IEEE Electrification Magazine - June 2015 - 19

Naval power systems
are designed to
enable continued
operation with a
single line to
ground fault.

considerations and for survivability
considerations. An important QOS
consideration is the ability to preserve power to loads when a generator set trips offline while damage
to the distribution system and the
ability to preserve power to vital
mission systems loads is of major
interest in the survivability analysis. For QOS, many ships operate
with their electric plant paralleled
in peacetime steaming and only
shift to the more survivable split-plant configurations
(where the ship operates two independent islands that
provide independent alternate sources of power to vital
loads) under threat conditions. QOS is implemented by
classifying loads as uninterruptible, short-term interruptible, and long-term interruptible. By properly
selecting the number and ratings of generator sets and
energy storage modules and implementing the QOS
concepts in the control system, the power system can
be designed with a high QOS.
xx
IPSs facilitate fuel-cell integration. Because of their
inherent efficiency, fuel cells promise to play an
important role in future naval power system design.
Since fuel cells produce electrical power, their integration into the IPS is natural since fuel cells are simply
generator sets with special characteristics. Before fuel
cells can be used, however, a number of technical and
ship integration issues require resolution. These
issues include the following:
■ Fuel cells cannot directly use the fuels currently
available aboard ship. A fuel reformer is needed to
convert diesel fuel marine to a fuel compatible
with the fuel cell. A fuel reformer suitable for naval
warship installation does not currently exist,
although prototypes have been developed.
■ Ship integration requirements such as the quantity of gas produced and the allowable back pressure do not enable proper sizing of the intake
and exhaust systems. Depending on the flow
requirements, this intake air could be provided
by the ship's normal ventilation system or may
require dedicated intakes.
■ Fuel cells typically behave slowly dynamically. Must
fuel cells be integrated with energy storage to provide better transient performance? If so, how does
one size and cost the requisite energy storage?
While current naval ship designs have not taken full
advantage of all the opportunities of an IPS, the continued
evolution of naval warships will likely see more of these
benefits realized in future designs.

Integrating Mission Systems
Integrating multiple advanced mission systems on a future
destroyer under 10,000 m will require employing the following

tactics to reduce the needed weight and
volume of the power system:
xxx
use controls (software implementation) to obtain required power
system design performance from
less hardware capacity than would
be needed otherwise
xx
x
energy storage resources that can be
shared by the multiple advanced mission systems (and the ships' power
systems) instead of each bringing
their own
xx
a power system, such as medium-voltage dc (MVdc),
capable of providing greater energy dynamics than
the classic ac power systems
xx
power system components that can fulfill more than
one power system function simultaneously, e.g.,
power converters, which also limit and isolate faults
obviating the need for distinct circuit breakers.
High-power mission systems, particularly those characterized as pulse loads or highly dynamic stochastic
loads, may lead to the use of MVdc generation and distribution aboard future naval ships. The primary reasons for
employing an MVdc system are as follows:
xx
The speed of the prime mover is largely decoupled
from the power quality of the bus. The generator can
be optimized for each type of prime mover without
having to incorporate reduction gears or speed
increasing gears so generators are not restricted to a
given number of poles. The speed can even vary
across the power operating range of the prime mover
to optimize efficiency and/or responsiveness.
xx
Power conversion equipment can operate at high frequencies, resulting in relatively smaller transformers
and other electromagnetic devices.
xx
Without the skin effect experienced in ac power transmission, the full cross section of a dc conductor is
effective in the transmission of power. Additionally,
the power factor does not apply to dc systems.
Depending on the voltage selection, cable weights
may decrease for a given power level.
xx
Power electronics can control fault currents to levels
considerably lower than ac systems employing conventional circuit breakers. Lower fault currents also
reduce damage during faults.
xx
Since there is no common frequency of vibrating
equipment, the acoustic signature has a broader signature with fewer tonals that can be observed in the
acoustic signature of the ship when compared to a
ship operating at a constant ac frequency.
xx
Paralleling power sources only require voltage matching and do not require time-critical phase matching.
This enables generator sets to come online faster after
starting, thereby reducing the aggregate amount of
energy storage needed to enable operating with a single generator set online.

IEEE Electrific ation Magazine / j une 2 0 1 5

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2015