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.
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