MIL-HDBK-1004/5
common base. The 20-percent series voltage compensation (-j0.692 per unit) is
on the low-voltage base. The series compensation per-unit impedance value has
to be greater than the total inductive reactance on the same per-unit base.
Referring to Figure A-24 (service cable length equals 200 feet), the
total per-unit inductive reactance from bus 3 to bus 7 is j0.2454 + j0.2668 +
j0.0793 = j0.5915. Since the series compensation of 20 percent is equal to -
j0.692 per-unit reactance (Table A-3), the minimum voltage at the airplane
interface connector will be greater than 113 volts RMS. The resistance of the
cables and the utilization service assembly transformer also have to be
included in the analysis.
Table A-3
90-kVA Line Drop Compensator's Per-Unit Impedance
______________________________________________________________________________
Compensation
Per-Unit Ohms on 118-Volt Base
Percent
90-kVA Base
312-kVA Base
______________________________________________________________________________
5
-j0.023
-j0.172
6
-j0.028
-j0.209
7
-j0.033
-j0.246
8
-j0.037
-j0.276
9
-j0.042
-j0.313
12
-j0.055
-j0.412
14
-j0.065
-j0.485
16
-j0.073
-j0.545
18
-j0.084
-j0.627
20
-j0.093
-j0.692
Per-Unit Ohms on a 90-kVA Base = (0.204)squared/0.09 = 0.4624 ohms
Example: 5-percent ohms = 0.05 x 0.4624 = 0.023 ohms
______________________________________________________________________________
If four unit-loaded utilization service assemblies (100-ampere, 0.8-
power-factor load) were on a feeder cable of 15,000 feet, the maximum service
cable length is No. 4/0 AWG - 270 feet and the length for the aircraft cable
to the airplane interface connector is No. 2 AWG - 40 feet.
Results. Table A-4 shows the effects of the various parameters on
3.3
the maximum service cable length for 100-ampere, 0.8-power-factor unit loads.
The beneficial effects of paralleling service, thus reducing the impedance of
the load circuits, is the same as increasing the series compensation. When
the series compensation is set at 20 percent, the service cable maximum
lengths, as given in Table A-4, should be adequate for most installations.
56
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