MIL-HDBK-1003/11
8.4.3.1 Generator Protection. The normal protection required for
medium-voltage generators is shown on Figure 6. Control power is supplied
from the station battery system.
a) Differential Relaying (ANSI Device 87): Since differential
relaying utilizes a current difference between two points to indicate a
fault, differential current transformers should not be used to supply other
devices. The current transformer location points are shown on Figure 6.
The generator current transformers can be located on either side of the
generator circuit breaker in accordance with the manufacturer's standard
practice. The lockout feature (ANSI Device 86) is standard for differential
relaying.
b) Ground Relaying (ANSI Device 51G): The lockout feature is
desirable for ground relaying, but it is not necessary in plants having
adequately trained personnel.
8.4.3.2 Incoming Line and Feeder Protection. The minimum relaying
requirements shall consist of overcurrent protection as is shown on
definitive drawings (refer to Section 1). Although time-overcurrent
relaying (ANSI Device 51) may be sufficient for protection, it normally also
provides the instantaneous element, (ANSI Device 50), an accessory feature
in the same enclosure with the time-overcurrent relay. This unit can be
blocked, if not needed, but is available for changing system conditions.
8.4.3.3 Load Shedding Capability. A load shedding system capability can be
provided based on sensing underfrequency or a rate of frequency decline on
the system caused by sudden load changes. System balance can be established
by temporarily dropping selected feeder loads. Underfrequency schemes are
usually arranged in steps to continue dropping load until the system is
stabilized. The use of undervoltage sensing is inadvisable since the
generator voltage regulators will tend to compensate for voltage decay.
8.4.3.4 Analysis. To determine actual protective relaying requirements, an
analysis should be performed concerning requirements for new systems and
coordination with existing systems. Fault calculations may indicate the
need for protection in addition to the minimum requirements covered
previously. Additional protection may be indicated because of either the
size of the new distribution system or to match the existing distribution
system. See NAVFAC MO-204, Electric Power System Analysis, for guidance on
assembling the information necessary for a coordination study.
8.4.3.5 Control Power. Direct-current closing and tripping for
medium-voltage circuit breakers should usually be provided by a 125 V
station battery system. For low-voltage generating plants, 24 V or 48 V
systems should normally be supplied, except where very small systems utilize
automatic transfer switches for commercial to generating system transfer.
Lead calcium cells should be utilized except when maintenance requirements
justify the use of the more costly nickel-cadmium cells. Batteries are
highly reliable devices when properly maintained. Provision of a second
battery system will usually not provide any more reliability, since such its
system maintenance will be on the same level as the system it backs up.
However, for very large plants consider supplying one-half of the plant
loads from separate battery systems which can interlocked so either or both
systems can supply the load but systems cannot be paralleled.
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