IEEE Electrification Magazine - March 2014 - 61

Loads

Communication
Flow
Power Flow

Communication Bus

Higher Control Level:

dc−dc

dc−dc

dc−ac

Power
Line

* Secondary Control
* Supervisory Control (EMS)

PV Array

Measurements

Common dc Bus

Small Wind Turbine
and Generator

dc−dc

Battery Bank

Local
Control
Level

dc−dc
dc−ac

Flywheel

Distributed Generation
Figure 6. A communication infrastructure coordinates converters.

Enhancement of Conventional
dc Power Distribution Architectures
the main factor contributing to the high reliability and
robustness of telecom systems, consumer electronics, and
heVs, i.e., the direct connection of the battery to the common bus, entails several major drawbacks that come
more into play in cases when more expandable and flexible dc systems are required. the most prominent problems include mandatory rigid design of a battery pack,
decentralized charging that causes circulating currents,
and inability to directly control voltage of the common
bus. in this regard, it is the connection of battery stack to a
common bus via dedicated dc-dc converter what distinguishes fully flexible systems from the applications
addressed in previous sections. this simple yet effective
topological change not only allows for complete control
over the battery recharging and common bus voltage but
also greatly facilitates the system's extensibility. however,
even though the battery can now have a dedicated charger that cancels out the circulating current effect, it cannot
control the common bus and its own voltage at the same
time; hence, other converters need to take care over the
common bus voltage regulation during the regulated
charging process. Moreover, the effective capacity of the
battery is no longer seen at the main terminals, as it is
now replaced by the several orders of magnitude lower
capacitance given only by the output filters of dc-dc converters. therefore, the control design of bus-regulating
converters now becomes a much more challenging problem from a stability point of view. additionally, a

communication infrastructure between converters is
often adopted to coordinate and synchronize their actions
in realizing functionalities such as secondary or supervisory control, as depicted in Figure 6.
the controllability of a singular dc bus voltage also
results in the controllability of tie-line current flows
toward other dc buses. consequently, by adapting the voltages of respective buses, it becomes possible to control
complex dc power distribution architectures. Functionally,
such a structure can then be identified as a microgrid
(MG), a concept that attracted considerable attention in the
academic community over the past decade. the following
section looks back at the origin of the MG and explores the
possibilities of tailoring the results from that research field
to advanced dc power distribution architectures.

Changing the Energy Paradigm: Distributed
Generation and Microgrids
since the very beginning of the introduction of distributed
generation, coordination among various distributed generators (dGs) was recognized as a key prerequisite not
only for the full exploitation of their potential benefits, but
also for avoiding negative impacts on utility. in that sense,
a MG concept emerged as one of possible resolutions for
efficient integration of a growing number of dGs scattered
throughout the network. it is basically a small grid that
gathers local loads and dGs, and it may operate in both
grid-connected and islanded modes. Being an independent entity, which optimally coordinates local dGs and
loads, MG was envisioned to greatly reduce the number of
	

IEEE Electrific ation Magazine / MARCH 2 0 1 4

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https://www.nxtbook.com/nxtbooks/pes/electrification_december2021
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