Generator response for a Fault 1

[CuriousElectron]

Greetings,
For a line-to-line or line-to-ground fault that occurs external to generator stator, is there a possibility that generator could experience internal damage to winding or core due to high magnitude short external to it, if the contribution from the unit is not cleared fast enough?
I understand that if a bolted fault occurs at generator terminals, then there will likely be damage to the stator that will require rewinding or replacing at least some of the winding coils on the unit.
What about scenarios where the fault occurs on generator bus, would the generator just experience short-duration temperature rise to its windings(tolerated)until gen breaker and exciter breaker trip?
Just wanted to get the high level theory first. The main question was if the unit can exhibit any considerable internal damage for an external fault, assuming protective devices operate in the time frame that they suppose to.

Thanks guys,
EE

 

[TugboatEng]

There should be no damage to the generator by fault downstream of protective devices. I do know of a recent incident where racking tool was left in the switchboard enclosure and fell on to the bus bars. The generator was not damaged but the switchboard is no longer operable. That does render the generator useless as well.

My sister company has experienced a fault within the generator enclosure and that caused an unprotected fault which destroyed the generator. The wiring was too stiff and layer over a terminal. After 10 years if chaffed and shorted.

It's exceedingly important that you construct your wiring between the generator and the first protective device in the most reliable and secure method possible.

 

[waross]

It depends a lot on the excitation.
Older, self excited sets would often suffer voltage collapse and the result may be to little current to operate the protective devices, and often no damage to the generator.
Much worse than a fault is an out of phase closure.
Closure on a 90 degree phase error develops the greatest torque transients.
Closure on a 180 degree phase error develops the greatest current and magnetic forces.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

It's possible, but unlikely. Generators are normally designed to withstand the mechanical forces generated by a three-phase fault at the generator terminals. That is a common design specification. Single line to ground fault are another issue, since the zero sequence reactance of a generator is often less than the subtransient reactance and this results in higher fault current - but most larger generators are not solidly-grounded, so this isn't a concern.

 

[cranky108]

Generator damage, like that of transformers adds up over time, so there would be some damage, but the unit would work.
The winding become loose, and some heat stress of the windings, but not much that is repairable at the time.
Same kind of thing when it is overloaded.

 

[electricpete]

I don't know the answer for your machine but there's a lot you didn't say. What size generator. What is the grounding configuration. Is this fault you're postulating upstream or downstream of the generator circuit breaker that will clear the fault.

For the large turbine driven generators that I'm familiar with, they cannot mechanically tolerate the forces from phase to phase short circuit fault and there is a long distance between the generator terminals and the interrupting device. Hence isolated phase bus construction is used to prevent/minimize the possibility of phase to phase fault. Phase to ground short circuit fault current magnitude is limited by the neutral grounding configuration.

=====================================
(2B)+(2B)' ?

 

[TugboatEng]

There was an incident last week that I was not involved with but a circuit breaker racking tool was left in a switchboard and as soon as the ship started taking rolls the racking tool fell across the main bus bars. These systems would have had a generator protection breaker as well as a bus tie breaker in between the generator and the fault. My understanding is that it still detonated the switchboard but the generators are okay.

 

[CuriousElectron]

[highlight #FCE94F]@electricpete[/highlight],
This is a Hydro generator, high impedance grounded. I was considering a hypothetical fault, Line-Line that occurs between the generator terminals and GEN circuit breaker. Obviously, we'd see a short duration temperature rise in the stator that would be proportional to the magnitude of the fault and the time it takes to trip the exciter breaker. I understand that this event would likely shorten the life of the winding coil insulation, but otherwise no 'serious' damage would occur to the stator. I'd imagine faults like these are extremely rare especially if generator bus is composed of iso-phase bus.
Am I right on this.
Thanks,
EE

 

[TugboatEng]

There is a certain point where you must rely on quality and not protective devices. You already rely on the quality of protective devices to protect your generator from downstream faults. However, there is no protection between your generator and your first protective device. In this area you must rely on quality of installation. It is not difficult to prevent faults in the initially installed equipment. Invest in your cable routing and strapping so you don't need to invest in worst case scenarios. When I connect generators I use stand offs you put wires in safe planes so that they cannot chaffe. Also have a final inspection of the enclosure to ensure there is no loose material that can cause failure.

 

[cranky108]

Iso-bus is good for isolating phases from phase to phase faults, however, in the GSU there maybe a concern depending on how it was designed.
The high impedance grounding is to limit fault current for a ground fault, which can happen in the iso-bus.