4
in Fig. 5, a ground fault at the generator high
voltage bushings elevates the sound phase line to
ground voltages to a nominal 173% of normal line
to neutral voltages. Also, the neutral to ground
voltage will rise to the normal phase-ground
voltage levels. The closer the ground fault is to
the generator neutral, the less the neutral to
ground voltage will be. One method to sense this
neutral shift is with the 59N relay (Fig. 4) monitor-
ing the generator neutral. The 59N will sense and
protect the generator for ground faults over about
95% of the generator winding. The selected 59N
(Basler relays per Table 2) relay should be
selected so as to not respond to third harmonic
voltage produced during normal operation. The
59N will not operate for faults near the generator
neutral because of the reduced neutral shift during
this type of fault.
FIGURE 5. NEUTRAL SHIFT DURING GROUND FAULT
ON HIGH IMPEDANCE GROUNDED SYSTEM.
Faults near the generator neutral may be sensed
with the 27-3N. When high impedance grounding
is in use, a detectable level of third harmonic
voltage will usually exist at the generator neutral,
typically 1-5% of generator line to neutral funda-
mental voltage. The level of third harmonic is
dependent on generator design and may be very
low in some generators (a 2/3 pitch machine will
experience a notably reduced third harmonic
voltage). The level of harmonic voltage will likely
decrease at lower excitation levels and lower load
levels. During ground faults near the generator
neutral, the third harmonic voltage in the generator
neutral is shorted to ground, causing the 27-3N to
drop out (Fig. 6). It is important that the 27-3N
have high rejection of fundamental frequency
voltage.
FIGURE 6. GROUND FAULT NEAR GENERATOR
NEUTRAL REDUCES THIRD-HARMONIC VOLTAGE IN
GENERATOR NEUTRAL, DROPPING OUT 27-3N.
The 27-3N performs a valuable monitoring
function aside from its fault detection function; if
the grounding system is shorted or an open
occurs, the 27-3N drops out.
The 59P phase overvoltage relay in Fig. 4
supervises the 27-3N relay, so that the 86
lockout relay can be reset when the generator is
out of service; otherwise, the field could not be
applied. Once the field is applied and the 59P
operates, the 27-3N protection is enabled. The
59P relay should be set for about 90% of rated
voltage. An “a” contact of the unit breaker can be
used instead of the 59P relay to supervise 27-3N
tripping. Blocking the 27-3N until some level of
forward power exists also has been done.
However, use of the 59P relay allows the 27-3N
to provide protection prior to synchronization
(i.e., putting the unit on line), once the field has
been applied.
In order to provide 100% stator winding cover-
age, the undervoltage (27-3N) and overvoltage
(59N) settings should overlap. For example, if a
generator-terminal fault produces 240V, 60 Hz
across the neutral voltage relay (59N), a 1V
pickup setting (a fairly sensitive setting) would
allow all but the last (1/240)*100 = 0.416% of the
winding to be covered by the overvoltage
function. If 20V third harmonic is developed
across the relay prior to a fault, a 1V third-
harmonic drop-out setting would provide dropout
for a fault up to (1/20)*100= 5% from the neutral.
Setting the 59N pickup too low or the 27N
dropout too low may result in operation of the
ground detection system during normal operating
conditions. The third harmonic dropout level may
be hardest to properly set, since its level is
dependent on machine design and generator
excitation and load levels. It may be advisable to
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