When you have a standby genset with an alternator that is larger than what the generator is rated for, do you use the generator rating or the actual alternator rating?

Example. A genset is rated for 500kw. The actual alternator used is rated for 660kw/825kva. The diesel engine driving the alternator is not large enough to produce 660kw. It is only capable of 500kw. In your model do you model as 660kw or 500kw (along with all the decrement curve and time constant data from the alternator data sheet) for the purposes of fault contribution calcs, not load flow?

Thanks for the suggestion, Mike. Not sure that is appropriate or not. I've run into this twice in one week. Seen it a few times before. Here's why I'm skeptical of this. If the alternator is spinning at 1800 rpm when the fault occurs and the fault contribution is so brief due to the fast decay, does the HP of the engine really matter for fault contribution purposes?

I just did a quick test, and the KW you input into SKM definitely affects the contribution with all the decrement curve and time constant data being identical between the two. What am I missing here?

This is what the generator rep told me when this came up a year ago. I don't recall what I did for that model. Like I said, I've had this two more times within the last week. Once more with a Kohler generator and once with a Cummins.

After thinking about this and looking at the data sheet for one of the generators, I think I may have answered my own question. I think you have to use the alternator rating, not the entire generator set rating. My reasoning for this is, the alternator will have a "base" KVA and the decrement curve data is often given in per unit.

The alternator manufacturer doesn't necessarily know what engine the alternator will be paired with and what the final genset output rating will be. They only know what their KVA rating is at a particular voltage. Data sheets are published for the alternator. I think therefore, you must use the alternator kva rating in the SKM model for the per unit decrement curve information to be accurate.

At least for fault current modeling, not for load flow.

Make sense anyone? Agree / disagree?

Here is the nameplate and the data sheet for a 480V, 500 kw genset. As you can see, the alternator is 660kw rated at 480V. The engine-alternator combo is only 500 kw.

I don't know. Can it? We're talking fractions of a second. Just a few cycles or so, or until a breaker tripped. Does the engine HP make any difference? I think the inertia of the alternator and its kva rating would be the determining factor.

That's great!

Never thought to use AI for an answer. In fact, I have yet to use ChatGPT or any other AI. Might have to start though.

Thanks for sharing that!