IEEE Electrification Magazine - June 2015 - 20

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Gas turbine engines with a high-speed power turbine

mated with high-speed generators that produce more
than 60-Hz frequency power are easily accommodated. A combination of high-speed power turbines and
generators enables a shorter generator set. Shorter
generator sets enable shorter machinery space
lengths, which assist ship designs in meeting floodable length requirements for damaged stability.
Enabling the integration of higher speed gas turbines
into power systems also provides additional opportunities for competition and potential cost savings. Note
that the impact of skin effect on conductor size can be
mitigated with minimizing the distance between the
generator and rectifier.
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High-power, highly dynamic, demanding electric mission loads (such as EMRGs, lasers, high-power radars,
and electronic warfare systems) are more easily accommodated with MVdc. Because the speed of the prime
mover does not directly affect power quality at the MVdc
bus (as is the case with ac systems), the rotational inertia
of the generator and power turbine
(for multispool gas turbines) can be
employed as energy storage, minimizing total ship impact of additional energy storage.
Before MVdc can be employed on a
naval warship, a number of technical
issues require resolution.
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Bus regulation and prime mover
regulation: In classic ac power
systems, real power is regulated
through prime mover governor
control of speed (frequency regulation). Reactive power is regulated through the generators' voltage regulation. DC power systems
only regulate voltage. By supplying an MVdc bus with no frequency regulation requirement, the power control of
the prime mover, gas turbine or diesel, is provided
with an additional degree of freedom, which can be
used to increase dynamic response. Other prime
mover developments under consideration could provide additional improvements to the dynamic
response of the prime mover itself. A number of
approaches to regulating the MVdc bus are available
and must be decided upon for standardization. One
simple approach would be to employ a droop
response wherein the bus voltage would reflect its
per-unit load. An integrated dynamic would have the
prime mover source respond to longer time-scale,
average, load changes and the fast energy storage
response to rapid pulse load changes. Criteria for
assessing the different approaches for regulating the
MVdc bus are necessary before selecting one. A droop
response may be an appropriate reversionary mode

when control communications are not present to provide commanded regulation set points.
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System grounding: While grounding considerations in
an MVdc system are analogous to those in ac systems,
the location of the system ground point is different. In
an ac system, grounding (high impedance or otherwise)
associated with the neutral point of the generator, or
related point, is a logical choice. A corresponding
approach for an MVdc system would be to install a resistive midpoint between the two poles. The midpoint
could then be grounded (high impedance or otherwise).
Other alternatives with respect to the ground point
should also be considered. Distributed system capacitance is understood and specified for ac systems, however, the effect of distributed system capacitance on
MVdc power systems is less studied. Guidance for specifying maximum components and system capacitance
within an MVdc system is still an open research area.
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Fault detection, localization, and isolation: In ac systems, time and fault current magnitude are employed
by circuit breakers to detect, localize,
and isolate faults as part of an overall
circuit protection system. In an MVdc
system, all sources connect to the distribution bus through power electronics that can limit fault currents. Furthermore, because dc circuit breakers
cannot take advantage of the zero
crossing of an ac waveform to extinguish an arc, the ability of dc circuit
breakers to interrupt large fault currents is limited. Using the inherent
capability of power electronics to limit
fault currents and new methods for
fault detection, localization, and isolation is an obvious choice for MVdc. The
details of how to accomplish this
requires additional investigation. Reliable methods of fault detection, localization, and isolation must be developed.
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Magnetic signature: A dc current creates a constant
magnetic field that can leave a residual magnetic
field in ferrous materials. This residual magnetic field
contributes to the overall ship magnetic signature
and is susceptible to mines and magnetic influence
sensors. The creation of residual magnetic fields can
be minimized by physically locating conductors that
are close to one another carrying currents in opposite
directions so the magnetic field from one conductor
can cancel out the field from the other. Ideally, a
coaxial power cable would completely eliminate the
magnetic field. One concern will be in the design of
terminations and the routing of conductors within
the power system and load equipment. Creepage and
clearance requirements to prevent arc faults will
require separation of conductors and locally result in

An HED adds a
propulsion motor
to the gearbox of a
mechanical drive
propulsion system to
allow the electrical
distribution system
to power the ship at
low speeds.

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2015

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