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

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3.6 Input Power
Specifications
Noninterruptible
Power

Noninterruptible power is not a
requirement set forth by any military standard
but is an equipment specification levied by the
procuring agency. The two most commonly
specified interrupt times, through which the
equipment is required to continue operating,
are a short-term interrupt (10 to 300
microseconds) and a long-term interrupt (20 to
150 milliseconds).

Operation for a short-term interrupt of prime
power is intended to provide adequate time,
after recognition of a power interrupt, to
perform a "put-away" routine to store data,
such that an automatic recovery occurs upon
restoration of power. Noninterruptible power is
somewhat of a misnomer since it is really not
intended to provide performance through a
short-term power transient as the name might
imply.

The short-term interrupt capability is
usually inherent in the input capacitor storage
capability of the power supply along with proper
selection of the input power detection level.
Accordingly, the impact on the power supply is
minor. Operation through a long-term interrupt
is intended to accommodate transfer from one
power bus to another as implemented by a
break-before-make transfer of power buses on an
aircraft or a shipboard automatic bus
transfer.

One method of operating through a bus
transfer is to add capacitance within each
power supply. For example, adding a capacitance
of 6500 microfarads after the rectifiers of a
3-phase, 60 Hz, 115 VAC, 500 W shipboard power
supply would allow 55 milliseconds of hold-up
time at 20% low line. This capacitor would have
a volume of 67 cubic inches and a weight of 3.3
pounds. The capacitor would occupy 40% of the
volume of a 3 watt/cubic inch power supply and
require the rest of the circuitry to operate at
a 5 watts/cubic inch output power density. This
is not unreasonable, but as output power density
increases, the impact on the power supply
becomes substantial.

One alternative to adding capacitors to each
power supply for storing energy is to provide
for auctioneering or diode OR'ing more than one
DC bus together and providing an appropriate
amount of centralized or distributed capacitance
for energy storage to control the common bus
voltage during the automatic
transfer.