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Unexplained momentary power outages

Unexplained momentary power outages

Unexplained momentary power outages

I was recently asked to look into momentary power outages at a facility that has been experiencing re-occurring momentary outages that is causing motors to drop out and electronics to reset.

This facility is served at 4.16kV from the local utility through a M-T-M 4.16k Switchgear lineup. We recently connected a meter to the secondary metering equipment (Pt's and CT's) at the 4.16kV Switchgear main in order to try to capture one of these momentary outages. (Other main is open with Tie Breaker Closed. During these outages no relays pickup nor do any breakers trip.

We were able to successfully capture one of these momentary outages and found that the voltage drops all the way to 0V for a duration of approximately 600ms before it is restored back to 4.16kV. Approximately another 566ms later the voltage drops close to 0V again this time for a shorter duration of only 92ms. This was the only event we were able to capture during the monitoring period.

I've included an attachment that shows the voltage and current trends during this event. The first two trends show the voltage and current respectively and the third trend shows the voltage and current of one phase on the same trend. Looking at these trends it appears that immediately after the voltage starts to decay the load current quickly increases. This is likely to be expected with motor loads which it mostly what this facility consists of (chiller loads etc..)

Any ideas on what may be causing these momentary outages. It would appear that it has to be something happening on the utility end? Could this perhaps be a re-closer or some other automatic protective device on the utility side opening and then re-closing quickly?

RE: Unexplained momentary power outages

A fault between the serving substation and the facility. The 600ms sag/outage is an instantaneous trip followed by a high-speed reclose. The 92ms sag is while a tap fuse blows to clear the fault.

RE: Unexplained momentary power outages

I had a similar problem.
Is this facility near the coast?
In my instance I talked to an engineer at the local utility.
He explained that they had some feeders running parallel to the beach.
During the dry season, winds would blow spry on to the insulators.
The moisture would evaporate and leave a film of salt on the insulator.
Getting into the autumn as the humidity increased and light rain started, the salt film would become conductive and flash over.
That would cause a serious voltage dip across the entire city.
As the rain became more intense the salt would be washed off the insulators and we would be good until the following autumn.
If you have a contact with the local utility he may be able to give an explanation.

"Why not the best?"
Jimmy Carter

RE: Unexplained momentary power outages

Some of my utility's high voltage circuits and medium voltage feeders have similar salt tracking problems, although not from being near a sea coast but from winter road salt; "power washing" [pressure washing with de-ionized water] is required to mitigate the problem. This tracking is a regular enough occurrence that insulatots have been placed in switchyards on wooden pallets and in proximity to where tracking and flashover are known to occur, and during the road salt season a technician routinely travels to these locations and tabulates the conductivity of said insulators to know when power washing is needed.


"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Unexplained momentary power outages


Can you explain the tap fuse blowing the clear the fault and the associated quicker time delay? Would this be the tap fuse on the overhead distribution line?

So if I understand correctly there appears to be a fault somewhere between substation and facility and when fault occurs the re-closer opens/closes first and when that doesn't clear the fault the tap fuse blows? I'm assuming this tap fuse is on another line/tap that is not serving the facility since it does not take the facility out permanently? What would cause the dip while the tap fuse is blowing?


This facility is fairly near the coast (aprox 10-15miles from beach). We have been experiencing a lot of seasonal rain lately so perhaps there may be a correlation between the rain and these dips/outages.

RE: Unexplained momentary power outages

So, fault on a tap somewhere along the feeder, probably closer to the substation than you are. The relays at the substation detect the fault and trip instantaneously, ahead of any fuses. Breaker opens and does a high-speed reclose. 50-75ms of depressed voltage followed by 0 while the breaker is open. Hopefully that's enough and the fault is gone but no fuses blown.

But the fault wasn't gone, so instantaneous is now blocked and the relay time overcurrent starts timing toward a second trip. While that's happening there's enough time for the tap fuse to blow and the event ends without a second breaker trip.

The fault pulls down the voltage at the fault location and that impacts the voltage everywhere on that feeder and on any other feeder sourced by the same transformer at the substation.

RE: Unexplained momentary power outages


Your explanation now makes sense. Thank you.

The one follow-up question I had was for the time period between when the re-closer closes and the fuse blows. The voltage is restored to normal for approximately 566ms. If the fault is still there wouldn't the voltage be depressed immediately after the re-closer closes and not return to close to normal for 566ms?

Is 600ms a typical time for a high speed re-close?

RE: Unexplained momentary power outages

Sometimes it takes a bit for the fault to restrike. If it's vegetation the fault may track for just a bit and then flash over; going from a fault resistance of hundreds of ohms to very low in a step change.

600ms is in the ball park for high-speed reclose. Maybe a bit slow, but not bad. I've seen many cases where the whole fault-trip-reclose sequence takes less than 500ms.

RE: Unexplained momentary power outages

Or if your equipment that you are recording with is on a different phase than the fault,the voltage will appear to be normal, while a different phase may be depressed.

Don't worry about the cause, as it's the utilities problem. Salt spray, animals, back hoes, dump trucks, bad drivers, etc. Look at the frequency of the events, not at what people say is all the time.

RE: Unexplained momentary power outages

Thanks for all of the info. Yes it looks as if its definitely the utilities problem and I will recommend to the facility that the utility needs to investigate the source of the problem.

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