Feature 6
exploitation in combatant ship classes. Two For a commercial ship, speed profile is often
major classes – the British Royal Navy’s Type a determinant, although DC distribution
45 destroyer and the US Navy’s DDG 1000 is not generally a consideration for most
guided missile destroyer – are emerging tonnage.
with all-electric integrated power system Pulsed loads, the ability to operate with
architectures that include AIM propulsion minimal prime movers, and survivability
motors. also bring energy storage systems into
Podded propulsion (with the propulsion the picture, and control systems that
motor installed in azimuthing pods external have traditionally focused on power
to the ship) has become the configuration of management must now consider both
choice for cruiseships and some specialised power and energy management.
civilian tonnage. With the exception of some When considering what has changed in
relatively low-powered naval applications, power distribution since 1997 and what has
however, it has not been adopted for not, the effect of fault protection equipment
combatant ships because of their high power can be looked at first. The effect on the
requirements and relatively fine hullforms amount of power that could be handled
(compared with typically more full-bodied by a system due to the steady state and
commercial vessels). current interruption limits of commercially
Propulsion motors to date have proven to be available circuit breakers at that time was a
too large for such podded naval applications, stark limit to the overall rating of the power
although one technology that might be Rolls-Royce WR21 gas turbine installation in system.
applicable – superconducting homopolar a Type 45 destroyer. This hard limit has not been changed
motors – has been under development for appreciably. Medium voltage AC circuit
some time. If R&D proves successful, motors breakers which have a steady state current
with sufficiently small radii might allow frequency switching to levels of 70kHz – the capability greater than 4000A would be a
podded propulsion to be adopted for frigates MOS Controlled Thyristor (MCT) – did welcome development.
and destroyers. not prove successful and is no longer under One aspect that has changed since 1997
development. in contemporary naval electric power
Power converters Wideband gap technology using either system designs where medium voltage
Although new materials were a major factor silicon carbide or gallium nitride materials, AC is generated is that virtually no loads
in creating the advanced motor types, the however, does show promise for high voltage/ directly use the MV AC power. Virtually all
development of new power converters was high frequency applications. is rectified to DC and further converted to
equally important. The Voltage Source suit the loads or transformed to low voltage
Inverter (VSI) or Pulse Width Modulated Electrical distribution AC.
(PWM) converter has proven essential in The core feature of an integrated power The observed behaviour of electric ship
the success of the concepts; and the Insulated system is that common gensets can be used to designers seems to be to design the vessel
Gate Bipolar Transistor (IGBT) has been serve all loads (propulsion and auxiliary) on to use standard MV 60Hz AC gensets,
the power semiconductor that enabled such the ship. In some configurations, particularly subject to the constraints of commercially-
drives. where the motor operates at low voltage, available switchgear and circuit breakers,
Over the past decade the voltage capability architectures have been proposed that exploit and distribute the MV AC to locations and
of these devices has tripled, fostering both a common bus. load aggregations where it makes sense to
new topologies as well as reducing the Ten years ago, DC distribution using a convert the MV AC to the type of power
dimensions of the power conversion units. zonal architecture with power conversion that the loads can use directly. This has
The IGBT devices coupled with advanced modules (PCM) to provide the type and led to power systems designs with a large
control systems such as Digital Signal quality of power needed in each zone was power converter population.
Processors (DSP) allow the design of motor proposed; and there was debate on AC versus
drives to be precisely controlled over a wide DC. Destroyers now under construction have Power generation
range of speeds, even when the profile is a combination AC/DC architecture, where In most discussions regarding power
highly non-linear. DC is limited to the ship service circuit only generation, the focus has been on the rating
A new device, the Integrated Gate and zonal distribution incorporates PCM. of the prime mover relative to the sum
Commutated Thyristor (IGCT), can be The AC versus DC debate continues but is of the loads (and some load dynamics).
operated at even higher voltages and is now generally focused on how much of each The emphasis has been on prime mover
now being incorporated in Neutral Point is required and what factors influence such developments that decrease weight per
Clamped PWM drives, primarily for a decision. For a warship, combat system MW, volume per MW, and specific fuel
commercial applications. A technology requirements – particularly if high-energy consumption.
that was considered promising for high pulsed loads are included – are a major driver. Diesel and gas turbine prime movers
Warship Technology March 2008 29
WT Mar - p28+29+30.indd 29 03/03/2008 15:57:39
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