×
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

Differential vs. zero-sequence currents

Differential vs. zero-sequence currents

Differential vs. zero-sequence currents

(OP)
I have an application for a CT based protection system in a 115/200 VAC 400 Hz 3 phase electrical system in an aircraft. The spec requires both differential protection and zero-sequence protection. Differential protection in an aircraft uses 2 sets of CTs, one at each end of the feeder to make sure that what goes into the feeder comes out the other end. I think I understand that the zero-sequence current is the same as ground fault current. If there is a differential fault, where else would the current go but to ground, so how do these 2 requirements differ?

I see from my research that ground faults are often measured using a single CT with all 3 phases passing through it. My application is not a new one, only new to me and the existing design has only the 3 phase CTs.

RE: Differential vs. zero-sequence currents

The differential relay checks current in versus current out.  Any type of fault that involves current leaving a phase conductor will be seen by the differential circuit.  So if there was a three-phase fault in the differential zone, there is no zero sequence current, but plenty of differential current.

The flux summation or zero sequence CT just measures the resultant flux of the three phase conductors.  If the three phase currents are equal, the net current and the net flux is zero.  When there is a ground fault, some current in one of phase conductors is going into the earth or ground system, so the three phase currents will not sum to zero.   If you have three phase CTs, you can also sum the three CT secondaries and run this through another relay to provide ground fault detection.  This is called a residual connection.  The two methods each have pluses and minuses.  

Cheers,

Dave

RE: Differential vs. zero-sequence currents

(OP)
Thanks dpc. Shows the power of writing something down - as I was writing the original post, I started thinking about phase-to-phase faults, just hadn't finished the thought. Now that you remind me, I have seen the residual connection before. I just needed a nudge in the right direction. Thanks

Bill

RE: Differential vs. zero-sequence currents

Another difference is that differential current will only be produced for faults located between the two CT sets. Zero sequence current can be produced by ground faults external to this area, and also by unbalanced phase to neutral connected loads.

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