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NAVSO
P-3641: Navy Power Supply Reliability Design & Mfg
Guidelines

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3.6 Input Power
Specifications
Platform Power
Characteristics

Shipboard Electrical Power

The ship service power distribution system,
supplied by the ship's alternators, is 440
VAC, 60 Hz, 3-phase, 3-wire, delta connected,
ungrounded power whose characteristics are
delineated by MIL-STD-1399 Section 300 as Type
I Power. This is called primary shipboard power.
A deviation is required if the user of
electrical power requires more than 5 kW of
power with any characteristics different from
primary shipboard power (unless the power is in
certain categories and used for service of
aircraft, aviation support equipment, or
aviation units under test). For less than 5 kW
of power, the user can use 115 VAC single- or
3-phase power.

The deviation procedure requires a
cost-benefit tradeoff analysis which addresses
the impact on the ship platform versus equipment
impact in terms of weight, space, power
consumption, reliability, life cycle costs and
total ship effectiveness. Besides the primary
shipboard power, there are thirteen other
combinations of power specified by MIL-STD-1399
Section 300. Use of any of these alternatives
has some impact on the electrical power and
distribution system. The tradeoff
considerations involved in using any power other
than the primary shipboard power are
discussed.

Shipboard Three-Phase Versus Single-Phase
Power

The primary shipboard power is ungrounded but
has a virtual alternating current (AC) ground
established at each equipment by means of
capacitors to chassis in the equipment EMI
filters. For any unbalance in virtual grounds
between equipment, alternating current will flow
in the hull of the ship. These hull currents
are considered undesirable for a variety of
reasons. Use of single- phase power creates
the greatest possible unbalance.

Because power supplies can be designed to be
smaller and lighter using 3-phase prime power,
this is preferred from both the ship power
system and user viewpoints.

Occasionally, there is an advantage to using
existing commercial or military single-phase
equipment. In these cases, if the anticipated
location and magnitude of induced hull current
is considered satisfactory and the power
system balance requirements are met, there is
virtually no penalty to the power system in
the tradeoff analysis. Hence, MIL-STD-1399
Section 300 lists the single-phase alternative
immediately after each 3-phase alternative in
the order of preference in selecting system
voltages.

In the absence of any clear advantage to
doing otherwise, 3-phase power should be
specified for shipboard equipment and
systems.

Shipboard 60 Hz Versus 400 Hz Power

The use of 400 Hz power on a ship requires
that a frequency changer, in the form of a
motor-generator set or a solid-state frequency
changer, be added to the ship power systems to
convert 60 Hz to 400 Hz power. Before line
rectified (no isolation transformer)
switching-mode power technology, there was a
significant advantage to the user in using 400
Hz power due to reduced magnetic component size.
With line-frequency transformers eliminated
from the power supply, the only advantage is in
slightly smaller EMI filters. In cost-benefit
tradeoff analyses, frequency changers add
weight, space, power consumption, life cycle
costs and reduce reliability which totally
offsets any benefits in using a 400 Hz shipboard
power architecture. These tradeoffs have been
summarized in Reference 24

.

MIL-STD-1399 Section 300 and Navy policy
recognize the need for 400 Hz power for
aircraft, aviation support equipment and
aviation units under test aboard ships. Except
for aircraft/avionics-related uses, approval of
deviations requesting the use of 400 Hz power
on ships is expected to be rare.

In the absence of any clear-cut advantage for
doing otherwise, 60 Hz power should be
specified for shipboard equipment and systems
not related to aircraft.

Shipboard 440 VAC Versus 115 VAC
Power

The preferred voltage from the ship power
system viewpoint is 440 VAC since it is the
output of the ship's alternators. Unfortunately,
this voltage level is beyond the
state-of-the-art of active components used in
the technological approaches needed to
increase the output power density of power
supplies and is expected to remain so in the
foreseeable future. Therefore, the voltage
must be stepped-down passively by the use of
autotransformers or isolation transformers.
Once this is accepted, the real consideration
becomes the power level at which stepdown is
executed.

The volume and weight of transformers vary
approximately as (VA) 0.75, where VA is the
volt-ampere rating. For this reason it is
inefficient from a weight and volume (and
probably cost) viewpoint to step-down the
voltage in a typical power supply (500 W) and
conversion should be considered at the
equipment, cabinet, group of cabinets, or
electronic equipment bay level. By providing
DC (155 VDC or 270 VDC) to the individual power
supplies, the required line-frequency
transformers that perform the stepdown can be
connected in multiphase configuration, thus
reducing harmonic currents.

In the absence of any clear cut advantage for
doing otherwise, convert power from 440 VAC in
the largest blocks of power practical and
consider using the multiphase stepdown
transformers as part of the approach to control
harmonic currents.

Shipboard 115V/200 VAC Power

This power can only be used for services to
aircraft, aviation support equipment and
aviation units under test. The 3-phase, 4-wire,
wye-connected 400 Hz power is intended to be
compatible with similar power controlled by
MIL-STD-704, the controlling specification for
aircraft power.

Shipboard DC Power

Continuity of power and reduction of harmonic
currents caused by rectification of AC
voltages are two primary concerns of shipboard
electrical power distribution that are best
met by providing DC power to using systems.
The power supply designer would welcome the
change to DC input power because the
technology being used to provide high output
power densities is based upon DC- to-DC
converter technology. Candidate voltages that
are possible DC voltages from both the power
system and user's viewpoints are 270 VDC and 155
VDC.

The ship power system designer tends to favor
the higher voltage, 270 VDC, because power
distribution system losses are
minimized.

The power supply designer tends to favor the
lower voltage, 155 VDC, since there are more
options in selecting circuit topologies and
components.

DC power eliminates the 3% harmonic current
constraints and its impact on output power
density. As a function of how the DC is
developed, the harmonic current problem may
have just been moved to another location and may
have to be addressed wherever the conversion
takes place. There is growing interest in
providing a standard DC voltage on
ships.

Aircraft Electrical Power

MIL-STD-704 establishes requirements for
electrical power characteristics on aircraft
at the interface between the electrical power
system and the utilizing equipment.

Aircraft AC Power

The standard AC power is a 400 Hz, 115/200
VAC, 4-wire, 3-phase, wye-connected system
with the neutral of the wye grounded. Loads
greater than 0.5 kVA must use 3-phase power.
Single-phase loads must be connected
line-to-neutral. The only alternative, when
specifically authorized, is 230/400 VAC with
the amplitude requirements proportional to the
115/200 VAC limits.

Aircraft DC Power

The standard DC power is 28 VDC. An alternate
is 270 VDC. This is the nominal DC voltage
obtained when the standard 200 VAC, 3-phase,
line-to-line aircraft voltage is bridge
rectified. Studies have shown major overall
advantages for aircraft using 270 VDC as the
primary aircraft power. For this reason, a
large share of the Navy exploratory and advanced
development funds for aircraft electrical
power system development in recent years has
been expended in developing the components
necessary for a 270 VDC system.

Fortunately, power supplies can be developed
for 3-phase, 115/200 VAC input power that can
be directly used on 270 VDC with no
modification. If initially considered, these
dual-input power supplies have minimal impact on
the size, weight, cost and other attributes
when compared to power supplies that are only
designed for 3-phase, 115/200 VAC
input.