Sustained Brown Out

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alex111

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Last night around (Oct 4) 8PM our lights dimmed and a radiant space heater faded.(Singe phase home) I pulled the panel cover and shut all the branch breakers and got a stable 140 Volt across the bus and 69 -70V to the neutral from either side. I shut down all motor equipment, Fridge, freezer, pump etc. It lasted about 20 minute until we went to zero or full blackout. Power restored around 4 AM on the 5th. Everything is working as of now.

Anyone care to speculate on how this happens from the grid supply side? and how long motors and devices can tolerate this scale voltage drop.

Alex English
Enterprise Ont.
 
alex111 said:
Last night around (Oct 4) 8PM our lights dimmed and a radiant space heater faded.(Singe phase home) I pulled the panel cover and shut all the branch breakers and got a stable 140 Volt across the bus and 69 -70V to the neutral from either side. I shut down all motor equipment, Fridge, freezer, pump etc. It lasted about 20 minute until we went to zero or full blackout. Power restored around 4 AM on the 5th. Everything is working as of now.

Anyone care to speculate on how this happens from the grid supply side? and how long motors and devices can tolerate this scale voltage drop.

Alex English
Enterprise Ont.
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Do you know how big the brown out was ? If it was very large (city wide) you would think there would be information available.
 
There's dozens of things that can go on with the grid, starting at the transformer at your house on up to the substation and beyond.

If the voltage drops low enough the motor won't draw enough amps to try and start so it's possible there won't be any damage. Almost all electronics use switch mode power supplies so I would think those would just drop out on low voltage too.
 
I'm guessing a delta primary transformer in the system with 1 phase open. Common on 3 phase lines where one cutout blows.
 
synchro said:
240/140 = 1.71 is very close to sqrt (3), so a dropped phase somewhere sounds likely.
Click to expand...

Also keep in mind that the voltage on a dropped phase feeding a delta primary transformer will produce 50% voltage across two secondary phases assuming the load is equal. Typically it isn't, so you get varying voltage above and below 50%.
 
If a wye-wye lost a phase on the primary, then I think the L-L voltage on the secondary would drop by 1/sqrt (3). But I'm guessing Y-Y is not as common in the distribution system? MV distribution is not my forte.
 
We have lost one phase on a system. Not necessarily a fuse blown, but a jumper wire burnt into on a double dead end.
customers called in and said “ we have just a little bit of power. It’s like it’s not all there”..
their voltage was backfeeding through all the banks on the overhead circuit.

FYI, distribution systems are full of Wye-Wye. I would bet 90% of padmounts are.
For protection, we only put a delta Wye somewhere behind a three phase recloser or breaker close to the station. Never behind single phase OCRs or triple singles. Farther out on the circuit.
 
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