Emergency Generator

The generator might run as a bomb before it can be slapped into synchrony and run as a motor.

You will find the answer here. Its the first entry in the Google search result list.

From what you described and Tornado hints at, following is something of the proces if your generator is "synchronous".

• Breakers close and provide synchronous signal to the motor connections.
• Motor on receipt of excitation attempts to accelerate to synchronous speed of rotation in zero cycles causing HUGE current draw.
• This current draw collapses grid supply in your local area and a set of breakers (somewhere) will isolate either your building or your block.
• This current draw also damages generator coils.
• Your system senses power outage and kick starts motor.
• Motor drives (now damaged) generator in an attempt to overcome the power outage, probably causing even more damage.
• Lots of sparks,lots of smoke, lots of tears.

On a larger scale, this happened in the UK in 1970's. New generator at power plant being run up to speed. Once within 2hz,control is handed over to "automation" so that when phases are in synch the generator is connected to grid. Unfortunately there was a 50hz/60hz mix up and on the next phase crossing, the controller trigered. Accelerating that turbine set by 2hz pulled the local voltage down and tripped the other turbine at that plant for low voltage. This then tripped a nearby HV yard and the fault propagated across the UK causing significant blackouts. (Migh have specifics a little crossed, it was a case study in EE in the early '80s)

I will rephrase my question. Generator is off, breaker closes accidentally, power is now fed onto the generator, will the generator now run as a motor.

Yes, but not for very long until it is destroyed as per the above scenarios. Although there is no field current, the alternator will act as an induction motor driving a load (your prime mover) from zero speed. The resulting torque will snap the drive shaft, and/or violently destroy any bolts that are holding the units together/down.

In a less draconian scenario you should hope that all the upstream protective devices trip and remove the source(s) of power. Expect a plant blackout as a minimum, and a few days examining the equipment for internal damage.

It may try to but more than likely due to the high current draw it creates the breaker will just re open itself.

In normal working conditions trying to connect a dead generator to a live line is not much different than connecting a normal synchronous motor of the same KW rating to a live line in a DOL (Direct On Line) start.

More than likely nothing bad will happen but there is a possibility that the generators AVR (Automatic Voltage Regulator) that controls the rotor field may bet backfed hard enough to do damage to it.

Exactly what happened at one of our sites recently. A fault with the transfer switch back-fed the generator (not running at time), tripped the main breaker and blew the voltage regulator. And let some smoke out of the ATS.

Here's another way to look at your question:

Your system includes a protection system to prevent your system from being damaged by exactly your proposed scenario. If one bypasses this protection system then things can get damaged if your scenario happens.

The obvious question then is why do you want to bypass this equipment protection system? Maybe you are curious why the protection system is more complicated (and possibly more prone to failure) than the production system. You could also be wondering if the brief time that this scenario happened if hidden things are still damaged after repairs.

There's also an ambiguity in your description that the generator is "OFF" that is not helpful. Is the dynamo not electrically energized prior to breaker being flipped but still mechanically connected to the prime mover (gas turbine, ICE, treadmill, waterwheel, wind turbine, etc.) without a directional slip clutch or other mechanical disconnect being or open? If the dynamo is able to freely spin without any mechanical loading except bearing resistance and rotor inertia then one can easily get the grid to safely spin the rotor without any more damage than normal wear and tear. However, if the mechanical connection is there to a frozen prime mover then considerable damage can happen because the mechanical system will experience massive torques and forces opposite in direction than expected.

I agree with the previous statements. Even if the drive couplings do not shear between generator and the prime mover, many of the power transmission devices are designed to turn in a particular direction, not backward. Motorizing is very, very bad, even worse from a dead stop.

The prime mover will probably not be set up to pump oil when the oil pump is run "backwards", if you don't destroy the engine by literally spinning it off its stand, you will seize it up if it stays rotating backward with no oil pressure.

From time to time in our old, old power generating plant, I have heard about incidents where they were taking a steam turbine offline (due to end of dispatched run time), and when the load was low enough, trip was attempted, but did not disconnect. Field breakers did not disengage, and so the generator went into a motorized condition, which meant in our case it kept spinning in the same direction, but with a negative VARS condition, which leads to increasing speed dramatically, until overspeed either trips turbine/generator (or not). It is not fun or nice to play with 13 kV disconnects. If they decide to arc, copper vapor is sufficient for 4th degree burns (to the bone), and almost certain death of anyone in the path of the several thousand degree copper vapor.

This is what led me to this question, the fact that it over speeds, still trying to work out how it can go faster than its synchronous speed.

That's easy, the generator is larger than the total load it sees, and/or it's at the end of a relatively weak radial line. If the generator is the only source, then the governor needs to be in isochronous, not parallel or droop mode. If it's being paralleled with a much smaller generator, then they need the proper droop settings, otherwise the larger unit will try and pick up load from the smaller one, which in turn leads to frequency/speed instabilities.

This is also different than your original question, next time provide as much detail as possible up front, twenty questions doesn't play well around here.

I was not saying yours would overspeed. Our situation is different. It comes when the steam turbine/generator is online, and either is unloaded to minimum load and the field breaker does not open upon trip, or the unit suddenly trips loaded, but field breaker remains closed, in both cases of this, power is influent not out-flowing. In both these cases the "applied" current results in the generator speeding up past what it is being driven.

In your case, the influent power would attempt to accelerate the generator and prime mover up to speed, not necessarily overspeed, but the impulse of a near dead short on the field grid terminating at your generator would be exciting to watch from a distance, I suppose.

You state at the beginning this has never happened, but now imply that it has, and you observe results?

If it is an ac generator, it cannot be going faster than its synchronous speed at rated frequency when motoring, assuming some residual field magnetism, and no amortisseur windings. Perhaps it is a series field dc generator?

How do you know it is going faster? Your instruments may be misinterpreting the situation.

Is this a 3 phase synchronous machine? Small permanent magnet field generator?

Your generator, unless very small, should be tripped off by the reverse power protection, which is included for just that reason, loss of prime mover. But this protection assumes the field is on, because the machine is still connected to the bus. Loss of field will also prevent your main breaker from closing, and will open it, so your prime mover doesn't overspeed the machine to destruction.

Excuse me for that bag of gas being emitted. I had cranial intrarectumitis. True I have never, ever seen a unit spin backwards, but around here, even plugging could happen, given the near zombie like performance of some guys just back from weekend hangover.

Motoring a generator does not reverse its direction of rotation. The phase sequence determines rotation and that shouldn't change ever unless someone rewires it.

Best case, generator output breaker does it's job and trips, worst case, fire and brimstone.

Norm

One more apology for the absolutely balderdash answer I gave earlier. It can happen, but only by human intervention (somebody in zombie mode while attaching cabling/connectors.)

Just a case of too much arcflash or Texas moonshine, either way feel the burn!

Best not to use the equipment this way to artificially produce bottled smoke. My money says the smoke will get out every time. Have you ever noticed how corrosive smoke is to wires it came out of?

Is this even possible in your situation? Is your generator not being used with an automatic transfer switch with interlocks to prevent this?

Hmmm. You don't happen to work for an airline company do you?

http://dailycaller.com/2016/08/08/delta-says-atlanta-power-outage-triggered-computer-system-freeze/

IT WILL ACT AS A MOTOR AND THE MACHINE WILL BE DAMAGED

If the generator is three phase, the rotor of the generator will instantly try to align with the phase angle of the grid.

I saw this happen once on a submarine where the motor generator set and turbogenerator were "synced". Unfortunately the operator misread the syncroscope and synced them 180 degrees out. The mg tried to force the tg to match it's phase orientation and the tg tried to do the same to the mg. The mg lost. The forces on the mg ripped the mg off it's base plate, picked it up in the air and then in dropped into the hole in the bed plate.

Alternatively, the massive currents generated during the excursion will usually blow out circuit protection. The only question is how quick and what else fails first.

And no one can hear you scream underwater, unless it's on porpoise.

<...What will happen if this breaker accidentally closed when there is no Power loss [?]....>

The answer is nothing, because <...enough precautions [are] in place for this scenario not to happen...>.

The system is designed to prevent it happening, because were it to happen in the absence of protection, there would be a catastrophic self-disassembly of some part of the installation involving loss of business process, equipment damage or even fire at the facility, possibly involving loss of life or limb, which would invoke a measure of unpopularity among the facility's insurance cover provider, never mind the criminal negligence proceedings that would arise thereafter.

That's why properly trained and qualified Electrical Engineers are involved with the design, erection and commissioning of such installations, rather than give the job to any old proverbial Tom, Dick or Harry.

There are usually enough layers of electrical safety in place, it would be definitely a triumph of intent to get it to happen before the instigator got a wrench to the noggin.

The only way this could happen if all the good guys were previously knocked unconscious, or forced to leave.

With nothing in place but a chain link fence, it might happen someday, but I sincerely hope it does not, and that any plan to do so is thwarted, and interrupted.

Fool-proof is easy, idiot-proof is near impossible. Anybody who has commissioned a new power plant expects to find a forgotten jumper bypassing a critical subsystem sometime in their career. Been there, done that.

Darwin was right.

Our job is to provide the tools to breed a better idiot.

Do you mean breed idiots that are more Darwin efficient, or less likely to off themselves in the machinery?

Whichever way works better for you.

An example:

Once upon a time in the nuclear industry someone came up with a dosimeter that would chirp every so many microrems of exposure. The object being to allow the worker to find the safest place to position themselves for work in a hot zone. The dose rates went up dramatically and when investigated found the Nimrods were having contests on the job to see who could make the dosimeter chirp the fastest.

See? There is one born every minute or so. Relatively speaking, of course.

Reminds me of an experience I had at one of my first jobs at a pasta plant where i worked as a maintenance tech.

The guy who was my boss was the original tech they hired when the plant was built and one of his bragging points was how he singlehandedly got the whole plant and all the pasta machines wired and up and running a good two weeks sooner than expected.

Sounded pretty impressive in person and management sure liked him for it. Well they did until I showed up and started pointing every time they wanted to do an upgrade to anything that it was damn near impossible being over half the machines didn't have the majority of their wiring in place that would the allow newer systems to be added on to existing ones because everything but the most critical sensors, interlocks and most rudimentary of controls were all just jumpered out in the main PLC and electrical control cabinets.

Well they did until I showed up and started pointing every time they issued a workorder to do an upgrade to anything that it was damn near impossible being over half the machines didn't have the majority of their wiring in place that would allow newer systems to be added on to existing ones because everything but the most critical sensors, interlocks and most rudimentary controls were all just jumpered out in the main PLC and electrical control cabinets.

Upon review of my work and my job training, from his superiors without his initial knowledge no less, it was also found that I had never received a pile of training as well but that he had just forged my signature and filed the paperwork away claiming I had gotten it.

Stuff hit the fan between me and him at first and then between him and his boss shortly after that.

Always bad when "stuff" hits the fan in a pasta plant! Is that where that green pasta comes from? I call B.S. on this post. LOL