MIL-HDBK-1004/5
APPENDIX A
ANALYSIS OF 400-HERTZ
CENTRALIZED POWER DISTRIBUTION SYSTEMS
Section 1: MAXIMUM FEEDER CABLE LENGTH
1.1
Variations. Seven different feeder cable lengths were analyzed in
Case A to determine the maximum feeder cable length. The lengths varied from
5,000 feet for Case Al to 40,000 feet for Case A7. For a typical cable, the
only parameters determining the voltage droop are resistance, inductance, and
capacitance. The No. 2 American wire gauge (AWG) cable parameters used are as
follows:
Resistance
0.198 ohms per 1,000 feet
74 x 10-6 henries per 1,000 feet
Inductance
0.603 x 10-6 farads per mile
Capacitance
The capacitance of the cable is compensated for by shunt inductance
(shunt reactance). A 100-ampere, 0.8-power-factor load is assumed for each
case. The series compensation (line drop compensator) is fixed at 12 percent.
Feeder lengths are as follows: Case A1 - 5,000 feet; Case A2 -
10,000 feet: Case A3 - 15,000 feet; Case A4 - 20,000 feet; Case A5 - 25,000
feet; Case A6 - 30,000 feet: and Case A7 - 40,000 feet.
1.2
Discussion. The per-unit resistance and reactance for each
component are shown between the buses. Figure A-1 (Case A1) shows the
per-unit resistance and reactance of the feeder cable between buses 2 and 3.
Between buses 4 and 5, the 12-percent, voltage-compensation impedance shown is
-jO.412 per unit for capacitance. The frequency conversion assembly's
generator power input to bus 1 is in megavoltampere-ampere (MVA) units. The
power is 28.6 kilowatts (kW) and the reactive power is 20.8 kilovars (kvar).
All impedance parameters shown are in per-unit. The per-unit basis is one
Bus 2 on the high side of the frequency conversion assembly' s
0.9951 per unit or 4,140 volts 1-1. At bus 3, which is the end of the
distribution feeder cable, the voltage is 0.9924 per unit or 4,128 volts 1-1.
This is the logical point in the system where the optimum feeder length is
determined, since this is the point where the no-load voltage going to the
last utilization service assembly on the feeder must be determined. Bus 4 is
the low-voltage side of the utilization service assembly transformer. The
voltage here is 0.9725 per unit on a 208-volt per-unit base or 202.3 volts.
Bus 5 is the load side of the utilization service assembly's line
drop compensator. The voltage is 0.9998 per unit or essentially one per unit.
It must be remembered that only one utilization service assembly is on the
38
Previous Page
