IEEE Electrification Magazine - June 2016 - 39

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The expected proliferation of electric vehicles during

the next few years may represent one of the most
demanding challenges in electric power distribution.
Fast battery chargers can be simplified and made
more efficient if powered from a high-voltage dc bus.
xx
A dc microgrid is much less susceptible to failure due
to main grid disturbances because the ac-dc converter interface between the ac grid and the dc microgrid
provides an energy-storage buffer and can replace
the functionality of fast active switches used in the
ac microgrids.
xx
Power quality problems, such as sags, swells, imbalances, and flickering, among others, associated with
the high penetration of renewable resources that
show varying power output, are an important issue in
ac microgrids but are easily mitigated in dc systems
with robust control of the dc bus voltage.
xx
Stability depends on maintaining the dc bus voltage
within the normal operating range for all the devices
connected to the bus under every transient and
steady state condition. Although stability is more challenging as larger and more complex dc microgrids are
implemented and different dc microgrid clusters are
connected together, the solutions are simpler than for
ac microgrid systems.
xx
Advances in dc-dc converter technology have resulted
in highly efficient and reliable converters providing
the "dc transformer" effect that counterbalances the
decisive factor that favored ac systems in the 1900s. In
fact, high-voltage dc transmission lines are now being
used to link separated ac grids with different ratings
where the ac power is rectified into dc power that is
then converted to a higher dc voltage level for transmission. This provides space savings and removes the
need for synchronization.
Consequently, strong arguments indicate that implementing microgrids by using a common dc voltage is simpler, more reliable, and more efficient than using ac to
implement microgrids.

Obstacles to dc Microgrid
Demonstration Projects
Although, conceptually, dc microgrids are much simpler
than ac microgrids, they are still seen by many as a futuristic concept that needs technical advances in many
fields. With the theoretical advantages of dc microgrids
validated by calculations, analysis, and simulations, the
next step is the implementation of dc microgrid demonstration projects to validate the different concepts in real
conditions. Several demonstration projects have been initiated around the world during the last few years. Most of
them are relatively simple projects focused on a specific
application such as telecom or data servers, or photovoltaic (PV) solar plus battery storage installations. Direct-current distribution systems are also being used successfully
on ships and airplanes operating independent of the ac

grid. Larger, more complete demonstration projects
including variety of loads are now being subsidized by
governmental organizations mainly in Europe and North
America. Some of the challenges that project developers
interested in building dc microgrid demonstrations are
facing are
xx
difficulties in finding electric loads that will operate
on dc due to the investment required to remove acrectifying elements that are an integral part of the ac
products and due to the wide range of internal dc bus
voltages in modern off-the-shelf loads
xx
resistance from local authorities to approve dc installations due to the limited experience with this type of
project and the lack of references in safety codes
xx
the need for custom power electronics equipment
incorporating controls and communication features
specific to dc microgrids that demands extensive R&D
effort (this increases the cost and extends the project
execution time)
xx
unknowns regarding the protections needed in dc systems and the lack of protection devices designed specifically for dc systems
xx
a wide variance of ratings and specifications and the
repetition of efforts instead of collaboration that
would accelerate the goal of more practical implementations of dc microgrids (this is because dc
microgrids have been conceived and designed by
small groups in academia and industry with very little
cooperation among them).
Efforts are now being made to overcome these obstacles and limitations. DC microgrid conferences and workshops are being organized to bring together different
experts, to increase awareness, and to promote dc
microgrids with the governments, the press, and the general public.
Engineers working on dc microgrid concepts start with
a blank page, building a puzzle based on their own experiences and preferences without a set of basic common
rules. As a result, components developers interested in dc
systems are unable to find a common ground on the specifications they receive from project integrators, leading to
high cost and lack of interest in providing custom hardware. The EMerge Alliance is an industry association helping in accelerating the adoption of dc power distribution
in commercial buildings. Among other functions, it brings
together companies interested in dc microgrids with the
purpose of finding a common ground that will result in
standardized requirements and systems and ensure that
all the players are aware of what other companies are producing (Figure 4). In addition, the EMerge Alliance reaches
out to manufacturers of conventional electric loads to
show them the benefits of using dc power and to stimulate them to develop dc-rated loads.
At the same time, compliance organizations are working on safety studies and standards that will reduce hurdles and enable the implementation of commercial-scale
IEEE Electrific ation Magazine / j une 2 0 1 6

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