MIL-HDBK-1004/5
Section 2: SINGLE AND MULTIPLE UNIT LOADS
2.1
Variations. The effects of single and multiple 100-ampere
0.8-power-factor unit loads were analyzed in Case D to determine if the feeder
cables could handle multiple loads and still comply with voltage-drop
limitations. The voltage range of 108 volts minimum to 118 volts maximum, as
specified in MIL-STD-704, is the aircraft operating range. This voltage range
allows for a 0- to 5-volt drop in the internal electrical aircraft
distribution system. Therefore, the minimum voltage at the interface
(aircraft connection input) point should be 113 volts RMS and the maximum
voltage should be 118 volts RMS. Figures A-8 through A-12 (Case D) are a
series of computer runs based on the results of Section A.1. The analysis is
concerned with only a minimum, steady state, no-load utilization service
assembly voltage of 113 volts 1-n, but the 113 volts minimum should also be
maintained at the aircraft interface point. The minimum per-unit voltage at
the end of the feeder cable will be 0.942. The series compensation will not
affect the results of this section. Figure A-8 (Case Dl) has one utilization
service assembly or a single unit load of 100- ampere 0.8-power-factor. The
bus 6 per-unit voltage is 0.9759 per unit or 117.1 volts. The bus 3 per-unit
voltage is 0.9865 per unit or 118.4 volts on a 120-volt per-unit base. When
two utilization service assemblies are supplied by a feeder cable (Figure A-9)
each with a 100-ampere 0.8-power-factor unit load, bus 3 and bus 6 per-unit
voltages are 0.9726 and 0.9618, respectively. With four unit-loaded
utilization service assemblies (Figure A-10), each with 100-ampere,
0.8-power-factor load per 75-kVA transformer, the bus 3 and bus 6 per-unit
voltages are 0.9432 and 0.9321, respectively. The maximum number of
unit-loaded utilization service assemblies permitted on a 15,000-foot feeder
cable is four in order for voltage not to drop below 113 volts at utilization
service assemblies. The 113 volts minimum must be maintained at the aircraft
interface point also. This can be achieved by setting the line drop
compensation high enough to offset the inductive voltage drop which occurs
from the point of the utilization service assembly input to the aircraft
interface point. This is the limit imposed by the droop in the bus 3 voltage.
The minimum per-unit voltage at bus 3 is 0.942. The results of this section
are comparable to those of Section A.1. Table A-2 summarizes Case D data.
2.2
Discussion. The service cable length and series (line drop
compensator) compensation have little effect on feeder cable lengths.
Table A-2 indicates that four unit-loaded utilization service
assemblies on a 15,000-foot feeder cable will have a bus 3 voltage of 0.943
per unit and provide a no-load utilization service assembly voltage of 113.2
volts+1-n,. The required 113- volt minimum, no-load, steady state voltage
aircraft interface point must also be checked. This will be covered later in
this analysis. All tests were made with one unit load per assembly. Two unit
loads on the same assembly will produce the same results as one unit load on
two assemblies.
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