300% Generator Short Circuit Current

[mbrooke]

How does generator produce 300% of its rated current for 10 seconds? Does the engine actually output over 300% more torque or do some other physics kick in to produce that level of current without stalling the pistons? I'm lost

350REOZJD, 60 Hz | Industrial Diesel Generators | KOHLER

kohlerpower.com

 

[xptpcrewx]

How does generator produce 300% of its rated current for 10 seconds? Does the engine actually output over 300% more torque or do some other physics kick in to produce that level of current without stalling the pistons? I'm lost

350REOZJD, 60 Hz | Industrial Diesel Generators | KOHLER

kohlerpower.com

View attachment 2556606

Generators generally produce fault current that is lower than the full load amperes of the machine. Look at the decrement curve.

Field forcing is a function in some AVR’s which intentionally raises the excitation voltage of the machine during a fault and hence increases the fault current by 300%. This is prescribed with the goal of operating overcurrent protective devices. Without field forcing, the fault current just looks like load and a fault would persist indefinitely.


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[LarryFine]

How does the generator's own breaker not trip first?

 

[xptpcrewx]

How does the generator's own breaker not trip first?

Like I said, the fault current produced by a generator is typically lower than the full load amperes of the machine, and the pickup settings or rating of the OCPD is set above the full load amperes of the machine.


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[mbrooke]

Generators generally produce fault current that is lower than the full load amperes of the machine. Look at the decrement curve.

Field forcing is a function in some AVR’s which intentionally raises the excitation voltage of the machine during a fault and hence increases the fault current by 300%. This is prescribed with the goal of operating overcurrent protective devices. Without field forcing, the fault current just looks like load and a fault would persist indefinitely.


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What is a decrement curve?

I can understand field forcing, but does this not put more load in the engine such that it must use more fuel to keep the same rpm?

 

[mbrooke]

Like I said, the fault current produced by a generator is typically lower than the full load amperes of the machine, and the pickup settings or rating of the OCPD is set above the full load amperes of the machine.


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Lower due to the reactive component of the fault loop?

 

[kwired]

Most generator sets will have a prime mover sized to generator output.

On farms it can be common to see tractor PTO driven generators. Some guys run these with a tractor that is much larger than needed for the generator rating.

Back in 2005 we had ice storm that took power out in large area and some out of power for week or so. I had a guy that to get some corn out of a bin to feed cattle, but this bin was not on same utility service as other parts of the farmstead and had no transfer switch on it.

He decides to directly wire the auger motor to the generator, turns breaker on the generator on to start motor and it breaks universal joint on the PTO drive shaft.

He has me out there and asks what happened there, I looked and he miswired the motor connection - somehow got incoming L1 and L2 tied to the same point so bolted fault. Had he been running more appropriate size tractor for the generator output it maybe would have killed the engine, or at least be a noticeable load on it?

 

[xptpcrewx]

Lower due to the reactive component of the fault loop?

Lower because the synchronous reactance is usually greater than 1 pu and under no load, the excitation is also low.


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[xptpcrewx]

What is a decrement curve?

I can understand field forcing, but does this not put more load in the engine such that it must use more fuel to keep the same rpm?

The generator decrement curve describes the time varying fault contributions of a generator.

I’m not sure about “more” load on the engine. The power factor of the machine and short circuit are not making 100% real power.

I would imagine 300% field forcing at 10s is acceptable and within the rating of the machine. Maybe someone can check me on this.


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[mbrooke]

Most generator sets will have a prime mover sized to generator output.

On farms it can be common to see tractor PTO driven generators. Some guys run these with a tractor that is much larger than needed for the generator rating.

Back in 2005 we had ice storm that took power out in large area and some out of power for week or so. I had a guy that to get some corn out of a bin to feed cattle, but this bin was not on same utility service as other parts of the farmstead and had no transfer switch on it.

He decides to directly wire the auger motor to the generator, turns breaker on the generator on to start motor and it breaks universal joint on the PTO drive shaft.

He has me out there and asks what happened there, I looked and he miswired the motor connection - somehow got incoming L1 and L2 tied to the same point so bolted fault. Had he been running more appropriate size tractor for the generator output it maybe would have killed the engine, or at least be a noticeable load on it?

I wonder if someone can answer this. It seems like reactive power actually causes a generator to speed up.

 

[mbrooke]

The generator decrement curve describes the time varying fault contributions of a generator.

I’m not sure about “more” load on the engine. The power factor of the machine and short circuit are not making 100% real power.

I would imagine 300% field forcing at 10s is acceptable and within the rating of the machine. Maybe someone can check me on this.


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So how much kw load does the engine "see" for a 3 phase bolted fault vs a line to ground fault during field forcing? What about if we start to introduce some conductor resistance and reactance? How does the torque requirement of the two compare?


That part which confuses me is that while the R goes down, at the same time more current can flow through the windings and as such more I2R losses which in theory can only be achieved through actual "work" (watts instead of va).

 

[topgone]

IMO, field forcing has minimal effect of transient and subtransient fault because of the time constant of the field circuit! The field control cannot effect changes in the terminal voltage (hence the fault current) during the period from fault inception to about the subtransient period! The driving force for the fault current will the remanent fluxes in the system that will try to keep the system voltages from decaying fast. The fault current will be high because of the very low fault impedance that the faulted circuit presents during faults!