IEEE Electrification Magazine - June 2016 - 47

as studies have progressed, researchers and engineers
have noticed inadequacies in the ac power architecture
that can be summarized as follows:
xx
generator sets have to work in fixed speed and thus
limit further improvement in fuel efficiency
xx
the ac power architecture introduces unwanted reactive power flow and power quality problems (e.g.,
three-phase imbalances and harmonic currents)
xx
the bulky conventional transformers occupy too much
valuable space and weight onboard
xx
there is a potential risk of systemic disintegration
when supporting emerging pulsed electrical loads.
These problems also plague terrestrial power distribution systems, resulting in the current trend toward returning to dc-based power distribution systems. Edison's dc
power system has once again led the second Industrial
Revolution and brought a new era of light as well as
electrification to humankind. It was overshadowed for
more than a century after losing
the famous "battle of the currents"
due to its inherent inability (at that
time) to change voltage levels without the addition of multiple motor-
generator sets, thus making the
system uneconomical to operate
compared with the ac power system
(which had at its disposal the simple
transformer for changing voltage levels). But thanks to the rapid development of modern power electronic
technologies, the high-frequency dc-
dc converter has already qualified for
taking on the role of transformer in
dc systems. It therefore may allow
Edison's invention to change the
world once again. Just as Edison once
strove to prove, it is becoming clear that the dc power system has several major advantages over the ac system, and
even some newly recognized advantages, such as
xx
replacing bulky ferromagnetic transformers with
compact power electronic converters
xx
easier parallel connection or disconnection for dc
power sources
xx
elimination of harmonic and imbalance problems
xx
elimination of synchronization problems
xx
elimination of reactive power flow.
Additionally, considering the specific needs of shipboard power systems, the dc-based IPS could bring a
broad range of advantages for both commercial and mission-oriented ships. Generally, the dc power architecture
will eliminate bulky low-frequency transformers and
reduce the rating of switchgear, thus reducing the occupied space and overall weight of the whole system,
which may result in extra cargo space. The commercial
sector focuses on the 15% fuel saving due to allowing
variable-speed diesel generators, whereas the military

sector is interested in support for advanced electrical
equipment and weapons, which are characterized by
high-power pulsed loads. For these vessels (mainly warships), meeting these objectives requires a highly
secured power supply. Moreover, a dc power architecture
could provide better survivability, limitation of fault current, and reconfiguration capability. Besides that, the
integration of advanced high-speed, high-efficiency diesel generation (i.e., gas turbine generation) could also be
easily achieved within the dc power architecture, which
could effectively improve the fuel efficiency of the system. Due to the higher power levels required in AES
applications, the only available design option for a dcbased IPS is the medium-voltage dc (MVdc) solution with
a dc bus voltage above 1 kV.
The typical power architecture of terrestrial dc
microgrids is shown in Figure 2(a), where the RESs, energy-storage systems (ESSs), and local electrical loads are
packaged together with the dc bus to
enable islanding operation, which
makes the system fully resistant to
major blackouts in the main grid. The
elimination of reactive power and
synchronization problems makes the
whole system much simpler to
design, control, and coordinate. Moreover, with a well-selected nominal bus
voltage, the overall efficiency will be
generally higher than its ac counterpart. The three-wire, bipolar-type dc
microgrid power architecture is
shown in Figure 2(b). The architecture
evolves from Edison's three-wire dc
power distribution system, which was
initially designed to save conductors.
Compared with the typical architecture, the positive bus and negative bus can work independently if a fault occurs, which result in inherent
redundancy and higher reliability. Moreover, it allows
using a neutral bus with a low rated current if the loads on
the positive bus and negative bus are roughly equal.
Figure 3 shows a ring-bus-based dc microgrid power
architecture proposed for a critical load with higher security requirements (e.g., a data center). The ring bus allows
energy flows along either the shortest path or a suboptimal path. That is, wherever a single fault occurs in the system, it can be isolated by switching off the nearest circuit
breakers, allowing other parts to work as normal. This feature guarantees system survival from single-point failures.
In addition, the ring bus allows the critical load to obtain
energy from multiple nodes by applying either a conventional multiple-contact point switch or multiterminal converters. Accordingly, the critical load is highly secured to
achieve uninterrupted operation.
A similar architecture can apply to the maritime
power system, but the inner part of the system will be

Just as Edison once
strove to prove, it is
becoming clear that
the dc power system
has several major
advantages over the ac
system, and even some
newly recognized
advantages.

IEEE Electrific ation Magazine / j une 2 0 1 6

47



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