×
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

Overcurrent Protection Setings for Zig Zag Grounding Transformers

Overcurrent Protection Setings for Zig Zag Grounding Transformers

Overcurrent Protection Setings for Zig Zag Grounding Transformers

(OP)
I have 50/51 overcurrent relay protecting a Zig Zag grounding transformer. What are the recommended settings for the realy?

RE: Overcurrent Protection Setings for Zig Zag Grounding Transformers

Low enough to trip on all faults, high enough not to trip on normal load.

RE: Overcurrent Protection Setings for Zig Zag Grounding Transformers

I don't think David read the question properly. The earthing transformer draws a very small magnetising current in normal operation, until called upon to pass earth fault current. It needs to be the last thing to trip on your network under earth fault conditions otherwise the other earth fault protection relays on the network will effectively be disabled, and you will end up with an unearthed system. Other than that, it needs to trip before it gets damaged by the fault current.
Regards
Marmite

RE: Overcurrent Protection Setings for Zig Zag Grounding Transformers

No, I read the question just fine. Of course the relay should trip something other than the transformer for neutral/residual overcurrents, maybe/maybe not trip a lockout; no idea what's being grounded. The phase overcurrents on the transformer, set above the phase current associated with the maximum ground current, should trip a lockout that removes the whole circuit from service until the transformer damage gets repaired.

All that said, my original answer still stands. There is not enough information in the original question to actually recommend settings.

RE: Overcurrent Protection Setings for Zig Zag Grounding Transformers

I don't think you want to trip out the zig-zag transformer.
The supply transformer should be tripped out before the I2T crosses the the zig-zag transformer's damage curve.
An alternative is to insert a series resistor in the ground path to limit the fault current and alarm the fault. This depends a lot on the application, and the protection philosophy of the protection group.

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

RE: Overcurrent Protection Setings for Zig Zag Grounding Transformers

Hi,

How are the CTs connected?

A good method of protecting a zig-zag transformer is to
connect the overcurrent CTs in delta. The overcurrent
relay will then ignore the zero sequence current which
must flow for an external fault, and can be set low and
fast.

Thanks,
Alan

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