×
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 Currents through single phase to ground connected load

Zero Sequence Currents through single phase to ground connected load

Zero Sequence Currents through single phase to ground connected load

(OP)
I work for a PUD in Washington and we had two feeders out of one substation that tripped at the same time.  One was faulted, the other wasn't.  I'm now reviewing the causes of sympathetic tripping and my notes (from an SEL class I took a couple years ago) state that zero sequence fault current from a phase to ground fault can flow through single phase to ground connected load.  There's no further explanation in the notes as to how this can happen.  Can anyone provide an explanation?  I understand how the zero sequence current can flow through a three phase bank with grounded Y on the high side and delta on the low side, but struggling with the single phase connections.

RE: Zero Sequence Currents through single phase to ground connected load

http://www.selinc.com/techpprs/6061.pdf shows a single feeder example on page 18. Guess you could put the load on the unfaulted feeder. Zero sequence current through a load, though, sounds like load current to me. I think your unbalance would need to be pretty severe on the unfaulted feeder prior to the fault for the fault to have an effect. Sure you didn't go circuit to circuit?

RE: Zero Sequence Currents through single phase to ground connected load

Are you saying you had induced currents on the non-faulted feeder due to mutual coupling?  

RE: Zero Sequence Currents through single phase to ground connected load

samreye,
Could you describe in a little more detail the events that occurred?  What type of relay tripped each feeder?

RE: Zero Sequence Currents through single phase to ground connected load

and also where was the fault location?

RE: Zero Sequence Currents through single phase to ground connected load

(OP)
Thanks for the quick response and link to the SEL paper.  Here's more details on the fault.  The faulted feeder had some UG cable that went bad, so we believe it was a single line to ground fault.  The cable was approximately a half mile from the substation.  The feeder relays are Westinghouse Type CO relays, so there's no fault data available.  Additionally, all targets dropped (all three phases plus ground), but we have reclosing on the circuit and the targets hadn't been reset in a while, so there's no way of knowing which targets were associated with the bo cable fault.  As for the other feeder that tripped at the same time, the ground relay had the only target that dropped, which to me suggests that we had 3Io fault current coming back on the unfaulted feeder.  There are no three phase grounded Y - delta distributions transformers on that feeder though, hence the inquery into single phase grounded distribution transformers as a path for zero sequence current.  

RE: Zero Sequence Currents through single phase to ground connected load

I assume that the ground relay that tripped was fed by the residual from the phase CTs.  You should check to see if the CT neutral is grounded in more than one location.  Return current from a fault on one feeder could flow through the other feeder's ground relay if there are two ground points.

RE: Zero Sequence Currents through single phase to ground connected load

Do the circuits share some structures on the overhead portions? Did the (presumably) unfaulted feeder successfully reclose or keep tripping to lockout? UG fault causes overhead conductor movement. Circuit 1 A phase contacts circuit 2 B phase. To each relay it looks like a line to ground fault.

RE: Zero Sequence Currents through single phase to ground connected load

(OP)
Yes, the two circuits share structures for a few spans out of the substation.  Four over two construction with a shared neutral.  I originally dismissed a circuit to circuit fault due to conductor movement because the presumable unfaulted circuit ground relay tripped.  But if a phase to phase fault between circuits looks like a ground fault to the relays, then that's certainly a possibility.  The presumably unfaulted circuit did successfully reclose after 10 seconds, so what ever caused the relay to operate was no longer present when the breaker closed.  As for the CT grounds, drawings indicate that it's grounded at the CT, which is actually fairly close to the relay in the metal clad switchgear.  Thank you for the information and suggestions.

RE: Zero Sequence Currents through single phase to ground connected load

Drawings do not always reflect the true state of CT wiring, but if it is in metal clad switchgear, this makes induced currents in the CT neutral a lot less likely.  

RE: Zero Sequence Currents through single phase to ground connected load

We had an occasion where a trip on a feeder caused another feeder to trip, and we found that the CTs were grounded at more than one point. Suggestion: Remove all earths from the CTs on the feeder that tripped and make sure that CT sircuits are not grounded (megger or voltmeter check), then re-assert the ground only in one point. Most of the times you will find the reason in the secondary system rather than primary.

RE: Zero Sequence Currents through single phase to ground connected load

Suggestion to samreye (Electrical) Jan 27, 2004 marked ///\\\
I work for a PUD in Washington and we had two feeders out of one substation that tripped at the same time.  One was faulted, the other wasn't.  I'm now reviewing the causes of sympathetic tripping and my notes (from an SEL class I took a couple years ago) state that zero sequence fault current from a phase to ground fault can flow through single phase to ground connected load.
///Yes, this is correct. The single phase fault will inject current into ground. Any single phase load of the same faulted phase to ground fault will provide the path to the zero sequence current back to the source of the zero sequence current. Often, it is easier to find the answer in textbooks that cover this topic, e.g.:
William D. Stevenson, Jr., "Elements of Power System Analysis," 3rd Ed., McGraw-Hill Book Co., 1975, Chapter 13 "Unsymmetrical Faults"\\\
  There's no further explanation in the notes as to how this can happen.  Can anyone provide an explanation?
///Try to get a good textbook since the literature that you are reading is presupposing that you have a pertinent background.\\\
  I understand how the zero sequence current can flow through a three phase bank with grounded Y on the high side and delta on the low side, but struggling with the single phase connections.
///This needs background covering Unsymmetrical Faults.\\\

RE: Zero Sequence Currents through single phase to ground connected load

Theoretically, part of the fault current can flow back through line-ground connected loads on the unfaulted circuit.  Practically, the load impedance is so much higher than the source impedance that this current is negligible.  

RE: Zero Sequence Currents through single phase to ground connected load

If the load is drawn between phase and neutral of the transformer, there may be other restrictions than those given by the rated current of the winding. Subclause 8.1 of IEC 60076-1 requires that the neutral terminal shall always be dimensioned for the recognised earth fault current and for continuous load current through the neutral when specified (this is a normal condition for distribution transformers).

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