IEEE Electrification Magazine - March 2014 - 62

nodes under the jurisdiction of distribution system operators and to simplify the top-level-communication infrastructure. this is so because the internal coordination of
groups of final consumers and small dGs consolidated
within the intelligent MGs could be done independently
from the distribution system, and the operator may only
consider power exchange with the MG at
the Pcc.
early MG efforts were largely
focused on ac so as to line up with
the existing power system infrastructure. to that end, significant efforts
focusing on improvement of current
sharing, power quality, stability, energy management, and smooth mode
transitions have been undertaken to
perfect and standardize the operation
of inverter-based ac MGs. the topic
has further expanded, even to the
level of a hierarchical control classification. hence, ac MGs began to be
perceived as full-scale distribution systems compatible
with the main utility. however, the initial motivation has
led to a somewhat misleading judgment that the standardization of MG topology and control should be carried
out literally following the guidelines imposed by the large
ac power systems. this is because of fundamental differences between today's technology and that at the turn of
20th century, the time when solid-state power converters
did not yet exist. indeed, the outcome of the famous Battle of the currents, which decided the global direction of
power system evolution in favor of ac architecture, could
have been different if they did exist. now, inspired by new
trends in electricity production and consumption as well
as remarkable technological improvements in power electronics, the same ac versus dc debate is taking place
again. however, this time, it appears that the odds are on
the side of dc, at least on the low-voltage level. indeed,
merging local dc devices into fully controllable and flexible dc MGs arises as an attractive possibility for this paradigm shift.

multiple buses with different voltages. in that sense, 380 V
is used as a rule for the high-voltage bus since it is known
to match the industry standard for consumer electronics
with the power factor correction circuit at the input. Moreover, this voltage offers the best efficiency gains in comparison to ac and is hence predicted to serve as the
principal distribution bus that supplies
high demanding loads such as heV
chargers, washing machines, rotating
esss, etc. a number of lower-voltage
buses (48, 24, or 12 V) that power lessdemanding loads such as electronic
devices and led lighting and ventilation are foreseen to be built upon it
using dedicated power electronic
interfaces. the structure that comprises a single high-voltage bus and a
number of low-voltage buses is
depicted in Figure 7.
nevertheless, when one considers
a number of mutually interconnected
dc subsystems, which is preferably done on high-voltage
buses (i.e., 380 V), each one of them needs to have flexible
control over its internal voltage. this property may be
achieved by sources whose converters are designed for
voltage support, i.e., voltage-source converters (Vscs). if
Vscs are programmed in a nonstiff manner, a number of
sources may control the bus simultaneously. this voltageregulating strategy is commonly referred to as the voltage
droop (Vd) and is used for primary control of dc buses
MGs. the corresponding control law may be expressed as

Tie-line current
between two MGs
can be regulated
by imposing the
appropriate voltage
drop between
their buses.

DC Microgrids: Full-Scale Electricity
Power Distribution Systems Adjusted
to Modern Trends
advanced low-voltage dc distribution systems for household and commercial appliances have been under consideration for quite some time in both academia and
industry. recent comparative analyses between performances of traditional ac systems and their dc counterparts have raised a question about the most appropriate
dc voltage level, which is to be adopted as a future standard. however, as different types of modern consumer
electronic appliances are operated on distinctive voltage
levels, it is generally agreed that future dc systems should
not be formed around one standardized bus, but of

I E E E E l e c t r i f i c ati o n M agaz ine / MARCH 2014

v dg = v ref,MG - R d i o,

(1)

where v dg and v ref,MG are the common bus and reference
voltage, respectively, i o is the output current, whereas R d is
the virtual resistance, which defines the steady-state
slope. as soon as it is ensured that at least one converter
operates in voltage-regulating mode, other kinds of units
may be connected to the same bus. in that sense, it is
desirable that the ress exploit as much renewable energy
as possible and, hence, their converters are run by the
respective MPPt algorithms in normal operating mode.
this means that for given environmental conditions (sun
and wind), the res will follow the reference imposed by
the algorithm, which is typically executed several orders of
magnitude slower than inner loop controllers. therefore,
the res can be considered a constant power source in a
dynamic sense. this point is depicted in Figure 8, where
one may see the operational principle of three converters
connected to one common bus, with two converters being
operated in Vd mode (with different slopes) and one converter in MPPt mode.
however, if the voltage of the bus is regulated exclusively by the law stated in (1), its deviation from the reference value is unavoidable. this feature does not represent

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2014