POCO power meets Genny power.

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chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
Let me tell a quick story before I get to my question. I was called into the office yesterday morning to look at a problem with an ATS we installed. Guy has a 200A 1? main breaker panel. Out of this panel we installed a 2 pole 60A breaker to feed the 'Normal Power' lugs in the ATS. From the load lugs in the ATS, we install conductors back to the feed thru lugs at the bottom of the bus in the 200A panel. OK, time to test.

1) Our guy opens the 200A main breaker
2) ATS senses loss of normal power
3) ATS sends start signal, genny starts
4) Genny is up to speed and energizes bus in 200A panel
5) 60A breaker now energized on EM power
6) ATS now sees the EM power as Normal power.
7) ATS switches back to Normal power
8) Genny powers down, buss de-energizes
9) ATS senses loss of Normal power...

You get the rest, click, clack, clack, clack....

So on to my question. As I stand there in disbelief listening to these two talking about turning the 60A off here and the 200A off there and so on, one guy says, "Well if the HO did forget to turn off the main when POCO comes back on line, it will buck phases with the genny and trip the genny breaker. This is where I jump in and say that won't happen as there is no fault current path between the genny windings and POCO tranny windings. That is correct right?

In the above scenario, what would actually occur? Genny VR goes nuts, damage to POCO tranny?

Sorry for the long post but sometimes you see things that make you think, " Are you guys for real???"
 
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Jraef

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Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
... From the load lugs in the ATS, we install conductors back to the feed thru lugs at the bottom of the bus in the 200A panel. ...<snip>
You have nothing synchronizing the generator power with the utility power. The result will be a 99.999% chance (or greater) that they will connect out of synch with each other, the result of which is possibly a massive voltage and current spike during the brief path to destruction of the generator and a very nasty phone call from the utility if they can figure out where you are. You also run a serious RISK OF KILLING A LINE WORKER SOMEWHERE!

Someone has all but completely missed the POINT of the ATS; it is supposed to ISOLATE the two sources, not feed back into itself and allow them to connect at the same time. There must be NO manual process of someone remembering to turn breakers on or off to maintain that isolation. Anything like that is a code violation.

So you must do one of two things;
1) put the ATS AHEAD of the entire load center (if you want to power ALL of the loads for example), or
2) separate out a portion of the loads that are critical enough to need to be powered by the generator. Those loads are then fed by a sub-feed breaker in the main load center, THROUGH the normal side of the ATS, then the generator feeds the Emergency side of the ATS and the output (load) side of the ATS feeds another* panel with the breakers JUST for those critical loads.

* Some mfrs now offer load centers with a split bus to where those critical load breakers are separated from the main bus so that you can feed them with the ATS. The ATS still needs to keep the generator and utility power totally isolated though.
 

bob

Senior Member
Location
Alabama
What happens when the utility has an interruption in power
1) ATS senses loss of normal power
2) ATS sends start signal, genny starts
3) Genny is up to speed and energizes bus in 200A panel with 200 Amp main breaker on
4) 60A breaker now energized on EM power
5) The genny sends reverse power thru the service to the utility system
5) Utility lineman thinks the system is dead
6) Lineman touches line and now he is dead

You got a problem with the installation.
JR posts just ahead of me.
 
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chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
You got a problem with the installation.
JR posts just ahead of me.
I am well aware of this gentleman, as I stated I could just stand there in disbelief listening to these guys. Jr touched on my question briefly stating catastrophic failure. This is my question, what actually happens here?
 

Jraef

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Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I am well aware of this gentleman, as I stated I could just stand there in disbelief listening to these guys. Jr touched on my question briefly stating catastrophic failure. This is my question, what actually happens here?
You have 2 situations here:

1) The generator is connecting to the utility grid while the grid is down, therefore the generator is trying to supply the entire grid up to the point where the utility isolated that section for whatever reason. This is where you run the risk of killing someone, and the generator will be attempting to run countless flat screen TVs in living rooms and hair dryers in bathrooms in your neighborhood and maybe your entire town. It will, if everyone is lucky, trip on overload. More likely though the utility has "Reverse Feed" protection relays at some point which will further isolate the grid when they see this happening, but not in anywhere near enough time to save that lineman's life. If you are lucky enough not to have killed anyone, at the very least you will have cause a much further delay in returning power to the users as the PoCo searches for the source of the reverse feed and has to keep that section of the grid isolated.

2) The generator is on-line and connected to the grid trying to run the TVs when the grid comes back on. The grid ipower is alternating at a different point in the sine wave when that happens. The grid is MUCH more powerful than the generator, so it immediately pulls the magneitc fields in the generator into synch, but that will cause a big current surge, which causes a voltage dip, which causes another big current spike, immediately followed by a big voltage spike at the system "rings" into synchnonicity. At the same time the grid power turns the generator into a motor. The massive torque spike that results can (and does) twist the shaft off of the benerator because the prime mover, the engine, cannot change speed that fast. If you are VERY lucky, the two systems were not much out of phase when they connected, but if they are 180deg out (the worst case scenario), that's when the shaft snaps and the genset is destroyed.

The significance of all of that stuff in #2 is lost on the family of the line worker that was killed though.
 
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bob

Senior Member
Location
Alabama
I apologize for the size of the dwg. I was trying to re size it when I ran out of time. JR has answered your question. I just wanted to comment. The way the installation is installed, when the genny starts it energized the 200 amp panel bus and the service cables connected to it. These cables are connected to the utility grid thru the service conductors which backfeed thru the transformer which energizes the primary conductors. The larger utilities caution their linemen to be carefull during storms when they will likely encounter generators that may backfeed into the system. However accidents still happen.
 

Rick Christopherson

Senior Member
This is where I jump in and say that won't happen as there is no fault current path between the genny windings and POCO tranny windings. That is correct right?
Chris, this is a common misconception with electricity. It is common because that is what we've had drilled into our heads from early electrical instruction.

Contrary to what you were taught in basic circuits, electrons don't care if there is a circuit or not (as in return path). They only care (move) when there is a difference in voltage. When the generator is out of phase with the POCO system, there will be a significant difference in the instantaneous voltages of the phases/legs of the two systems. This will be the equivalent of a dead short but the actual voltage difference will vary depending on the phase difference between the systems. When this does happen, it can be very violent and can permanently damage the physical mechanics of the generator and engine system.

For example, at one instant in time, the voltage between the A & B phases on the generator may be 10 volts, but the voltage between the A & B phases at the POCO may be 200 volts. That constitutes a dead short of 190 volts (instantaneous) between those two systems.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Rick,
I don't understand. If there is no circuit there is no current path and there is no current flow, an increase in voltage does not change this until the voltage is high enough to overcome any insulation or "air gaps".
In this case there are solid paths. There are at least two conductors of the generator and at least two conductors of the utility that are directly connected to each other. There is a "return" path for both systems.
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
This is called a bolted fault and will draw the max. amp as in the AIC rating, blowing things up. If I understand what you are asking.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Rick,
I don't understand. If there is no circuit there is no current path and there is no current flow, an increase in voltage does not change this until the voltage is high enough to overcome any insulation or "air gaps".
In this case there are solid paths. There are at least two conductors of the generator and at least two conductors of the utility that are directly connected to each other. There is a "return" path for both systems.
The current path is the windings on the transformer, usually the overcurrent protection will catch the fault, unless the fault current is too great, and the breaker fails catastrophically. I troubleshot a manual transfer switch on a brand new grocery store where the maintance electricians were testing the portable connection (I think it was a 800 or 1 meg genset) where they would transfer the load (or they thought) to the generator. The breaker in the transfer switch kept tripping, after about three tries, they decided something was not right.:roll: They thought they had a bad breaker, and I went out to see what it would take to change it out, after looking at the connections, I knew something was not right. Tracing back all of the feeds, I found that the generator was connected to the utility terminations, the utility was connected to the load terminations, and the load was connected to the utility input terminations. The manufacture did not have any of the connections labeled, so I can't fully blame the installing electricians, but they should have asked if they were not sure.
 

kingpb

Senior Member
My point was that Rick seems to be telling us that we can have current flow without a circuit and I don't agree with that.
Don, I agree. Even when a breaker opens and there is an arc, there is a circuit path and that path has some amount of impedance.

Back to the OP: As already stated, the install has an issue. It can be made to operate in parallel with the utility, but it takes some additional equipment. The utility company should have interconnection requirements for you to follow, but for safety reasons, i.e. liability and lawsuit, you would be best to disconnect it yourselves; immediately. The good news, it can be resolved because anything is possible with enough time and money. :thumbsup:

HO standby generator systems should be connected such that they are an open transition transfer, IOW - the bus they are connecting to is a dead bus. See picture in previous post. That is a standard set-up. So, as power is lost, the 200A bus goes dead, the Gen starts and wha-la, power gets restored. When utility power comes back and is stable for the set amount of time, the ATS will disconnect from the Gen, making the 200A panel go dead, then the Utility power connects.

There are multiple fixes, but they are in varying degrees of complexity. The easiest solution is get a 200A ATS, and a Gen big enough to handle the panel loads. Make sure it is SER, Eaton makes a nice one with breakers, and hook it up according to the picture in the previous post.
 

Rick Christopherson

Senior Member
Rick,
I don't understand. If there is no circuit there is no current path and there is no current flow, an increase in voltage does not change this until the voltage is high enough to overcome any insulation or "air gaps".
I believe you are confusing the terms "Circuit" and "conductor" (or circuit path). I didn't say that there was no conductor. I said that current doesn't require a complete circuit.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I believe you are confusing the terms "Circuit" and "conductor" (or circuit path). I didn't say that there was no conductor. I said that current doesn't require a complete circuit.
I still don't understand...you need a conductor to make a circuit. That conductor may not be a wire, but you need a conductor. The current does require a complete circuit to flow...if it didn't switches would not work.
 
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