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Short Circuit Current of Standby Generator

Short Circuit Current of Standby Generator

Short Circuit Current of Standby Generator


How do I determine short circuit current (L-G and L-L-L) of standby generator set, in the 125-750kw range? How long can a set handle or export this current? What happens when it goes beyond 10 seconds? Does the current differ the first few cycles vs the rest of the duration? What happens to the output voltage when a generator is shorted? Does the short circuit power factor change any dynamic?

In so far I'm getting 7.5 times the rated current for a bolted L-G fault not to exceed 5 seconds and 3 times the rated current for a bolted L-L-L fault not to exceed 15 seconds before stator coils exceed 300*C.

I want to make sure down stream circuit breakers clear before the gen set stalls out or becomes damaged.

RE: Short Circuit Current of Standby Generator

"Sub Transient Reaction."
"Transient Reaction."
Googling "Generator Reaction" often brings up all three values.

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

RE: Short Circuit Current of Standby Generator

Typically most protection engineers I work with ask for the units generator data sheet or decrement curves, I attached a sample one I had open this morning.
That works if you already have a unit, if you are specifying a unit, then many of the manufacturer's sizing softwares will also provide that info.
Just be aware some of the info you are asking for will be affected by the excitation system type, such as PM, SE or AREP. Also most units used to ship with AVR's that had fixed excitation limiters, newer digital AVR's have adjustable excitation limiters that can be adjusted to affect the maximum time the AVR will go full field.


RE: Short Circuit Current of Standby Generator

Do you mean reactance? I was hoping something more specific for the common makes Kohler/Cummins/Cat. And the accompanying voltage drop when shorted.

Can AVR excitation time limits be adjusted on off the shelf units? If I am not mistaken modern gens are programmed to drop the field after 10 seconds.

RE: Short Circuit Current of Standby Generator

You should get the GENERATOR DECREMENT CURVE(s) from the gen. manufacturer for
NL, FL, with AVR on MANUAL & AUTO etc. Those standard curves will provide all the info that you are
looking for.

RE: Short Circuit Current of Standby Generator

All right.

But- how do I apply them? Is converting a specific short circuit current and its associated output voltage into an ohm value sufficient in determining breaker opening times?

RE: Short Circuit Current of Standby Generator

Also, I have a question here. How is a generator capable of outputting 300% current for 10 seconds when the engine is only rated for 100kw output? Does the engine actually output 3x more torque or is there more into this?

RE: Short Circuit Current of Standby Generator


It is current at a very low power factor, so not many kw in the fault. [a very general response]

RE: Short Circuit Current of Standby Generator

Roughly speaking, do you have any idea of the voltage? I ask because in theory a lower voltage means less current flow through the conductors between the genset and fault point.

RE: Short Circuit Current of Standby Generator

Those two docs are gold, diamond and silver bowsmile I will read through them, I have a lot to learn and will certainly enjoy it- at first glance very concisely written with the reader in mind.

RE: Short Circuit Current of Standby Generator

Alright- those papers have been immensely helpful. It is beginning to make sense, including field forcing.

However, I want to add a bit on top of the knowledge pyramid. Having the decrement curves, how do I obtain the current magnitude and voltage at the generator terminals as I head gradually away from the generator and my conductors become more resistive rather than reactive? For example sizes 6 AWG and declining in physical size resistance dominates.

Also- what happens to the engine? I'm guessing torque required goes up as more real power than reactive power is required to push V through R vs V through X.

RE: Short Circuit Current of Standby Generator

Simplistically you have to model the generator as a voltage source, shunted by the generator internal impedance in series with the line impedance and fault impedance. Generators tend to be fairly reactive (high X/R) and lines more resistive.

The governor will attempt to maintain constant engine speed while the voltage regulator will attempt to maintain voltage at the terminals (assuming this is a single generator). If the generator is supplying significant load pre-fault, the torque may actually go down with a close in (low impedance) fault because the voltage collapses and there is no longer a resistive load taking real power from the machine.

There are entire books and college courses on these topics. It can't really be covered adequately in a forum like this. Get yourself a good introduction to power systems text.

The engineer's first problem in any design situation is to discover what the problem really is. Unk.

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