Consider an illegal (no transfer switch) portable gasoline generator back feeding an otherwise deenergized portion of the grid when utility power is restored. How bad can it get for the generating homeowner and his neighbors? Trip the generator breaker? Destroy the generator? Ignite the generator fuel tank burning up the generator and any nearby structures? How bad can it get, and what have you seen? This is not Aurora where they hammered away at it, just a one time close. Thanks.
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Portable generator versus grid. How bad? 1
- Thread starter stevenal
- Start date
- Moderator
- #2
A small set will not be able to back feed the grid.
I would expect an instantaneous trip of the generator breaker.
I saw a 6 KW generator closed into a dead grid.
Instant generator breaker trip. No damage.
The energization surge of even a small distribution transformer will be seen by the breaker of a small set (6 KW or less) as a short circuit.
Add to that the energization surge of all the distribution transformers on the primary circuit.
Closing into a hot grid out of phase?
At the least an instantaneous trip.
If you are around 90 degrees out, there may be mechanical damage to the generator.
At 180 degrees out, the breaker will be subject to twice the rated voltage and may fail catastrophically.
On small sets I don't expect to see breakers rated for a higher voltage than the minimum needed.
I would not expect any damage to be external to the set.
However, in this case, I would also expect the unexpected.
The old standby:
"It depends."
A Poor man's transfer switch:
Siemens has these available to convert a standard load center into a transfer switch.
It interlocks the top mounted breaker with the upper lefthand breaker position.
The picture should be rotated 90 degrees clockwise.
The opening fits over the main breaker and the pivoted rocker interlocks the breaker handles.
Here is the Square D version.
Bill
--------------------
"Why not the best?"
Jimmy Carter
I would expect an instantaneous trip of the generator breaker.
I saw a 6 KW generator closed into a dead grid.
Instant generator breaker trip. No damage.
The energization surge of even a small distribution transformer will be seen by the breaker of a small set (6 KW or less) as a short circuit.
Add to that the energization surge of all the distribution transformers on the primary circuit.
Closing into a hot grid out of phase?
At the least an instantaneous trip.
If you are around 90 degrees out, there may be mechanical damage to the generator.
At 180 degrees out, the breaker will be subject to twice the rated voltage and may fail catastrophically.
On small sets I don't expect to see breakers rated for a higher voltage than the minimum needed.
I would not expect any damage to be external to the set.
However, in this case, I would also expect the unexpected.
The old standby:
"It depends."
A Poor man's transfer switch:
Siemens has these available to convert a standard load center into a transfer switch.
It interlocks the top mounted breaker with the upper lefthand breaker position.
The picture should be rotated 90 degrees clockwise.
The opening fits over the main breaker and the pivoted rocker interlocks the breaker handles.
Here is the Square D version.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Those interlock plates are the bomb! You can get them for about any make of breaker panel. Heck you can get them for any make on Amazon! There is no excuse at all for not making your generator install safe-for-morons. Gotta be the cheapest liability insurance there is.
Keith Cress
kcress -
Keith Cress
kcress -
- Thread starter
- #4
The generator in question did in fact back feed a small portion of the grid. I'm not sure how big the generator is, or how many services were being fed by it. The generator did not close into a hot grid, just the opposite. I guess the guy got lucky.
I have one of those Siemens interlocks myself, and it has worked great.
I have one of those Siemens interlocks myself, and it has worked great.
I heard about an incident like this.
The generator did not trip - apparently the controls were not smart enough to detect that there was too much load, and the generator went to full throttle and slowed down, and stalled. It was trying to carry the entire street branch back to where ever the blown fuse was.
Paralleling to the utility without following their rules is Really Hazardous to the linemen trying to repair the system.
Be careful the interlock is listed for your panel.
The generator did not trip - apparently the controls were not smart enough to detect that there was too much load, and the generator went to full throttle and slowed down, and stalled. It was trying to carry the entire street branch back to where ever the blown fuse was.
Paralleling to the utility without following their rules is Really Hazardous to the linemen trying to repair the system.
Be careful the interlock is listed for your panel.
- Moderator
- #6
I forgot about voltage collapse.
All current and very little voltage.
I once saw a crew looking for a fault in a street light circuit.
They powered the circuit with a small generator, about 2 KW.
The generator slowed down, the voltage collapsed, but the genny kept running.
Then they went down the street testing with a clamp meter at each lamp standard.
It worked well.
It was obvious that they had done this often.
Bill
--------------------
"Why not the best?"
Jimmy Carter
All current and very little voltage.
I once saw a crew looking for a fault in a street light circuit.
They powered the circuit with a small generator, about 2 KW.
The generator slowed down, the voltage collapsed, but the genny kept running.
Then they went down the street testing with a clamp meter at each lamp standard.
It worked well.
It was obvious that they had done this often.
Bill
--------------------
"Why not the best?"
Jimmy Carter
- Moderator
- #7
Given the amount of conditions that may cause a back feed into an open line, consider everything hot until the ground chains are on.
Delta transformers, phantom deltas, motors running single phased, induction, did I miss anything?
Bill
--------------------
"Why not the best?"
Jimmy Carter
Delta transformers, phantom deltas, motors running single phased, induction, did I miss anything?
Bill
--------------------
"Why not the best?"
Jimmy Carter
- Thread starter
- #8
- Moderator
- #9
I surmise that the first point of failure may have been the main breaker on the genset.
A closure at 180 degrees out of phase will subject the breaker to twice the normal voltage and the current may well exceed the interrupting rating of the breaker.
A lot of small generators have barely adequate main breakers in respect of the generator short circuit current capability.
Back feeding the generator from the grid may easily subject the breaker to voltage and current well above the capability of the breaker.
I suspect that the owner was the only service on that transformer.
The street light may have been energized blocks away.
If the owner was able to get a usable voltage out of his connection, he would have been feeding all of the customers on the transformer and all of the customers who were on transformers still in parallel with the owners supply transformer.
I am sure that that would have been noted in the report.
The size of the portable generator is not mentioned.
That would make a difference, it would take a large generator to back feed any amount of load.
That the owner shut off a number of breakers suggests limited capacity of the gen-set, supporting my theory that the owner was the only customer on the transformer and that the primary fuse that was out was for that transformer only.
That is no back feed to other than one transformer.
My thoughts.
Comments?
Bill
--------------------
"Why not the best?"
Jimmy Carter
A closure at 180 degrees out of phase will subject the breaker to twice the normal voltage and the current may well exceed the interrupting rating of the breaker.
A lot of small generators have barely adequate main breakers in respect of the generator short circuit current capability.
Back feeding the generator from the grid may easily subject the breaker to voltage and current well above the capability of the breaker.
I suspect that the owner was the only service on that transformer.
The street light may have been energized blocks away.
If the owner was able to get a usable voltage out of his connection, he would have been feeding all of the customers on the transformer and all of the customers who were on transformers still in parallel with the owners supply transformer.
I am sure that that would have been noted in the report.
The size of the portable generator is not mentioned.
That would make a difference, it would take a large generator to back feed any amount of load.
That the owner shut off a number of breakers suggests limited capacity of the gen-set, supporting my theory that the owner was the only customer on the transformer and that the primary fuse that was out was for that transformer only.
That is no back feed to other than one transformer.
My thoughts.
Comments?
Bill
--------------------
"Why not the best?"
Jimmy Carter
A domestic single phase gen set randomly paralleled to a live grid would result in many scenarios, from no noticeable result to catastrophic failure of the set, dependant upon phase difference at moment of paralleling
A large 3 phase set - spectacular
if not synch’d properly!
I.Eng MIMMM MIET MIPowerE AIOSH
A large 3 phase set - spectacular
if not synch’d properly!I.Eng MIMMM MIET MIPowerE AIOSH
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1
- Moderator
- #11
I used to see a 350 KVA set routinely sync'ed at about 30 degrees out of phase.A large 3 phase set - spectacular
This was in a small power house where sets were added online and taken offline several times a day as the load varied.
The breaker tripped, the operator reset the breaker and tried again.
There was a 30 degree error hard wired into the synchro-scope.
They operated that way for a number of years until I got involved and made corrections.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Late to the party . . .
In my view this could only occur if a technically/electrically ignorant [ not necessarily stupid ] person is attempting to supply power to their residence / farm / small business during a power outage. It would take them not realizing that opening their main grid supply breaker is a necessity in these circumstances, lest they firstly [ as was stated ] cause hazardous conditions for their local utility's lines personnel, and secondly attempt to supply load beyond the capability of the generator, thirdly . . .
Flagrantly illegal need not necessarily mean unsafe, provided one knows what one is doing, which can certainly be a BIG "IF."
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
In my view this could only occur if a technically/electrically ignorant [ not necessarily stupid ] person is attempting to supply power to their residence / farm / small business during a power outage. It would take them not realizing that opening their main grid supply breaker is a necessity in these circumstances, lest they firstly [ as was stated ] cause hazardous conditions for their local utility's lines personnel, and secondly attempt to supply load beyond the capability of the generator, thirdly . . .
Flagrantly illegal need not necessarily mean unsafe, provided one knows what one is doing, which can certainly be a BIG "IF."
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
bacon4life
Electrical
Although I have not seen an actual failure, I had assumed any size generator could break the shaft and catastrophically fly into nearby structures when closed out of phase.
Really bad case: If a fallen tree had broken one of wires in an overhead service drop to the house, it is possible a tree trimmer removing the tree could inadvertently restore the connection. In this case, there would be no transformer inrush so the generator would see full out of phase voltage.
Bad case: A slightly less severe case would be a blown cutout feeding a single transformer with a single customer. The transformer inrush would slightly decrease the out of phase voltage.
Perhaps lucky case: In a scenario where the utility restores an entire feeder or an entire substation, the overall inrush from many transformers may reduce the voltage enough to avoid catastrophic mechanical damage to the generator.
Really bad case: If a fallen tree had broken one of wires in an overhead service drop to the house, it is possible a tree trimmer removing the tree could inadvertently restore the connection. In this case, there would be no transformer inrush so the generator would see full out of phase voltage.
Bad case: A slightly less severe case would be a blown cutout feeding a single transformer with a single customer. The transformer inrush would slightly decrease the out of phase voltage.
Perhaps lucky case: In a scenario where the utility restores an entire feeder or an entire substation, the overall inrush from many transformers may reduce the voltage enough to avoid catastrophic mechanical damage to the generator.
- Moderator
- #14
I think that this was the case, except that after several tries, the homeowner managed to energize the transformer.
After several tries the utility crew was able to close in the transformer cutout, and you know the rest.
Bill
--------------------
"Why not the best?"
Jimmy Carter
- Moderator
- #15
There were also a pair of 600 KW sets that were paralleled about once a day with the same sync-scope error.Waross said:
Is this close enough?
I forgot to mention;
About once a year one of the 350 KW sets would shear a key in the coupling.
Bill
--------------------
"Why not the best?"
Jimmy Carter
LittleInch
Petroleum
well this all sounds to me like a question - I'm driving along at 60 MPH and then go round a corner and there's a big rock in the road. What's going to happen?
Well your automatic sensors might recognise it and slow the car down stopping at the rock ( Your breaker drops out)
You hit the brakes and the anti lock works well and you nudge the rock and break a few lights (You blow a few fuses as well)
You hit the brakes too late and hit it so hard the airbags go off (You break the engine and coupling)
You don't see the rock and run into it at 60 MPH, the car breaks and bursts into flames and everyone dies (your disaster story you linked to above)
All are possible, but some are more probable than others. Take your pick.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Well your automatic sensors might recognise it and slow the car down stopping at the rock ( Your breaker drops out)
You hit the brakes and the anti lock works well and you nudge the rock and break a few lights (You blow a few fuses as well)
You hit the brakes too late and hit it so hard the airbags go off (You break the engine and coupling)
You don't see the rock and run into it at 60 MPH, the car breaks and bursts into flames and everyone dies (your disaster story you linked to above)
All are possible, but some are more probable than others. Take your pick.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
- Moderator
- #17
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