MIL-HDBK-1004/5
3.4.1
Low-Voltage System Equipment. In addition to a utilization service
assembly as used on a medium voltage system, each low-voltage system includes
the individual aircraft's supply source or fixed service point unit.
3.4.1.1
Utilization Service Assemblies. To assure satisfactory operation,
provide utilization service assemblies with components as shown on Figure 7.
a) Step-down transformers. Normally, provide step-down
transformers rated 75 kVA and with a three-phase 208Y/120-volt output. The
terminal rating of 208Y/120 volts is consistent with the usually higher
voltage rating of distribution equipment over the typical 200Y/115 volts of
utilization equipment. The higher distribution voltage level allows for
voltage drop between the distribution and utilization points. The load served
is generally no more than two, 100-ampere, 0.8-power-factor unit loads (34.5
kVA each). In special cases, larger load requirements (such as for TACAMO
loads) may have to be served. In such cases, criteria shall be provided by
the using agency to the designer. For TACAMO loads, utilize 400 kVA
transformers with the same maximum percentage impedance values shown in Table
4 for 75-kVA units.
b) Line drop compensators. Set the medium-voltage feeder length to
give a per-unit voltage droop (absolute change in voltage between steady
state, no-load and steady state, full-load) to 0.942 at the end of the feeder
cable or 113-volts line-to-neutral on a 120-volt utilization assembly terminal
voltage base. Therefore, compensate the drop from the utilization service
assembly to the aircraft connector so that no less than 113 volts are provided
to the aircraft at the interface point. Provide line drop compensators as
indicated in Table 4 for 75-kVA transformers and 460 to 480 kVA for 400-kVA
transformers.
c) Passive-element filter assembly. Install Passive-element filter
assemblies to reduce harmonics which can be generated in the system. Standard
performance requirements are based on systems which do not provide additional
harmonics from the presence of rectified direct-current loads. Provide
passive-element filters on equipment terminals of an aircraft whose load
produces harmonics.
3.4.1.2
Fixed Service Point Units. Generally, fixed service point units
provide disconnecting devices for two aircraft; that is, they provide two
circuit breakers. Provide power quality in accordance with the requirements
of MIL-STD-704.
3.4.2
Low-Voltage Cable Limitations. The location of the parked aircraft
and the 60-foot aircraft cable limits the location of fixed service point
units. Each fixed service point unit requires a utilization service assembly
or solid state frequency converter for its supply, with the location limited
by the maximum 250-foot service cable length.
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