×
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

ZCT's secondary connected in parallel to protection relay

ZCT's secondary connected in parallel to protection relay

ZCT's secondary connected in parallel to protection relay

(OP)
The secondary of the transformer is connected to the MCC by two parallel cables (single core, 400sqmm, 2 cables per phase). The first set of cables have a separate Zero sequence CT (ZCT1) and the second set of cables have a separate Zero sequence CT (ZCT2),
Obviously the contractor has made like this because it was convenient to accomodate all the cables in this way. My question is there any possibility of errors by this connection?

RE: ZCT's secondary connected in parallel to protection relay

The CT will have a relatively high impedance with regards to each other. The relay will have a very low impedance relative to the CTs.
Virtually all the current will flow through the relay element.
A similar application in that it depends on the high impedance ratio between the CTs and the meter element is an accepted connection for metering three phase energy with a two element meter.
"A" phase is metered by a CT connected across the "A" phase element,
"C" phase is metered by a CT connected across the "C" phase element,
The Third CT is connected across the other CTs in delta so that "B" phase current is forced through both the "A" phase and "C" phase elements of the meter.
The "A" phase element meters the "A" phase component of "B" phase and the "C" phase element meters the "C" phase component of "B" phase.
This is significant in that it illustrates that the very high ration of CT impedance to meter element impedance delivers metering quality results.
I have used this connection successfully.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: ZCT's secondary connected in parallel to protection relay

If you never have to worry about saturation, then I think Bill's answer pretty much sums it up.

But if you can get unequal saturation, I find myself just a bit leery. Generally you'd use a ZCT when the expected ground fault current is much less than the expected phase fault current; so you get things like 1200:5 phase CTs and a 50:5 ZCT. Now say that you have a high resistance connection at one end of the upper 'R' conductor so that the current split between the sets is not identical. Both CTs will have some standing current, but it will cancel at the summation point. Now, have a phase-phase fault somewhere downstream and if the CT that has too much of the 'R' current saturates your relay may see current that it shouldn't. It might trip on ground for a fault not involving ground one or more zones away. Maybe you can get away with it, maybe you can't .

RE: ZCT's secondary connected in parallel to protection relay

I'd run this by the relay supplier. These ZCT and GF relays are often customized combinations and not really standard CTs and protective relays.

RE: ZCT's secondary connected in parallel to protection relay

Make sure both CT's are orientated the same way and correctly aligned in the correct direction.

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