FIGURE 28. SYNCHRONIZING PARAMETERS: SLIP,
ADVANCE ANGLE, AND BREAKER CLOSING TIME.
Integrated Application Examples
Figs. 29 through 33 show examples of
protection packages.
Fig. 29 represents bare-minimum protection, with
only overcurrent protection. Generators with such
minimum protection are uncommon in an era of
microprocessor-based multifunction relays. Such
protection likely would be seen only on very
small (<50kVA) generators used for standby
power that is never paralleled with the utility grid
or other generators. It may appear to be a
disadvantage to use CTs on the neutral side as
shown, since the relays may operate faster with
CTs on the terminal side. The increase in speed
would be the result of a larger current contribu-
tion from external sources. However, if the CTs
are located on the terminal side of the generator,
there will be no protection prior to putting the
machine on line. This is not recommended,
because a generator with an internal fault
could be destroyed when the field is applied.
FIGURE 29. EXAMPLE OF BARE-MINIMUM
PROTECTION (LOW-IMPEDANCE GROUNDING).
Fig. 30 shows the suggested minimum protection
with low-resistance grounding. It includes
differential protection, which provides fast,
selective response, but differential protection
becomes less common as generator size
decreases below 2MVA, on 480V units and
below, and on generators that are never paral-
leled with other generation. The differential relay
responds to fault contributions from both the
generator and the external system. While the
differential relay is fast, the slow decay of the
generator field will cause the generator to
continue feeding current into a fault. However,
fast relay operation will interrupt the external-
source contribution, which may be greater than
the generator contribution. Fast disconnection
from the external source allows prompt restora-
tion of normal voltage to loads and may reduce
damage and cost of repairs.
FIGURE 30. SUGGESTED MINIMUM PROTECTION
EXAMPLE (LOW-IMPEDANCE GROUNDING).
The differential relay (87G) may protect for
ground faults, depending upon the grounding
impedance. The 51N relay in Fig. 30 provides
back-up protection for the 87G or will be the
primary protection if the differential relay (87G) is
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