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TM 5-685/NAVFAC MO-912
grounding transformer for delta type systems where
rent of about 8 A or less. This, however, should not
a system neutral point does not exist. However,
be construed to mean that ground faults of a mag-
because grounding through direct high-resistance
nitude below this level will always allow the suc-
entails having a large physical resistance size with
cessful location and isolation before escalation oc-
a continuous current rating (bulky and very costly),
curs. Here, the quality and the responsiveness of
direct high-resistance grounding is not practical
the plant operators to locate and isolate a ground
and would not be recommended. High-resistance
fault is of vital importance. To avoid high transient
grounding through a grounding transformer is cost
overvoltages, suppress harmonics and allow ad-
effective and accomplishes the same objective.
equate relaying, the grounding transformer and re-
sistor combination is selected to allow current to
(1) High-resistance grounding accomplishes
the advantages of ungrounded and solidly grounded
flow that is equal to or greater than the capacitive
systems and eliminates the disadvantages. It limits
charging current.
transient overvoltages resulting from single phase
j. Ground fault current can be reduced in distri-
to ground fault, by limiting ground fault currents to
bution systems which are predominantly reactive
through reactance grounding. A reactor is connected
approximately 8 A. This amount of ground fault
current is not enough to activate series over-current
between the generator neutral and ground. The
protective devices, hence no loss of power to down-
magnitude of the ground fault is directly related to
the reactor size. The reactor should be sized such
stream loads will occur during ground fault condi-
that the current flow through it is at least 25 per-
tions.
(2) Special relaying must be used on a high-
cent and preferably 60 percent of the three phase
resistance grounded system in order to sense that a
fault current. Because of the high level of ground
ground fault has occurred. The fault should then be
fault current relative to resistance grounded sys-
located and removed as soon as possible so that if
tems, reactance grounded systems are only used on
high reactance distribution systems.
another ground fault occurs on either of the two
unfaulted phases, high magnitude ground fault cur-
k. Whether to group or individually ground gen-
erators is a decision the engineer is confronted with
rents and resulting equipment damage will not oc-
when installing generator grounding equipment.
cur.
(3) High-resistance grounding is normally ap-
Generators produce slightly non-sinusoidal voltage
plied on electrical systems rated 5kV and below. It
waveforms, hence, circulating harmonic currents
is usually applied in situations where:
are present when two or more generating units with
unequal loading or dissimilar electrical characteris-
(a) It is essential to prevent unplanned sys-
tics are operated in parallel.
tem power outages.
(1) The path for harmonic current is estab-
(b) Previously the system has been operated
ungrounded and no ground relaying has been in-
lished when two or more generator neutrals are
grounded, thus providing a loop for harmonic circu-
stalled.
lation. Because of the 120" relationship of other
(4) NEC Articles 250-5 Exception No. 5 and
250-27 have specific requirements for high imped-
harmonics, only triple series (3rd, 9th, 15th, etc.)
ance grounding for system voltages between 480
harmonic currents can flow in the neutral. Har-
and 1000
For those system voltages the following
monic current problems can be prevented by: elimi-
nating zero sequence loops (undergrounding the
criteria apply:
generator neutrals); providing a large impedance in
(a) The conditions of maintenance and su-
the zero sequence circuit to limit circulating cur-
pervision assure that only qualified persons will
service the installation.
rents to tolerable levels (low or high resistance
grounding the generator neutrals); connecting the
(b) Continuity of power is required.
generator neutrals directly to the paralleling
(c) Ground detectors are installed on the sys-
tem.
switchgear neutral bus and grounding the bus at
one point only; or, grounding only one generator
(d) Line-to-neutral loads are not served.
neutral of a parallel system.
(5) Depending on the priority of need, high re-
(2) An effective ground grid system in power
sistance grounding can be designed to alarm only or
plants or substations is highly important and one
provide direct tripping of generators off line in order
that deserves careful analysis and evaluation. The
to prevent fault escalation prior to fault locating
primary function of a ground grid is to limit volt-
and removal. High-resistance grounding (arranged
ages appearing across insulation, or between sup-
to alarm only) has proven to be a viable grounding
posedly non-energized portions of equipment or
mode for 600 V and 5 kV systems with an inherent
structures within a person's reach under ground
total system charging current to ground
of
fault conditions. Reducing the hazard ensures the
about 5.5 A or less, resulting in a ground fault cur-
2-7





 


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