Ignorant questions from "quallified persons"

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iceworm said:
Let's take the case of a 139kv line in a substation dropping across the 7200V line going out to house transformers. The 7200V line goes to about 80kv ~ 11X7200. The house 240 suddenly jumps to 2600V. And we are waiting for the utility fuses to trip. -----

Still waiting -------

Still - ah there they go. It finally cleared. And most anything that was turned on in the house is toast. What did this house ground rod do? After the house equipment shorted the house main tripped unless the equipment burned open quicker than the main could open.

I don't see where the house ground rod made anything better.

Got to go to work for a while - I'll check back in later

ice
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In the station I'm sure the differential would clear pretty fast, but even out on the system, do you think the 80 kV would clear faster or slower without grounds?
 
George Stolz said:
Bump to try to clear a forum glitch.

Edit:
Hey, everybody - if you can't see the last page of a thread, post a reply, it seems to work...
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That's a feature, not a glitch.;) I re-queried "today's posts" then re-opened the thread and it cleared it up for me.
 
mivey said:
In the station I'm sure the differential would clear pretty fast, but even out on the system, do you think the 80 kV would clear faster or slower without grounds?
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I have a post on here bout 2006 where a delivry truck hit a guy wire to a pole droping the SEC drop accross a road to another pole that went underground to two new houses, one of the phases also came down and landed on the bare neutral 7200 volts, each house was on plastic well, and had two ground rods each for a total of 4 ground rods, this 7200 volt line never cleared, and just sat there arcing but it welded to the bare neutral still going to these houses, when the utility crew got there the line man quickly grabed a jumper to which he connected one end to a pole ground and the other end he touched to the arcing line forcing the cutout to open, all the time the house had arcing in the main panel and to every TV that had a cable box, the cable to the house was melted and droped to the ground eventuly, we had to replace many circuits, and the panel, meter sockets, and phone and cable wiring, as most of the insulation was blowen off, all the phones were also damaged as well as the phone lines, a few of the cable box's was just melted into a blob, the poor home owners with arcing inside the home and outside didn't know which way to run, it was a mess, but a good point that shows even at 7200 volts, 4 groundrods and the added impedance of the cable from each house back to the MGN would not open this line, only a direct short to the MGN did, which at least the lineman knew this was the fastest way to get the line shut down.

I tried to find my older post but the forum search funtion would only hang.
 
hurk27 said:
...one of the phases also came down and landed on the bare neutral 7200 volts, each house was on plastic well, and had two ground rods each for a total of 4 ground rods, this 7200 volt line never cleared, and just sat there arcing but it welded to the bare neutral
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The bonding of the neutrals should have provided a clear fault path, but keep in mind they were not designed to handle the primary system. I would think the higher voltage would provide even better fault clearing current than the secondary voltage for which they were designed (unless it just burnt them open).
hurk27 said:
when the utility crew got there the line man quickly grabed a jumper to which he connected one end to a pole ground and the other end he touched to the arcing line forcing the cutout to open
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Evidently the system was weak or was never intended to handle this type fault. I would expect it to be able to handle a transient delivered to it. This transient was located at the secondary system, not just delivered through its connection to the primary through the transformer.

Even so, "touching" the ungrounded conductor to part of the MGN should not be a better connection than a connector made for the purpose of bonding the primary and secondary neutrals together.
hurk27 said:
but a good point that shows even at 7200 volts, 4 ground rods and the added impedance of the cable from each house back to the MGN would not open this line, only a direct short to the MGN did, which at least the lineman knew this was the fastest way to get the line shut down.
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The neutral bond to the MGN should have been the main path for current. The protection scheme for the primary would normally consider a high-impedance ground fault. This estimated impedance might be on the order of at least 40 ohms to a 100 ohms or so. This minimum fault design criteria should certainly have been more than the impedance seen by the bonded neutrals in parallel with the 4 rods (the 4 rods being the smaller contributer to higher fault current).

Finally, it could just be that the primary system protection was not adequately designed to handle minimum faults or that the "weld" was a high impedance connection. What did the analysis show?
 
mivey said:
The bonding of the neutrals should have provided a clear fault path, but keep in mind they were not designed to handle the primary system. I would think the higher voltage would provide even better fault clearing current than the secondary voltage for which they were designed (unless it just burnt them open).
Evidently the system was weak or was never intended to handle this type fault. I would expect it to be able to handle a transient delivered to it. This transient was located at the secondary system, not just delivered through its connection to the primary through the transformer.

Even so, "touching" the ungrounded conductor to part of the MGN should not be a better connection than a connector made for the purpose of bonding the primary and secondary neutrals together.
The neutral bond to the MGN should have been the main path for current. The protection scheme for the primary would normally consider a high-impedance ground fault. This estimated impedance might be on the order of at least 40 ohms to a 100 ohms or so. This minimum fault design criteria should certainly have been more than the impedance seen by the bonded neutrals in parallel with the 4 rods (the 4 rods being the smaller contributer to higher fault current).

Finally, it could just be that the primary system protection was not adequately designed to handle minimum faults or that the "weld" was a high impedance connection. What did the analysis show?
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I think you missed that the secondaries (SEC's) broke there connection at the transformer and fell to the ground, but was still attached to the pole across the road where they went underground to each house, one of the primaries also fell and landed on the bare SEC neutral welding to it laying on the ground, the only return path to the MGN was through the TV cable shield that went from each house to another pole down the road, they were bonded both at the house and to the MGN at the pole, at least till they burned through. our POCO not being a CO-OP are under the gun to not have outages and over fuse everything, like t-15's on transformers feeding only one house. I have no idea what these primary line cutouts are set for, but I'll bet they are at least 125% of the line rating, maybe more.
 
A long time ago, way back in industry
We pulled along a neutral and then conductors three.
But then we nicked a cable and all the plant did shout.
And so we found some cutters and the fourth one did without.

So in our mighty plant did the work go round and round.
And never did we worry that a phase might go to ground.
But when all the whirling motors induced some voltage gain,
Tom shook hands with Jerry and an arc there was to blame.

And so we take our columns and we drive them deep in dirt.
We weld them all together so no one else gets hurt.
We take the metal parts and we tie them in a knot.
So now we have a GEC and no more funeral plot.
 
hurk27 said:
landed on the bare SEC neutral welding to it laying on the ground...
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Again, I would use the term "weld" loosely. Arc welds without shielding gas are probably full of contaminants.

There must have been a high impedance path up to the ground rods. Normal protection would trip a ground fault for impedances below 40 ohms. As you mentioned before though, the POCO there may not use what I would consider normal protection schemes.
 
mivey said:
Again, I would use the term "weld" loosely. Arc welds without shielding gas are probably full of contaminants.

There must have been a high impedance path up to the ground rods. Normal protection would trip a ground fault for impedances below 40 ohms. As you mentioned before though, the POCO there may not use what I would consider normal protection schemes.
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The primary aluminum line and the secondary neutral was so well melted together the POCO engineer had the line man cut it out and save it.

As far as the impedance to the ground rods, there was 250, 250, 4/0 across the road, to another pole which split to 4/0, 4/0, 1/0 to each house then #6 from each meter to the two ground rods at each house, I doubt the resistance of the ground rod was very low as the soil in this area is just beach sand.

Our distribution around here is 14,400/7200 Y so I don't think you can have ground fault protection on a grounded Y, the MGN is Earthed 4 times a mile or more.
 
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