Parallel Generators Grounding

Hello all,

I have a small question concerning grounding parallel generators. I found three ways to do it: either connect each one of them to ground, or connect all of them to one neutral bus and connect that bus to ground, or simply ground one of them to avoid any ground loops. Each method has its own advantages and disadvantages. Can someone provide an explanation of when do we use each method? What are the conditions needed to deploy any method of these? Which method is generally better? Can someone please provide some good references?

Thank you

Mohammad