For years working around the engines (gas and diesel) and generators I mostly deal with in the .5 to 5 MW range, the default or rule of thumb type setting for reverse power was 10% of rated for 10 seconds. You saw it not only in the protective relays but in multifunction load controllers like the Woodward DSLC and Basler DGC-2020.
Now look at all the improvements in horsepower to weight/size ratios in modern gas and diesel IC engines, a 500kW genset was a big chunk of iron, and not just the engine but the tail end as well. And internal improvements like smaller bearing/crankpin surface area per BHP than older engines, improved piston ring to liner interface, improved cam to lifter profiles, all this stuff made the engines frictional horsepower go way down. But did that really get passed on down the trail to the folks doing the electrical protection? Do the current modeling softwares like SPM and ETap take that into account? I have an older protection handbook that states "Maximum motoring power of a diesel engine is 25% of rating", that was probably a good guideline for a large slow speed engine many years ago, not sure it would be of any use today.
I still see a lot of units in the field with 10/10 reverse power protection, on a high speed highly turbocharged engine and a smaller, lighter generator end, I will bet money that engine has nowhere near a 10% of rated parasitic horsepower demand. But they still showup, and almost every time I bring it up it, it becomes an argument. Of the 9 units I mentioned last year, I was involved on three of them early in their service life. In all three cases I expressed my concern, in all three cases the settings were not changed. On top of the fact the engines have reduced frictional losses, modern engines also have more extensive protection system with very large numbers of active shutdown protections. Take an "old" style mechanical diesel engine with a standard protection package, it likely had low oil pressure, high water temperature, overspeed and emergency stop shutdowns. It was also in most cases an Energize to Shutdown protection system, so a loss of control power would allow the engine to continue to run. Take an engine driving a similar size generator today with modern electronic engine controls operating under some sort of emissions compliance. I would bet at a MINIMUM there are at least 20 active shutdown protections, and if control voltage drops while the engine is running it will operate erratically or shutdown altogether. I'm not sure if this is an issue on the smaller end of the size range or because it driven by and IC engine. Sounds like the gas and steam turbine folks are more diligent in their protection schemes.
Nice thing about a forum like this is the differing perspectives everyone has based on what they work around, not everyone may have all the "right" answers but it sure brings in a lot of great info.
Hope that helps, MikeL.