×
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 Wired in Delta for Differential Protection

CTs Wired in Delta for Differential Protection

CTs Wired in Delta for Differential Protection

(OP)
In a system with nearly all CTs wired in wye (excluding those used to account for the phase shift and zero sequence current in Delta-Wye transformer protection), there are three sets wired in delta for the differential protection of an outgoing line.

The outgoing line comes out of a ring bus. The two breaker positions on either side of the outgoing line have CTs wired in delta. The outgoing line connects to an autotransformer (Wye-Wye) and then to a breaker. The CTs on the far side of the breaker are part of the differential circuit and are also wired in delta.

Why would this be the case? I've heard that it could be due to a delta tertiary on the autotransformer. If so, do the CTs on the delta tertiary need to be wired in wye while the CTs on the wye windings need to be wired in delta to account for phase shift and elimination of zero sequence currents?

The tertiary does not seem to be involved in the differential circuit at all, but could the other CTs be wired in delta just in case this was added later?

Thank you for any clarification you can provide.

RE: CTs Wired in Delta for Differential Protection

Delta connected CT filter out zero sequence current, which can cause a misoperation on an external fault. It is very common to use delta connected CT's on wye-wye and auto transformers for this reason.

The tertiary may not be connected to the differential if it is not used, or no other protection is available for the circuit tied to the tertiary. Some times the differential may not be sensitive enough to detect tertiary faults, so the point is moot.

RE: CTs Wired in Delta for Differential Protection

(OP)
Is there a reason why it's so common to see CTs wired in wye? What is the disadvantage to the delta connected CTs?

RE: CTs Wired in Delta for Differential Protection

A wye CT connection provides the actual phase currents to relays and meters. A delta connection introduces the root 3 factor and phase shifts the values. With wye CT's the digital display on your relay matches the meter readout.

Delta connections are a little more complicated than a standard wye and a little more difficult to design, install and commission correctly.

Where do you put the ground (earth) connection on the CT secondary circuit? It is intuitively obvious the wye point is grounded on a wye, but which corner is grounded on a delta?

With modern relays being able to do the zero sequence filtering digitally, it makes it much easier to keep everything in wye. The same CT can feed multiple protection or metering devices (assuming your reliability procedures allow that).

RE: CTs Wired in Delta for Differential Protection

Quote:

Is there a reason why it's so common to see CTs wired in wye? What is the disadvantage to the delta connected CTs?
Zero sequence currents do not appear on the secondary circuits of a delta CT set. So there is no place to put a ground relay.

RE: CTs Wired in Delta for Differential Protection

I perfer wye connected CT's because there are fewer wiring errors, and I can use a ground relay.

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