×
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

CTs problem

CTs problem

CTs problem

(OP)
We have three CTs on a feeder rated 400/1, 36 kV. The CT on one phase failed and we can't get one urgently. We have one rated 200/1, 36 kV. Is it possible to use the 200/1 in conjuction with the others considering that the relays are electromechanical on which the plug settings can be adjusted.

RE: CTs problem

At full load, assuming full load to be 400A, you will have 2x nominal current flowing in your relay. Can your plug settings accommodate this? Do you have a residually connected earth fault element? If so then forget it.

You've told us so little about the relaying scheme that it is impossible to give a yes/no answer.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

RE: CTs problem

(OP)
The scheme is an overcurrent with residual earthfault with the o/c setting at 100% and E/F setting at 0.15%. The plug settings can actually accomodate the required settings from 10% to 200%

RE: CTs problem

Does the CT thermal factor allow using the 200/1 ratio?  Do the 400/1 CTs have a 200/1 tap?  If so, then you can set all to 200/1 and change the plug setting to suit.  You cannot use the residually connected earth fault element if you use unequal ratios.

RE: CTs problem

A CT group used with a residually connected E/F relay must be made up of 'identical' CTs. Identical in this case primarily concerns ratio, winding resistance, and kneepoint voltage. Unfortunately there is no way to incorporate unmatched CTs of differing ratios into a residual group for an electromechanical relay.

Sorry.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

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