MIL-HDBK-1004/5
The bus 6 value (utilization service assembly output) is 119.3 volts; the bus
3 value (end of feeder cable) is 0.9863 per unit. The voltage at bus 3
indicates that four 75-kVA utilization service assemblies could be added to
the feeder cable, based on Table A-2; that is, one per-unit load gives the bus
3 voltage indicated on Figure A-21. However, four per-unit loads, as shown
by Table A-2, will not decrease voltage below the criteria. The maximum
service cable length is used to determine the voltage at the end of the feeder
cable (bus 3). After the steady state minimum voltage is established as 0.942
per unit at bus 3, the maximum service cable length can be determined.
Assume that four unit-loaded utilization service assemblies on a
feeder cable produce voltage greater than 0.942 per unit at bus 3 or 113 volts
on the last utility service assembly transformer's low-voltage side at the
no-load condition. The percent series compensation is set to compensate for
the fixed utilization service assembly's transformer impedance, the variable
service cable impedance, and the fixed aircraft cable impedance. The
procedure adds all the inductive reactances from bus 3 to the aircraft
interface connector input. Choose a series voltage compensation percentage
that will cancel these reactances and some of the feeder cable reactance. It
is possible too overcompensate for the inductive reactance. This will produce
a capacitive reactance drop in the system. The final desired result is that
the voltage at the aircraft interface connector does not go below 113 volts
RMS or above 118 volts RMS, in the steady-state condition.
Figures A-22 through A-26 (Cases Bll through B15, respectively) are
for the cable characteristics given below:
No. 2 AWG cable 0.198 + jO.197 ohms per 1,000 feet
No. 4/0 AWG cable 0.085 + jO.178 ohms per 1,000 feet
The series compensation is 12 percent. The voltage effects
resulting from the No. 4/0 AWG service cable and the No. 2 AWG aircraft cable
are separated for each figure's single-line diagram.
The voltage at bus 3 has been selected to determine the feeder cable
maximum length. This voltage has been selected so that the no-load voltage on
a utilization service assembly near the end of the feeder cable will be no
less than 113 volts 1-n, as discussed in Section A.1.
The maximum service cable length for a 100-ampere, 0.8-power-factor
unit load is determined as follows. It is assumed that the feeder cable
length and number of unit-loaded utilization service assemblies have been
determined. This sets 113 volts as a minimum steady state voltage at a non-
loaded utilization service assembly transformer.
The inductive reactances of the utilization service assembly
transformer, aircraft cable, and the service cable are added together on a
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