to design the system to prevent the line CTs from full saturation during external phase faults. So beware of various clandestine calculations!

RE: CT calculation

can anyone show me with an example how to calculate
the dimensioning required for a CT  in order to keep
it from saturating due to transient effects?

RE: CT calculation

See Chapter 3 of ANSI/IEEE Std. 242 (IEEE Buff Book).  Basically, standard CTs for relaying (those in accordance with ANSI/IEEE C57.13) will be accurate within 10% at currents up to 20 times rated current at rated burden.  If your maximum fault current is going to exceed 20 times the CT rated amperes, then you may have a saturation problem.

RE: CT calculation

2
The CT can saturate for currents below 20x rated, due to transient DC component of the fault current waveform.  
I have a copy of the 1976 edition of the Westinghouse Applied Protective Relaying book, which gives the following formula -

Transient saturation is avoided if :
Vk >= 6.28*I*R*T

Vk = CT kneepoint voltage
I= symmetrical secondary current, A RMS
R= total secondary resistance
T= dc time constant of the primary circuit in cycles
  T=(Lp/Rp)*f
  Lp = primary circuit inductance
  Rp = primary circuit resistance
  f = system frequency

This formula will give a reasonable approximation.  More details are available from the folllowing reference: http://www.geindustrial.com/pm/notes/ger3973.pdf

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