IEEE Electrification Magazine - June 2015 - 18

Figure 9. Contrarotation through a traditional propeller, shaft, and pod.
(Photo courtesy of the U.S. Navy.)

reversible and can operate over the entire range from zero
to maximum speed, IPSs can employ FPPs. Additionally,
an IPS is well suited for incorporating pod propulsion and
contra-rotating (CR) propellers. Pod propulsion is well
established in the cruise line industry but has not yet
been introduced in surface combatants for the U.S. Navy.
CR propellers are desirable because of improved efficiency
that results from the second propeller recovering wake
energy from the first propeller's wake that would otherwise be lost. Since IPS configurations typically employ
independent motors on the same shaft to improve reliability, dedicating each motor to its own propeller to yield
improved efficiency does not add significant complexity
or cost. Alternately, a traditional propulsion shaft and propeller can be
paired with a pod to provide contrarotation without using inner and
outer shafts (Figure 9).
xx
IPSs provide general arrangements flexibility. Mechanical drive
ships locate the prime movers low
in the ship to align with the propeller shafts. An IPS-configured
ship offers the designer the flexibility to put the power generators
in almost any location (after taking stability constraints into consideration). The shaft line can be
simplified with direct drive
motors. Future ship designers
could also improve the longitudinal separation of propulsors to
improve survivability without

incorporating long shaft lines. The volume required for
combustion air and exhaust will likely decline because
of the reduced number of prime movers and can be
reduced further if generator sets are located higher in
the ship.
xx
IPSs improve ship producability. For example, the
elimination of long shaft lines enables the ship builder
to change the build sequence to simplify the erection
schedule and thereby reduce the ship's construction
schedule. By locating generator sets higher in the ship,
the in-yard date when these items are needed can be
delayed, reducing the likelihood that the equipment
will be damaged during the ship's construction. Zonal
distribution systems shorten cable lengths and minimize the number of spaces a cable must penetrate.
xx
IPSs support zonal survivability. For a distributed system, such as the electrical power system, zonal survivability is the ability of the distributed system, when
experiencing internal faults due to damage or equipment failure confined to adjacent zones, to ensure
loads in undamaged zones do not experience a service interruption. Zonal survivability assures damage
does not propagate outside the adjacent zones where
damage is experienced. At the ship level, zonal survivability facilitates the ship to maintain or restore the
ship's primary missions when experiencing battle
damage. Zonal survivability enables the crew to focus
restoration efforts on the damaged zones, maintain situational awareness, and take appropriate restorative
actions. Zonal survivability is an inherent feature of
the zonal distribution system incorporated into IPS.
xx
IPSs improve electric power quality of service (QOS),
which is a metric being introduced into shipboard
power system design to measure the reliability of electrical power provided by the power system to loads.
QOS is calculated as a mean time between service
interruption (MTBSI), where an interruption is defined from the perspective
of the load. A service interruption is
measured in terms of the maximum
duration that the power quality can be
outside normal limits and the load can
still operate properly. An interruption
in service shorter than this time duration is not used in the calculation of
the MTBSI. The time used in the MTBSI
calculation is usually specified by an
operating cycle, design reference mission, concept of operations, or an operational architecture. QOS does not take
into account survivability events such
as battle damage, collisions, fires, or
flooding. QOS does take into account
equipment failures and normal system
transients. The optimal configuration
of the electric plant may differ for QOS

The load that
shipboard power
systems must
provide has
experienced
considerable growth
with the introduction
of high-power
combat systems and
the electrification of
auxiliary equipment.

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