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Current on Faulted Phase Decreasing During Fault?

Current on Faulted Phase Decreasing During Fault?

Current on Faulted Phase Decreasing During Fault?

(OP)
A couple of weeks ago we had a fault on one of our 230kV lines.  My initial analysis (at 2 in the morning) was that we had a broken conductor.  This was based on a relay event report at one end of the line (both relays at that terminal) that showed the C-phase current decreasing and swinging from somewhat lagging (power in) to about 45 degrees leading (power out).  At the other terminal, the C-phase current became in phase with the voltage at a bit over 300A, or about 400 ohms of fault resistance.

36 hours or so later, the line was tested from both ends, one at a time.  From the first end tested, the line was energized for about 7 seconds and showed charging current for the entire line when closed, and no protective elements picked up before the breaker was manually opened.  When tested from the other end (the one that showed decreasing current on the initial fault) the line held for somewhat over 30 seconds then tripped.  Before fault initiation, this end also showed charging current for the entire line on all three phases.  After the line tripped, efforts to find the fault continued.  This involved the use of Sno-Cats and only patrolling a few miles of line per days.

Eventually a tree that had fallen into the line from up slope, out of the right of way, was discovered and removed.  No repairs were made to the line.

The question - What might have caused the decrease in current from one end other than a broken line?  Since there were no repairs, both ends showed charging current for the entire line during the subsequent line tests, and the line is now in service with no current imbalance, how did the current decrease?

RE: Current on Faulted Phase Decreasing During Fault?

Good evening David. Interesting. Is the normal current of the line above or below 300 amps? Was the fault closer to the supply end of the line? Normally line impedances are calculated with power flow in one direction, however, with forward power flow on two phases and reverse power flow on one phase, what may be the impedance seen by the reverse current and would this explain the currents that your relay recorded?
More questions, I don't know the answer but perhaps my questions may help point to a solution.
Thanks for sharing this with us.

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

RE: Current on Faulted Phase Decreasing During Fault?

(OP)
In the earliest "per fault" record, Ic was about 100A (compared to almost 150A for the other two phases) so something was already happening but not enough to trigger a recording.  In that first snapshot, Ic leads Vc by 156.5 degrees (the other two lead by a bit over 160 degrees).  During the fault, Ic drops to about 40A and leads Vc by 37 degrees.  The other two phases remain essentially unchanged.  That situation then remains stable for the nearly one second it took the directional ground overcurrent to respond.

(Angles in this post are from actual event reports - the first post was from memory.)

I've modeled this in OneLiner and was able to get something very close by opening the c-phase conductor and placing the right amount of resistance between the open end and ground.  But that wasn't what really happened.  Any thing I tried in the model without a broken conductor resulted in higher current in c-phase, or at least no decrease.

RE: Current on Faulted Phase Decreasing During Fault?

Does your model include the transformer characteristics? Is this a radial line or are there parallel lines which may supply power to a back-fed fault? With one phase grounded what happens to the transformer terminal voltage?  

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

RE: Current on Faulted Phase Decreasing During Fault?

Knowing that many of your lines have taps, does this line have any taps? Or more importantly is ther any zero sequence taps, or grounded wye capacitor banks?

RE: Current on Faulted Phase Decreasing During Fault?

(OP)
No taps on the line, mutuals are all accounted for in the OneLiner model.

RE: Current on Faulted Phase Decreasing During Fault?

Any repair sleeves in the area from the last tree that tumbled down the hill? I'm thinking the tree may have moved the conductor in the sleeve creating a temporary high impedance.

Looking forward to hearing about this at WPRC.

RE: Current on Faulted Phase Decreasing During Fault?

(OP)
Our transmission engineers aren't aware of a mechanism by which a sleeve could pull apart enough to cause an apparent open line and then self heal to the point of normal operations.

If I can't explain it, there certainly couldn't ever be a WPRC paper.

RE: Current on Faulted Phase Decreasing During Fault?

If this conductor is ACSR, any splice is a complex splice. It would have an inner connection for the steel, and an outer connection for the aluminum.

Not knowing everything about the splice, I would tend to believe it is possible for the aluminum to not have a good connection in the splice, but the steel would still support the line.

I have seen where old splices could not handle the rating of the line, and I had assumed it was because of age. Could this be how it starts?

RE: Current on Faulted Phase Decreasing During Fault?

On the same vain as the splice, what of the other line connections? Pads, bushings, breaker contacts, etc.

If you can get an infered photo, the substation group takes, this will remove this doubt very quickly.

RE: Current on Faulted Phase Decreasing During Fault?

David,

If your model needs an open circuit get this result, I agree. Any gap in the conductor will simply arc across. I was guessing instead that an increase in impedance might get the result observed, but you have the model. Self healing is the easy part, as the resistive heating welds the conductors together. Haven't you seen disconnect jaws welded together? Good excuse to go Sno-catting in this beautiful weather.

I have faith you will solve it before the paper submittal date.

RE: Current on Faulted Phase Decreasing During Fault?

David,
  What sort of grounding is used at each end of the line? Where was the fault located w.r.t. each end?

  On occasion, a tree in a line can present a high resistance L-G fault to the system and system behavior can be 'unusual'. At times, the fault current is insufficient to trip protection immediately and the tree just sits there and 'cooks'. As it does so, moisture is driven out and the fault resistance goes even higher. Its more common to see this on MV distribution systems, but given a resistance or reactance grounded HV system, lower energy faults like this may be possible.

RE: Current on Faulted Phase Decreasing During Fault?

(OP)
Line just under 100 miles, fault a bit beyond 48 miles from end that exhibited the reduced current.  230kV system is solidly grounded.

RE: Current on Faulted Phase Decreasing During Fault?

Is this line an interconnector between areas with significant generation capacity, or a feeder from an area with generation to an area with little / no generation?
If the latter, was the end with the reduced fault contribution the end with little / no generation?
 
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Current on Faulted Phase Decreasing During Fault?

(OP)
Line runs from generation to load, load end had the reduced current.  But, the load end has enough other sources that for a solid ground fault at the fault location it would have contributed over 1800A (and the generation end less that 2000A).  The whole fault, what ever it was, had a high enough resistance that the 230kV voltage at both ends was essentially unaffected.

RE: Current on Faulted Phase Decreasing During Fault?

(OP)
blushingblushingblushing

Well, I'm a bit embarrassed.  I was sure that I had looked at a high resistance fault with the conductor whole and found increased current at both ends.  But I just looked at again and find, much to my surprise, that I don't need a broken conductor to get the results seen by the relay.

I guess that an analysis done at 3 in the morning should be rechecked when fully awake.

Thanks to everybody who has contributed to the discussion.

RE: Current on Faulted Phase Decreasing During Fault?

Been there, Done that. No problem man.

RE: Current on Faulted Phase Decreasing During Fault?

3 a.m. is no time for mortal men to be working...
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

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