IEEE Electrification Magazine - June 2016 - 49

G
PV Panel

Wind Turbine

Utility Grid

Utility Grid

G
Diesel
Generation

Energy Storage

Figure 3. A ring-bus-based dc microgrid.

system (ZEDS). Figure 4(b) shows the layout of the equipment in an electrical zone. Note that a large number of
the loads in the zone are fed from both sides of the ship
to enhance survivability. As opposed to a terrestrial power
system, a maritime power system is inevitably restricted
by the cabin structure of the vessel or offshore platform,
so the size and weight of the overall system are important. To minimize the dc cabling size, voltage levels of
greater than 6 kV are proposed for future combatants. For
architectures as in Figure 4(a), the switches around the
ring bus are there to isolate faults that may occur on the
buses that distribute power to the zones. There are two
approaches: breaker-based and unit-based. With breakerbased architectures, the switches must be actively controlled solid-state circuit breakers (SSCBs) combined with
fast-acting no-load isolating mechanical switches. Such
systems have the potential to provide a high quality of
power during fault events (i.e., minimal power interruption), but the SSCB at these levels are still items in
development that carries with it considerable risk. Intercommunication between adjacent SSCBs is necessary to
isolate the fault because the dc ZEDS must be able to provide the same current from any direction.
With unit-based architectures, the power converters
that interface with the electrical sources to the port and
starboard buses play the primary role of driving the current to a fault on the bus to zero. The switches are all
no-load switches. To be unit-based, the architecture in
FigureĀ 4(a) cannot have cross-tie switches between buses
(i.e., where the battery-interfacing converters are),
because when a fault occurs on a bus system, operation
requires that critical loads within the zones autonomously shift their power sources to the healthy opposite bus.
This is accomplished by diode auctioneering of power
sources fed from both sides of the ship into the loads.
Intercommunication between the switches and converters is necessary to determine where to isolate the fault.
Once a switch isolates a fault, the power converters on
the effective bus are reenergized, and all but the faulted

part of the system is restored to operation. Communication is considerably more complex with the unit-based
system, but the risks of implementation and system cost
will be much lower when compared to the breaker-based
model. The system in Figure 4(c) is an alternative architecture that utilizes SSCBs of different current rating levels on two buses and may be able to isolate faults using
SSCBs but with minimal intercommunication. If generators are distributed between buses, this architecture provides an opportunity for operation with a high power
quality bus on the inside, dedicated to feeding the lowvoltage systems in the zones under normal conditions,
and a lower-quality bus on the outside that is dedicated
to high power loads and pulsed loads. These two buses
can operate independently of each other if the SSCBs
have reverse current-blocking capability. The architecture
offers an opportunity for efficiency improvement in the
ship by allowing the output bus to operate at a lower voltage than the inner bus when it is not necessary to operate at full propulsion speed.
These different power architectures are all feasible
choices for the design of onboard dc power systems. However, there are always tradeoffs between reliability and
complexity. Complicated power architectures require
much more sophisticated control and coordination strategies, which need to be carefully evaluated during earlystage design. Generally, the crucial guidelines for power
architecture design and selection should be the reliability
and redundancy requirements and the shipboard mission requirements.

Onboard Distributed eSSs
Enabling Smart Grid Technologies
Due to the soaring price of fossil fuels and the practical
need to integrate intermittent renewables into the future
energy system, energy storage technology has been one
of the hottest research directions in the last decade. With
the presence of highly intermittent energy sources and
IEEE Electrific ation Magazine / j une 2 0 1 6

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