MIL-HDBK-1004/5
utilization service assembly load is added, should not be less than
approximately 0.942 per-unit voltage. This per-unit voltage will produce a
no-load voltage of 113 volts+1-n, at the low side of a utilization service
assembly transformer at no load when per-unit voltage is 208/120 volts.
This analysis was performed on a basis of a 113-volt, no-load
voltage on the low side of the utilization service assembly transformer. To
meet the criteria, the full-load and no-load voltages supplied at the end of
the aircraft cable, which is the interface (aircraft connection input) point,
must be no less than 113 volts or greater than 118 volts. The voltage drop
for the aircraft cable connection must be considered. Therefore, all data
presented in this appendix must be based on this requirement.
The other Case A runs, which are shown on Figures A-2 through A-7,
are for various feeder cable lengths up to 40,000 feet. These figures have
the same format as Figure A-1 (Case A-1), so their voltage characteristics can
be compared. The optimum feeder length should be selected on a minimum
voltage requirement for bus 3 (utilization service assembly's high-voltage
side). The minimum steady-state voltage at bus 3 is recommended to be
approximately plus 0.942 per unit when 120 volts root mean square (RMS) is the
base. This voltage will set the minimum no-load voltage at bus 5 for all
other utilization service assemblies on this feeder cable. This no-load
voltage has been set for 113 volts at the utilization service assemblies, but
the 113 volts minimum is also required at the aircraft interface point. The
maximum feeder length is determined by the steady state voltage at the feeder
cable end, which is determined by the cable length, cable parameters, and the
load currents of all utilization service assemblies on the feeder cable.
1.3
Results. The results of the Case A analyses are tabulated in Table
A-1. Voltage was regulated at the generator-transformer, high-voltage side
(4,160 volts).
For a voltage drop on a 40,000-foot feeder not to exceed criteria,
only two 75-kVA utilization service assemblies (each with a 100-ampere
0.8-power-factor unit load) can be supplied. Utilization service assemblies
are capable of serving two 100-ampere 0.8-power-factor unit loads. If both
loads are supplied from one utilization service assembly, then a 40,000-foot
feeder cable could only serve one assembly without exceeding voltage drop
criteria. For a 20,000-foot feeder, only three unit loads can be supplied;
for a 10,000-foot feeder, only five unit loads can be supplied. The optimum
length of feeder cable is determined by the number of utilization service
assemblies on the feeder. The acceptable number of utilization service
assemblies per feeder cable length is obtained by using the voltage drop at
bus 3 in per unit for the various lengths and dividing this quantity into
0.0583 per unit (specified steady state maximum droop at bus 3). This
specified steady state limit will produce a utilization service assembly
no-load minimum voltage of 113 volts --L-N (a minimum voltage which should
also be provided at the aircraft interface point).
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