Depends on how long. For test and exercise, not a problem but for long periods, you should use a load bank. Maybe you should explain why you are asking in more detail, it may make a difference. There are some members here who are very experienced with diesel generators.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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the reason i ask is we have 1500 kw gen they run about 1 hr a week without a load and were told that it could lead to wet stacking which is not good. don't know if this is true.
Wet stacking, (or slobbering) cylinder glassing, oil pumping.
The wet stacking may lead to fires with the possibility of burning lube oil being blown out of the exhaust.
If this is for testing of a standby generator, you should try to run under load. You may be able to do a very fast transfer and run you facility from the generator for a test. This involves synchronizing with the grid and then opening the grid connection and closing the generator connection within a cycle or two. Service is maintained, most equipment will ride through a missing cycle or two. It avoids issues with the utility as the generator is never in parallel with the grid. It also makes for a nice transition back to the grid after an outage.
Bill
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"Why not the best?"
Jimmy Carter
If you have the genset for emergency backup, and have an automatic transfer switch, your transfer switch may be able to synchronize the genset with your mains and switch to generator power with barely a flicker of the lights. I wondered why the startup tech set our 400kw generator @ 60.1hz. The first time I tried to make a synchronized transfer it never happened. That's because I had set frequency back to 60.0. The governor held frequency too well and the cycles never got close enough. At 60.1hz it worked great. I would make the transfer at about 25% of the genny's rating, and then add load incrementally until I was running at about 50%. This provides a better real-life test of your system and is better for the engine.
"...engineering is the art of doing for 10 shillings what any fool can do for a pound". Duke of Wellington Arthur Wellesley
Most utilities frown on that type of operation. It leads to serious protection issues. There is also the possibility of a lethal back feed if the power should fail during your load test.
When I say "Frown" this may include the full weight of the utility's legal department and may include disconnection of your service with little or no notice. David Beach may have some comments from the utility's perspective.
Better to use a very fast open transfer and let the generator take the full load.
Better for the engine is good but life is hard for a standby set. The engine may have to be started cold, accelerated at full throttle and then hit with a transient overload, followed by full design load, all in 8 seconds or less. Life is hard.
Bill
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"Why not the best?"
Jimmy Carter
If you want to exercise the generator against the utility you need to apply for an interconnection. Where it goes from there depends on lots of things. There will be protection requirements that are generally not necessary on stand alone systems. There may also be utility system upgrades to pay for. In our case we can't say no to an interconnection application, but the mitigations may make it impractical to proceed.
