It is common practice to ground all types of generators through some form of external impedance. The purpose of this grounding is to limit the mechanical stresses and fault damage in the machine, to limit transient voltages during faults, and to provide a means for detecting ground faults within the machine.
A complete discussion of all grounding and ground protection methods may be found in IEEE Std C62.92.2-1989 and IEEE Std C37.101-1993.
The methods most commonly used for generator grounding will be discussed in this guide. They are listed in the following four broad categories:
a) High-impedance grounding
b) Low-resistance grounding
c) Reactance grounding
d) Grounding-transformer grounding
Solid grounding of a generator neutral is not generally used since this practice can result in high mechanical stresses and excessive fault damage in the machine. According to ANSI C50.13-1989, the maximum stresses that a generator is normally designed to withstand is that associated with the currents of a three-phase fault at the machine terminals.
Because of the relatively low zero-sequence impedance inherent in most synchronous generators, a solid phase-to-ground fault at the machine terminals will produce winding currents that are higher than those for a three-phase fault.
Therefore, to comply with this guide, generators shall be grounded in such a manner to limit the maximum phase-to-ground fault current to a magnitude equal to, or less than, the three-phase fault current.
Generators are not often operated ungrounded. While this approach greatly limits the phase-to-ground fault currents, and consequently limits damage to the machine, it can produce high transient over voltages during faults and also makes the fault location difficult to determine.