×
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

Zero Sequence CT Understanding

Zero Sequence CT Understanding

Zero Sequence CT Understanding

(OP)
Hello guys,

I am hoping someone can help me understand zero sequence CT's (sometimes referred to as ground CT's) by answering a few examples for me. Any additional information someone can provide such as a reference or description would be appreciated.

1.0 If the zero sequence CT surrounds all three phase conductors and you have the following currents in each phase, what will the zero sequence CT read on the primary side?

Ia=100 A < 0 degrees
Ib=100 A < -120 degrees
Ic=95 A < +120 degrees

3I0=Ia+Ib+Ic=5 A < -60 degrees

In this example the CT would read 5A on the primary, correct? Mathematically can you add the currents in all three phases making sure to include phase angles in order to get the current the ground CT see's?

2.0 This example assumes the zero sequence CT surrounds all three phase conductors plus a neutral conductor.
2.1 If there is imbalance in the circuit due to line-to-neutral loads, would the zero sequence CT read zero amps? I would think that any imbalance in the three phases would be canceled out by the current in the neutral.
2.2 What about if there is a single-line-to-ground fault? Then the zero sequence CT would not read zero assuming the neutral is insulated from ground, correct?
2.3 What about if there is a line-to-line fault? Then what kind of currents would the CT read? I would assume it would read close to zero and you would rely on the phase protection.

3.0 If a neutral conductor is present, does the zero sequence CT typically surround this conductor in addition to the phase conductors? Are there any downsides to doing this? This kind of goes with question 2.


Thanks for the help guys.
coloradog1

RE: Zero Sequence CT Understanding

1. Yes.
2.1 If all of the unbalanced load current returns on the neutral.
2.2 Yes.
2.3 0
3.0 Yes. It is required if there is line-to-neutral load, otherwise the ground relay would sense line-to-neutral load as ground current.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

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