MIL-HDBK-1003/7
standard short circuit ratios of 0.58 at rated kVA and power factor. If a generator
has a fast acting voltage regulator and a high ceiling voltage static excitation system,
this standard short circuit ratio should be adequate even under severe system
disturbance conditions. Higher short circuit ratios are available at extra cost to
provide more stability for unduly fluctuating loads which may be anticipated in the
system to be served.
f) Maximum winding temperature, at rated load for standard generators, is
predicated on operation at or below a maximum elevation of 3,300 feet; this may be
upgraded for higher altitudes at an additional price.
Features and Accessories. The following features and accessories are
6.2.2
available in accordance with NEMA standards SM 12 and SM 13 and will be specified as
applicable for each generator:
Voltage Variations. Unit will operate with voltage variations of plus or
6.2.2.1
minus 5 percent of rated voltage at rated kVA, power factor and frequency, but not
necessarily in accordance with the standards of performance established for operation at
rated voltage; i.e., losses and temperature rises may exceed standard values when
operation is not at rated voltage.
6.2.2.2
Thermal Variations
a) Starting from stabilized temperatures and rated conditions, the armature
will be capable of operating, with balanced current, at 130 percent of its rated current
for 1 minute not more than twice a year; and the field winding will be capable of
operating at 125 percent of rated load field voltage for 1 minute not more than twice a
year.
b) The generator will be capable of withstanding, without injury, the
thermal effects of unbalanced faults at the machine terminals, including the decaying
effects of field current and DC component of stator current for times up to 120 seconds,
provided the integrated product of generator negative phase sequence current squared and
time (I22t) does not exceed 30. Negative phase sequence current is expressed in per unit
of rated stator current, and time in seconds. The thermal effect of unbalanced faults
at the machine terminals includes the decaying effects of field current where protection
is provided by reducing field current (such as with an exciter field breaker or
equivalent) and DC component of the stator current.
Mechanical Withstand. Generator will be capable of withstanding, without
6.2.2.3
mechanical injury any type of short circuit at its terminals for times not exceeding its
short time thermal capabilities at rated kVA and power factor with 5 percent over rated
voltage, provided that maximum phase current is limited externally to the maximum
current obtained from the three-phase fault. Stator windings must withstand a normal
high potential test and show no abnormal deformation or damage to the coils and
connections.
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