MIL-HDBK-1004/5
On this basis, the per-unit voltage at bus 11 shown on Figure A-14
is 0.9381 which is below the criteria of 0.942. The steady state result was
obtained using the 12-percent voltage compensation setting. For this
assembly, the setting needs to be increased to 18 percent and the per-unit
voltage will increase to more than 0.942.
Figure A-15 has an initial load at bus 11 equivalent to three
unit-loaded utilization service assemblies with a total 300-ampere
0.8-power-factor load. A 100-ampere 0.8-power-factor unit load is stepped on
at bus 7. This indicates that the steady state voltage at bus 11 is below the
minimum of 0.942 per unit (113 volts). It also indicates that the series
compensation needs to be increased from 12 to 20 percent for this condition
(four unit loads) to meet minimum voltage requirements.
Figure A-16 has an initial load at bus 11 which is equivalent to
four unit-loaded utilization service assemblies with a total 400- ampere
0.8-power-factor load. A 100-ampere 0.8-power-factor unit load is stepped on
at bus 7. This results in a steady state voltage below the minimum voltage
specified (0.942 per unit). Bus 11 will be approximately 0.897 per unit, and
bus 7 will be 0.903 per unit with 12 percent series compensation. When the
series compensation is increased from 12 to 20 percent, the voltage at the
load will be raised 2.2 percent.
Two voltage droops must be considered for the maximum and minimum
load changes which keep the system's steady state voltage between 113 and 118
volts (bus 7 and bus 11, respectively). These are the voltage droops on the
feeder cable from all unit-loaded utilization service assemblies and from the
feeder cable to the airplane interface point for the load on the service
cable. For example: If the droop on the feeder cable is 0.058 per unit for a
total 400-ampere load and the droop from the input of a utilization service
assembly to the aircraft interface point from a load of 200 amperes is
compensated by a series compensation (12 to 20 percent) so that no reactive
droop exists for the service cable, then the 0.058-per-unit droop for the
feeder cable will appear at the aircraft interface point and be within the
113- to 118-volt requirement. A droop of 0.058 on a 120-volt base is 7 volts:
thus, 120 volts minus 7 volts equals 113 volts RMS. All reactive droop from
the input of the utilization service assembly to the aircraft interface point
should be compensated by the series compensation circuit.
The bus numbers from Figure A-16 can be used to illustrate reactive
droop compensation. If the voltage droop to bus 3 is held at a given percent
for a given total load, then the compensation for the reactive droop from bus
3 through an assembly to an aircraft interface point can be provided. The
last assembly on a feeder cable must have an input voltage of no less than
0.942 per unit. Resistance in the service cable and aircraft cable will then
be the determining factor for the voltage at the aircraft interface point. In
the final design, resistance will also have to be evaluated as a limiting
factor in cable lengths.
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