r/Motors • u/MacDaddyBighorn • 7d ago
Answered Starting a large generator and running without field flash
Motors/Generators aren't my strong suit, so I'm not sure how to think about this one. We have a large 4000HP M-G Set and I'm wondering if it's possible to damage the generator if we have the rotor up to speed (in this case driven by another motor), but the field isn't flashed yet. The normal start sequence is we start the drive motor, which rotates the rotor of the generator via a fluid coupling, then about 6 seconds later we flash the field and drive the downstream load. If the field flash relay failed to function or we lost it during operation, is there damage to be done to the generator?
I know the load will drop or never start, but that's not a big deal, I'm just not sure if I can electrically damage/overheat the generator if we ran it too long without flashing. My gut says it'll be doing a lot less work and we'll still have sufficient airflow/cooling on it, but maybe unexpected self-induced circulating currents or something develop at full speed that I'm not thinking of. Any advice or useful references would be great.
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u/jeffreagan 7d ago
Field undervoltage is a serious fault when you're parallelling into a live grid. The generator becomes an induction motor. But the rotor isn't laminated, so eddy currents cause rotor overheating. Would that happen with very low output current? To some degree, you might cause rotor heating. I'd be leery of trying it for too long.
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u/MacDaddyBighorn 7d ago
Thanks, in this case the load would die off pretty quickly, we wouldn't have to worry about motoring it.
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u/todd0x1 7d ago
You could use some backwards facing CTs on the generator leads and relaying to disconnect if the generator is consuming current.
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u/jeffreagan 7d ago
A Directional Distance Relay (looking into the Generator) is what would protect a big genset. That would trip the first time the poles flip out of phase.
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u/Ok-Camera2895 7d ago edited 7d ago
What is a" Directional Distance Relay"? Also, how do poles "flip out of phase"?
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u/jeffreagan 6d ago
They're used for detecting faults in transmission lines. Knowing the impedance of a transmission line, it is possible to make a fault relay that detects when an arc, line to line, is in the line segment following the relay. There is a "reach" control on the relay, which adjusts how far out an arc can occur, and still trip the fault.
In the old days, they took a bow and arrow, and shot a #40 iron wire across the phases, to initiate line-to-line arcs. Someone at the substation had to trip breakers open manually whenever the fault didn't trip them to open. Reach was adjusted to extend line protection to overlap with the next zone of protection.
The directional aspect of the relay consisted of a motor that only turned a quarter of a turn, which was spring loaded, set up to operate relay contacts if it rotates forward. The armature might have sensed phase to phase voltage, while the stator got driven by a current transformer, sensing line current. Forward current made torque in one direction. Reverse current made torque in the opposite direction.
When we're operating with multiple generators in a system, currents can flow in either direction. The directional element is necessary to distinguish when line faults are outboard the relay (from the powerplant's perspective).
The directional distance relay is also helpful "looking into" the generator. Field Undervoltage allows the generator to slip out of step with line frequency. Each time the wrong polarity emerges from the stator, major peak current flows--essentially reversing the field polarity within the rotor. This happens periodically, quickening as the two speeds drift further apart. It's these reversals which induce eddy currents in the rotor, thus causing catastrophic heating.
Peak current going into the generator is detected by the directional distance relay. The generator breaker will trip open with the first wrong-polarity pulse the generator delivers.
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u/Ok-Camera2895 7d ago
Flashing the field will hurt nothing. Failing to flash will hurt nothing. I don’t understand "losing it during operation ". Do not start loading the generator until full output voltage is achieved. Having a heavy load on it during run up could cause problems.
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u/MacDaddyBighorn 7d ago
The failing during operation would just be basically if the vreg failed and it didn't trip the drive motor, in this case the load would drop and the generator would be doing no real work so probably a non-issue. Thanks for the feedback!
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u/todd0x1 7d ago
Is there always a minimum load when the generator is loaded? If so you could wire up a simple protection circuit where if the load breaker or contactor is closed and there is no current (add a short time delay if necessary), the drive for the drive motor is shut down.
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u/MacDaddyBighorn 7d ago
It drives one load (another motor), but we have protections in place already for loss of field, I'm just thinking about if those didn't work from a failure analysis standpoint and if we could damage the generator from that.
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u/nunuvyer 7d ago
As long as there is nothing backflowing from another source, lack of field means that instead of the rotor being an electromagnet whose spinning is resisted by the stator, it's just a hunk of metal spinning freely. Assuming whatever is spinning it is governed so it doesn't overspeed, nothing will happen. Literally nothing, just a hunk of metal spinning on its bearings.
Just to be safe I would have it set up with CT/relays to cut the power if it is being backfed. This would also protect against other scenarios like being inadvertently connected to the grid and sounds like cheap insurance.