What causes utility to drop a phase?

Status
Not open for further replies.
Inquiring Minds said:
Probably didn't word my question well enough.

I am looking for reasons why you would lose 1 phase out of 3. Like you still have the A & B phase but the C phase dropped.
Click to expand...
Your wording was fine and the answers above are real reasons.

Roger
 
Inquiring Minds said:
Probably didn't word my question well enough.

I am looking for reasons why you would lose 1 phase out of 3. Like you still have the A & B phase but the C phase dropped.
Click to expand...
What do you mean by C phase dropped? Do you have a voltage reading on the Phases? Could this be a Corner grounded Delta system?
 
Inquiring Minds said:
What causes phase outages from the utility side? Is it a fault or break in the transformer or is it all the way back at the generator? Some issue with the power lines?
Click to expand...

We had SCL drop a phase three times in as many weeks. Was an underground system. I never did hear what the exact problem was, but probably a bad connector in am UG vault. If you want to to really geek out on separable connectors and their common failure modes, watch this:

 
What causes one phase of primary to drop out?
oh let me count the ways…

In general though if only one phase drops out it doesn’t go back all the way to the source as most stations have three phase breakers or reclosers.

probably back to the first set of single phase bank or OCRs or a fuse on a XF bank pole
 
Inquiring Minds said:
So when you lose a single phase, its typically a conductor issue, not transformer fault or generator issue?
That makes sense.
Thank you sir
Click to expand...
Can be a transformer or generator, but most the time is more likely lines, connections, or other components that have been damaged or simply failed over time. Overhead lines and associated items sit out there in the weather 24/7. some places it maybe is amazing they last as long as they do, especially places that see frequent wind.
 
220215-1057 EST

I am supplied on the pole with a three phase delta. My entire block has about 28 homes, and almost all are single phase. The homes are about equally distributed on the three supply lines. My home on the block is the first load on that supply. Across the street from me a tree branch came down breaking only one primary wire. Thus, the block was fed with a single single phase source.

What happened? All homes connected to that single phase source had no interruption of power. But 2/3 of the homes had their transformers connected in series, and these seriesed transformer were shunted across the working single phase source. All the homes have refrigerators and freezers, and possibly some other similar loads ( motors with built in overload protection ).

A motor would receive inadequate voltage, not start, putting a load on the system, and after a short time trip out removing its load for some short time, and then reclose. Each motor was random in its timing. Thus, voltage fluctuated between about 10 to 20 V, and 90 to 100 V on the homes with the primary wire that was broken.

This would not happen on a wye source. Just one phase would be lost, and it would be completely dead. However, I like and prefer my delta source.

.
 
I once was working at a gravel pit out in the middle of nowhere in SoCal starting up some soft starters, and we had a phase loss. Checked every fuse and breaker all the way up to the main transformer, but nobody had anything to check the MV feed. Called SCE and they said everything was fine on their end. Spent the best part of a day trying to figure it out and finally left. On the 25+ mile road back toward civilization, the power lines were running along the road and we came across a car accident where someone had hit a pole, snapping it off at the base. The pole was being held up by 2 of the wires, but the third was on the car. The guy in the car was unconscious or dead, probably from the initial crash, but the line was still hot and sparking so we couldn't go in to check. Called 911 and then called SCE again to tell them that they needed to cut power, but they kept saying they didn't see the issue on their end. Must have been something about the resistance in the connection to the car that made it appear normal. When they finally cut power we went in and the guy was dead, we just don't know whether that was immediate or as a result of the electrical issue.
 
Just like gar said, lose a feed wire and voila, lost power. HV feeders have breakers on them. One can go open for many reasons, hence a big section of a neighborhood using that phase goes black, and that lost phase in 3ph panel in some buildings.

If it's critical systems, or perhaps a $10k motor, then some sort of phase monitoring is key. A phase drops off (or below threshold) and you get an alert.

If you are in TX, then when a cold icy storm comes, the power source freezes over and stoping making juice.
 
Status
Not open for further replies.
Top