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FIGURE 32. SUGGESTED MINIMUM PROTECTION
EXAMPLE (HIGH-RESISTANCE GROUNDING).
The Basler BE1-951, BE1-1051, BE1-GPS100,
and BE1-59N include a third harmonic under-
voltage function (27-3N), that provides supervi-
sion of the grounding system, protects for faults
near the generator neutral, and detects a shorted
or open connection in the generator ground
connection or in the distribution transformer
secondary circuit.
FIGURE 33. EXTENDED PROTECTION EXAMPLE (HIGH-RESISTANCE GROUNDING).
Fig. 33 shows the application of additional relays
for extended protection: overexcitation relay (24),
negative sequence overcurrent and overvoltage
relay (46 and 47), ground-overcurrent relay
(51GN), voltage-balance relay (60), field-ground
relay (64F), frequency relay (81) and the 27/50/
62 relay combination for inadvertent energization
protection. Relay 51GN provides a second
means of detecting stator ground faults or faults
in the generator connections or faults in the delta
transformer windings. Differential relay 87T and
sudden-pressure relay 63 protect the unit step-up
transformer. Relay 51N provides backup for ex-
ternal ground faults and for faults in the high-
voltage transformer windings and leads. This
relay may also respond to an open phase con-
dition or a breaker-interrupter flashover that ener-
gizes the generator. The 51N relay will be very
slow for the flashover case, since it must be set
to coordinate with external relays and is a last-
resort backup for external faults.
Figure 33 shows wye-connected VTs, appropri-
ate with an isolated-phase bus.
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