Interesting fault video

[Power0020]

As seen on Facebook, I downloaded the video (attached) for a tree hitting an OHTL.

I wonder how the protection didn't act quickly enough to avoid that much arcing? A flashover (not involving earth) seemed to appear pretty much as an Over Current to the substation that the relay didn't act on?

I don't know the line voltage here but looks like something around 115 kV or less, the tree/wooden pole isn't a good ground connection (Ground fault not working), it doesn't seem to be an auto-reclosure neither(isn't it?)..... phase-to-phase arcing is what I guess here..

 

[waross]

Google "Jacobs Ladder" and/or "Travelling Arc".
It doesn't take much to move an arc along parallel conductors.
I can't explain the final arc to the right.

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

 

[bacon4life]

The wires to the right are bouncing around all over the place before the arcs to the right. It occasionally happens that a fault downstream will cause phase conductors upstream to bounce around enough to cause additional faults.

 

[Power0020]

Yup, it is Jacobs ladder, but why overcurrent protection didn't act? poor relay setting?

bouncing due to short circuit forces is common.

 

[HamburgerHelper]

Maybe, the arc is too long to pick up anything. An arcs voltage drop is 440v a foot so maybe it would be hard to pick up phase to phase?


There is one just like this but up on transmission in Dallas in 2010-2011. It went on for minutes before it was cleared. It is on youtube someplace, I know.

 

[Mbrooke]

Arcs can have considerable impedance compared to a bolted fault. As a result over current and step distance may not pick it up, or not pick it up fast enough, especially when said over current is set close to the conductor's damage curve. Line length can also play a role. A fault at the end of a very long line can look like an mild over load to protection relays. In fact very long distribution lines often have a recloser added 2/3 or more down the line to catch faults at the far end because the substation relays may not catch it (just looks like heavy load). About the only element which cuts out under nearly all fault conditions is differential protection, but outside of 230kv and above or exceptionally short 115kv lines its often impractical.

 

[Power0020]

how a differential relay will pick that? current summation is still zero on these two phases.

I think a phase-balance will pick that, but looks like the line isn't protected with this function, not common to see it though.

 

[ScottyUK]

Differential applied to each phase would certainly see it, but a residually-connected differential wouldn't.

 

[itsmoked]

It's the big, lower impedance, all three phases arc at the end that finally gets the breaker's attention.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Mbrooke]

Current summation will not be zero on those two phases as current entering the line will not be the same as current leaving the line. Assuming there are no tap loads one could easily set the differential to be sensitive enough to pickup up on it and trip.

 

[piterpol]

Negative-sequence overcurrent relays can be set much more sensitive than phase overcurrent relays.

 

[Mbrooke]

@Piterpol: Very true, but in some cases there still can be limitations in regard to secondary faults on a delta-wye transformer feeding a distribution system.

 

[davidbeach]

Numeric time overcurrent relays with instantaneous reset. Each fault lasted less time that required to time out and every time it extinguished and started over again there was at least 0.25 cycles during which there was no fault current.

 

[Mbrooke]

Here is a popular video of a sustained fault on You Tube since it was brought up:

https://www.youtube.com/watch?v=_FsRZ-I9_F0

 

[HamburgerHelper]

Mbrooke,

I knew the protection engineer that was with the utility this happened on. There is a rural co-op that covers over 3/4 of Oklahoma. They only had one protection engineer. When he retired, his job got outsourced to a relay manufacture. The utility has been looking for a protection engineer for years and they are actually offering pretty decent pay ,extremely decent if you take into account the cost of living, but nobody wants to move to Oklahoma. If someone took the job, I think they could raise a family and save 2 out 3 or 3 out of 4 paychecks.

 

[Mbrooke]

Did he say what the problem was? Or was this an outsourced relay?

 

[HamburgerHelper]

I was with that utility when it happened but never heard him talk about it at the time. I was in another department.

 

[Mbrooke]

If you know his number ask, Id be really curious.


In regards to the job opening, is it something that someone can do from Home?

 

[HamburgerHelper]

Mbrooke,

Don't know his number or remember his name. I don't know how technical of a response you would have gotten. The guy worked his way up from a technician to being the relay guy. He retired at like 50 because he had kidney problems and had the money since he never had kids.

Maybe, I don't know. I think most utilities would like a guy on site for resolving outages. Maybe, they just need a guy for project work. I don't know.

 

[JensenDrive]

Hard to say, but maybe 69kV. Saw 2 ph-faults, with no obvious trip on the first ph-ph event, sort of implying impdance relaying was not in use. Assume 600A line, from size of conductors, indicating a 1000A pickup. One phase at 69kV = 40kV phase to ground. Just to pick up the overcurrent relay implies on the order of 40MVA. Those signle phase arcs were a tad short of 40MVA.