IEEE Electrification Magazine - June 2016 - 43

information regarding the state of
charge and the power balance in
Vdc
Vnom + ∆V3
the microgrid.
Vnom + ∆V2
One of the important advantages of the concept is that the energy
Vnom + ∆V
manager does not need to have
any knowledge about specific
Vnom + Vo
sources or loads connected to the
SL4 SL3 SL2 SL1
V
nom
bus. It simply changes the average
SH2 SH4 SOC
SH1
50%
SH3
dc bus voltage, understanding that
other elements on the bus will
Vnom - ∆V
react to the voltage deviations, creating the desired outcome. If a drift
Vnom - ∆V2
in the dc voltage commanded by
Vnom - ∆V3
the energy manager does not
result in a sufficient response from Figure 7. An example of dc bus signaling for microgrid battery management.
the elements on the bus, the energy manager increases the dc voltage deviation to trigger additional responses from the
another can be programed in steps so it shuts down secsame or other elements connected to the bus. As a result,
tions of the generator as the dc voltage increases. Power
loads and generators can be added to or removed from
converters used for generators and loads only need to be
the microgrid (within the installation rating) without
provided with the capability to adjust their production/
affecting the global operation of the microgrid and withconsumption based on a control signal. The control signal
out requiring any retuning or reprogramming. Since the
can be embedded in the power converter or provided by
sustaining controls are slow in nature, a fast response to
an add-on controller. This would make any power convertpower-demand changes from the different elements
er capable of following a power reference compatible with
(with the exception of the energy-storage devices) is not
ARDA's microgrid-management concept. Figure 8 shows a
important to the microgrid stability. In fact, the elements
representation of the microgrid-management concept
should react slowly to remove any effect of transient voltwith individual functions executed on each element conage changes in the dc bus and to give the energy managnected to the dc bus.
er time to evaluate whether the response has been
By using the energy storage as the backbone of the
sufficient. Figure 7 shows a possible algorithm for an
microgrid, the ac grid is considered simply as a bidirectional
energy-storage device, changing the dc voltage set point
source/load interfaced through a power converter. This
as a function of its state of charge.
means that the operation mode and control method are
In microgrids with different types of sources and loads,
the same in a grid-connected or grid-independent operaeach component would have different preferred operating
tion. This would decouple the microgrid from the ac grid
patterns or performance requirements. Different compoand eliminate the risk of a large disturbance on the grid
nents in the microgrid do not have to respond in the same
affecting the microgrid or resulting in power loss. Furtherway to variations in dc voltage. The smooth dc voltage
more, the power converter connecting the dc bus to the ac
communication principle enables programming different
grid can be communicating with the utility and receiving
power response patterns as a function of dc bus voltage
requests for ancillary functions. These ancillary functions
for the different components connected to the microgrid,
would represent the optimizing function for the grid power
adding a third level of control for the microgrid or a set of
converters and would be executed in the same fashion as
optimizing functions. Optimizing functions may include
for any other component on the bus. ARDA's concept can
algorithms to minimize the operating cost for the
also be implemented as a hybrid microgrid by incorporating
microgrid, extend the lifetime of the components, provide
traditional ac loads and sources using inverters and rectifienergy-consumption reduction, etc. The function limiting
ers provided with algorithms to respond to average dc voltor setting the power for a source does not necessarily have
age changes and with optimization functions. This way,
to follow a linear relationship with the dc bus voltage.
they would appear to the energy management as any other
Instead, it may include nonlinear terms (quadratic, expodc source or dc load.
nential, steps, etc.) or discontinuities, depending on the
Conclusions
specific properties of each distributed resource or load,
Increases in the use of renewable distributed generation
and it may also be seasonal, depending on market or enviand improvements in energy-storage technologies make
ronmental factors. For example, a specific generating
microgrids the best concept for addressing reliability, susresource could be preprogrammed to ramp down the
tainability, resilience, and energy-efficiency concerns.
power production as the dc bus voltage increases, while
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Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2016