MIL-HDBK-1004/5
a) Loading. Cover the percentage of the total calculated 400-Hz
load which can be supplied by diesel-engine generators, and justification for
that percentage. In addition, indicate the number of diesel-engine generators
proposed, reasons for the selection, and size (kilowatt and power-factor
rating) with the maximum revolutions per minute (rpm), maximum brake mean
effective pressure (BMEP), and horsepower rating of the engines.
b) Engine class. Cover the type of starting system, type and grade
of fuel, and approximate storage capacity. Justify the reasons for selection
of other than fully automatic diesel-engine plants.
4.3.4
Distribution. Determine the number of utilization service
assemblies which can be served by each medium-voltage feeder in a manner
similar to the example shown on Table 12. Base the proposed number of
medium-voltage feeders on meeting voltage-drop limitations. Figure 8 shows
the single line and formulas used in making the voltage-drop calculations in
Figure 9. The calculations were simplified by the use of a unity power
factor. The complex calculations involved when using a 0.8 power factor will
probably require the designer to access a computer power system analysis
model. This system was used for the Appendix A study.
4.4
Design Computations. Provide computations to indicate that
materials and systems are adequate, but not overdesigned, and are correctly
coordinated.
4.4.1
Capacity and Other Calculations. Calculate loads, number of
frequency conversion assemblies needed, transformer capacities, and each
medium-voltage feeder's allowable utilization connections (see Tables 10, 11
and 12 and paragraph 4.3.3.2). Voltage-drop calculations are necessary (see
Figures 8 and 9).
Short Circuits. In addition to calculating protective device
4.4.2
current rating, determine short-circuit effects of 400-Hz electric power.
4.4.2.1
400-Hertz Systems. 400-Hz systems generate relatively low-fault
currents, primarily because of the inherent impedance of the motor-generator
set portion of the frequency conversion assembly. The peak let-through
current of a motor-generator set always occurs on the first full half-cycle.
Thereafter, the current decreases exponentially to a steady state value which
tends to be approximately 60 percent of the first full half-cycle peak
current. This is a function of the 400-Hz motor-generator set design and,
particularly, the design of the generator damper cage.
Analysis. For simplicity in conducting short-circuit analysis, the
4.4.2.2
impedance of each motor-generator set can be assumed to offer a maximum
available short-circuit contribution equal to 12 times that of the rated
full-load capacity of each set. A short-circuit analysis of a 400-Hz
distribution system is shown on Figure 10.
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