Grounding practices in a power generating station is a vast subject, on which extensive research has been done by many authors over the years. The grounding stations in such areas are normally spread over the entire station, and sometimes may even extend beyond its boundary to achieve the desired results. Here we discuss briefly, the basic criteria behind the requirement of a grounding system in a power station and its design considerations. The magnitude of ground voltage in such areas in the event of a ground fault is very high, due to high system voltage. On a ground fault, the ground path resistance may become a source of a high potential gradient across the grounding conductors at a particular location. It may become high enough to prove fatal to a human operator coming into contact with it. To limit this potential difference at all locations within a tolerable value and achieve an equipotential distribution of a ground conductor over the station is the basic criterion on which is based the design of a grounding system for a power generating station. This discussion is also applicable to outdoor switchyards and large sub-stations. For detailed working, it is advisable to refer to IEEE-80.
Also there is an excellent IEEE color book about industrial and commercial power system grounding, IEEE 142 (IEEE recommended practice for grounding of industrial and commercial power system), for grounding of industrial generators guide you can refer to section 1.8 (as below) of mentioned green book.