MIL-HDBK-1004/5
Section 4:
MAXIMUM LOAD CHANGES ALLOWED ON THE SYSTEM
4.1
Variations. Load changes will affect the amount of load which a
feeder cable can handle. Case C analyzes the impact of load changes on the
steady state voltage. Analyses of transient and motor-starting runs do not
indicate adverse effects on the system when the guidelines are followed.
This analysis was made with one feeder cable circuit from the
central generation system. Each feeder cable with utilization service
assemblies shall be essentially a separate circuit in the steady state
condition.
4.2
Steady State Load Changes. Maximum load changes are limited by the
steady state requirement for load voltage.
MIL-STD-704 limits the steady state phase voltage to a range of
108.0 volts RMS to 118.0 volts RMS in the normal mode or 102.0 volts RMS to
124.0 volts RMS in the emergency mode. These voltage-range limits are for
equipment inside the aircraft, and these limits take into account the 0- to 5-
volt drop permitted internally. Therefore, the voltage at the airplane
connector shall have a minimum limit of 113 volts.
The series of runs given in Figure A-27 through A-30 (Cases C1 to
C4) consider the steady state requirement. The load changes considered are:
Case C1 - 100 amperes, 0.8-power-factor; Case C2 - 150 amperes, 0.8-power-
factor; Case C3 - 200 amperes, 0.8-power-factor; and Case C4 - 250 amperes,
0.8-power-factor.
These figures indicate that the total current load on a feeder cable
is a factor in determining the maximum load which can be switched and still
meet the steady state requirement. Refer to Table A-1. The results show that
with a 10,000-foot feeder cable length having five unit-loaded utilization
service assemblies with a total 500-ampere 0.8-power-factor load, no other
step loads should be applied. However, if three unit-loaded utilization
service assemblies are on the cable feeder with a 300-ampere 0.8-power-factor
load, than a 200-ampere 0.8-power-factor step load can be applied through a
fourth utilization service assembly on the same feeder.
4.3
Transient Effects. Transient runs were made with different initial
loads and different passive-element step loads to investigate the maximum load
changes on the system that will not have adverse results on the system or on
other loads with voltage regulation at the generator terminal. The system has
one utilization service assembly with a 100-ampere 0.8-power-factor load.
Step loads of 100, 200, and 300 amperes are applied to bus 7. Figures A-13,
A-16, and A-31 through A-34 show the voltage results for step loads.
Transient and steady-state requirements are met when the design follows the
guidelines.
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