25kV Distribution Protection & Fault Analysis-Collateral Faults, Additional Faults

[distributionPlanner]

For the distribution power system protection engineers:

I am investigating 25kV distribution faults on an overhead system. A known fault occurred downstream of a recloser, which was downstream of the substation breaker on a radial distribution system (see ASCII SLD below).

[SUB BKR]-------[???2nd FAULT??]------------[Dx RECLOSER]---------------[KNOWN FAULT, TREE]---[LARGE CUSTOMER]

The fault occurred downstream of a the distribution recloser, but the substation relay locked out the breaker.
According to my TCC, there is at least 0.3s of separation between the sub bkr and the electronic recloser (both phase and gnd curves).
So, this should not have miscoordinated, but it did.

When looking at the waveforms recorded at the substation, there appeared to be very large fault currents that could not exist at the location of the tree. There is no generation on this radial line.

It appears that another fault occurred upstream of the Dx RECLOSER. The voltage measured at the sub dropped significantly, and even went to 0.1Vpu, which could not have occurred for the fault at the known location. And, the fault levels are much too high for a fault at the known location. Has anyone seen anything like this before?

Next steps we will take include downloading event data from the Dx RECLOSER and confirming that it was in service. We are also considering a line inspection at the suspected second fault location.

 

[davidbeach]

Wire slap between the recloser and the substation. We've seen several instances of the fault moving much closer to the substation on reclose.

 

[distributionPlanner]

Wire slap between the recloser and the substation. We've seen several instances of the fault moving much closer to the substation on reclose.

Have you seen this cause the upstream breaker to trip to lockout? The breaker requires 4 trip operations to lockout (3 reclose attempts).
When you say instances of the fault moving closer to the substation on reclose, do you mean it moved a little further upstream each reclose attempt?

How did your utility mitigate this? Shorter ruling spans?

 

[davidbeach]

We typically only see it on the first reclose. Our first reclose is high speed, the second is 30 seconds. It it goes to lock out, the third fault is generally back to the original location. I don't have any visibility of what might be done with the distribution feeder after an event of that type is identified.

 

[distributionPlanner]

Interesting. This phenomenon would make sense given the fault data (volts, amps) that I have.
Our sub bkr reclose intervals are 5s, 35s, 35s, so I'm trying to rationalize how likely this is in this situation, but it's definitely something that could be considered.

I forgot to mention, an additional factor is that this line is double circuited with itself (one branch is directly protected by the breaker and another branch is protected by a recloser adjacent to the breaker). I wonder if this could be a contributing factor to the wire slap theory.

 

[magoo2]

What davidbeach said is correct. There's an IEEE on the subject titled Fault Induced Conductor Motion.

A phase to phase fault will cause the two faulted conductors to swing away from each other. With reclosing, they will swing back and sometimes contact each other. Long spans are most vulnerable.

So you can have a fault beyond the recloser and have operations and even lockouts of the substation breaker. We discovered this when we looked at the digital relays at the substation.

You can speed up the recloser characteristic to avoid this, or you can reduce the span lengths (which is fairly expensive) or install spacers from phase to phase on the longest spans.