Also I would just add that you need to be specific about what exactly you are trying to accomplish. I would guess that a majority of people in the industry would say a low or a certain resistance grounding system is important, but for many applications it doesn't really matter or provide any benefit. Just because a PV system is large, I don't see that that triggers any special need for low resistance grounding. Yes the substation itself has some specific important earthing detail, but I believe even that is more about an equipotential system not achieving a certain resistance.
But 500 KW is NOT even large. Do you really have a substation on site for this? What exactly do you mean by substation?