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Open Neutral

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Inside the Beltway
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Engineer
Anyone else following the MN Railroad debacle?

Basically, a catenary substation was fed with a buried 138KV line, with redundancy in the firm of a 2nd feeder. ConEd took one down for maintenance. Then, the remaining line faulted, closing down a ~10 mile segment of one of the busiest commuter lines in the country.

They kludged up a small sub in a parking lot, fed by an existing 13KV resi feeder, and tied it to the cat. That got them ~10% of the needed capacity. Meanwhile, ConEd was digging freeze pits and frantically getting down to the faulted section.

I'd not want to be a homeowner sharing that feeder. A few 9MW single phase loads will not make the local distribution happy....
 
BTW, there was another issue. In one of the NYC blackouts, maybe 1977 but I'm not sure.... A large impediment was the incoming feeders needed power for their pumps/cooling system before they could be re-energized. As a result, ConEd had to haul generators to vaults around the city, power up the cooling for N ?minutes/hours?, then bring up the feeder....and pick up & move to the next feeder's manhole. A NYC resident described this circus to me.

This was an unpleasant surprise as post-1965, the utilities had gone to lengths to pre-plan re: which plants could blackstart, and how transmission assets to restart others from them would then be configured..... But no one realized the issue went further than that. One of the post-blackout improvements resolved this issue; I don't know how.

No matter how you prepare, Murphy is there ahead of you.
 
Update:

It now sounds like the cause was ConEd was freezing the out of service cable, and the N2 migrated over to the in-service one.

Needless to say, if you have an HV cable cooled by circulating oil, and freeze the oil, you might just have ..issues....
 
Open Neutral said:
Update:

It now sounds like the cause was ConEd was freezing the out of service cable, and the N2 migrated over to the in-service one.

Needless to say, if you have an HV cable cooled by circulating oil, and freeze the oil, you might just have ..issues....
Click to expand...

First, thanks for that link. I was in geek heaven reading the "how to" on that. Second, how did the LN2 migrate? From the LA story, it sounds like the LN2 runs through a jacket; they don't just dump it into the pit.
 
gadfly56 said:
First, thanks for that link. I was in geek heaven reading the "how to" on that. Second, how did the LN2 migrate? From the LA story, it sounds like the LN2 runs through a jacket; they don't just dump it into the pit.
Click to expand...
Probably true for most cases where it is planned. The picture linked earlier seemed to show the going right into the pit.
But even when a jacket is used, there will be leakage of LN2, and being cold it will accumulate in the bottom of a pit or vault. If not precautions were taken to insulate/isolate the second cable running in the same vault or even same duct bank I could see there being a problem. Especially at a time when the load on the working line goes to zero (when all the trains stop running) and so there is no distributed heating of the oil. If you got it cold enough it could get gummy and not flow properly even if it did not freeze hard enough to allow opening the jacket.
I would like to know more about what really happened though.

Also, if the cold N2 gas is loose in a vault or pit, workers will have to be using breathing equipment or very reliable high flow ventilation.
 
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