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?
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?
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
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"Why not the best?"
Jimmy Carter
RE: Current on Faulted Phase Decreasing During Fault?
(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?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Current on Faulted Phase Decreasing During Fault?
RE: Current on Faulted Phase Decreasing During Fault?
RE: Current on Faulted Phase Decreasing During Fault?
Looking forward to hearing about this at WPRC.
RE: Current on Faulted Phase Decreasing During Fault?
If I can't explain it, there certainly couldn't ever be a WPRC paper.
RE: Current on Faulted Phase Decreasing During Fault?
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?
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?
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?
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?
RE: Current on Faulted Phase Decreasing During Fault?
If the latter, was the end with the reduced fault contribution the end with little / no generation?
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If we learn from our mistakes I'm getting a great education!
RE: Current on Faulted Phase Decreasing During Fault?
RE: Current on Faulted Phase Decreasing During Fault?
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?
RE: Current on Faulted Phase Decreasing During Fault?
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If we learn from our mistakes I'm getting a great education!