not sufficiently sensitive to the ground current
level.
The 51V voltage-controlled or voltage-restrained
time overcurrent relay in Fig. 30 is shown on the
CT on the high voltage/system side of the
generator. This allows the relay to see system
contributions to a generator fault. It provides
back-up for the differential relay (87G) and for
external relays and breakers. Since it is monitor-
ing CTs on the system side of the generator, it
will not provide any back-up coverage prior to
having the unit on line. If there is no external
source, no 87G, or if it is desired that the 51V
provide generator protection while the breaker is
open, connect the 51V to the neutral-side CTs.
Fig. 30 shows three relays sharing the same CTs
with a differential relay. This is practical with
solid state and numeric relays, because their low
burden will not significantly degrade the quality of
differential relay protection. The common CT is
not a likely point of failure of all connected
relaying. A CT wiring error or CT short is unlikely
to disable both the 87G and 51V relays. Rather,
a shorted CT or defective connection will unbal-
ance the differential circuit and cause the 87G to
trip. Independent CTs could be used to provide
improved back-up protection, although this
seems to be a minimal advantage here. How-
ever, a separate CT is used for the 51N relay
that provides protection for the most likely type
of fault.
The reverse power relay (32) in Fig. 30 protects
the prime mover against forces from a motored
generator and could provide important protection
for the external system if the motoring power
significantly reduces voltage or overloads equip-
ment. Likewise, the loss-of-field relay (40) has
dual protection benefits—against rotor overheat-
ing and against depressed system voltage due to
excessive generator reactive absorption. Ther-
mal relay (49) protects against stator overheating
due to protracted heavy reactive power demands
and loss of generator cooling. Even if the
excitation system is equipped with a maximum
excitation limiter, a failure of the voltage regula-
tor or a faulty manual control could cause
excessive reactive power output. Frequency
relaying (81O/U) protects the generator from off
nominal frequency operation and senses genera-
tor islanding. The under and overvoltage function
(27/59) detects excitation system problems and
some protracted fault conditions.
Fig. 31 shows minimum basic protection for a
medium impedance grounded generator. It differs
from Fig. 30 only in the use of a ground differen-
tial relay (87N, part of CDS220 or BE1-67N). This
protection provides faster clearing of ground
faults where the grounding impedance is too high
to sense ground faults with the phase differential
relay (87G). The relay compares ground current
seen at the generator high voltage terminals to
ground current at the generator neutral. The 51N
relay provides backup for the ground differential
(87N) and for external faults, using the current
polarizing mode. The polarizing winding mea-
sures the neutral current.
FIGURE 31. SUGGESTED MINIMUM PROTECTION
EXAMPLE (MEDIUM-IMPEDANCE GROUNDED).
Fig. 32 shows minimum basic protection for a
high impedance grounded generator. It differs
from Fig. 30 only in the ground relay protection
and the method of grounding. The voltage units
59N/27-3N provide the only ground protection,
since the ground fault current is too small for
phase differential relay (87G) operation. The 59N
relay will not be selective if other generators are
in parallel, since all the 59N relays will see a
ground fault and nominally operate at the same
time. If three Phase-Ground Y-Y VTs were
applied in Fig. 32, the 27 and 59 could provide
additional ground fault protection, and an addi-
tional generator terminal 59N ground shift relay
could be applied.
17
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