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Energy Management Systems inMicrogrid Operations
Microgrids are a promising technology that can increase
the reliability and economics of energy supply to endconsumers. Microgrid development is shifting fromprototype demonstration and pilot projects to full-scalecommercial deployment. Microgrid energy managementsystems are critical components that can help microgridscome to fruition.
Wencong Su and Jianhui Wang
I. Introduction
Economic and environmental
incentives, as well as advances in
technology, are reshaping the
traditional view of power
systems. The majority of the
current U.S. power grid
infrastructure was built in the1930s. The aging and
overburdened power grid has
experienced five massive
blackouts in the past 40 years
(Farmer and Allen, 2006). To
address these challenges,
microgrids have emerged as a
relatively new and promising
solution to restructuring the
current energy infrastructure and
ensuring the reliability of energy
supply.
A. Definition of microgrid
and energy management
system (EMS)
Technically speaking, amicrogrid is a low-voltage
distribution network that is
located downstream of a
distribution substation
through a point of common
coupling (PCC). Microgrids
consist of a variety of components
including distributed generators
(DGs), distributed energy storage
Wencong Su works as a researcher for Argonne National Laboratory, a U.S.
Department of Energy Laboratory in Argonne, Illinois. He has also worked as
a research and development engineerintern at ABB’s U.S. Corporate ResearchCenter. He is currently working toward a
Ph.D. degree in the Department of Electrical and Computer Engineering at
North Carolina State University. Hisspecialties and research interests include
Smart Grid, microgrid, renewableenergy, grid integration of plug-in
electric vehicles, computationalintelligence, and power system modeling
and simulation.
Jianhui Wang is an energy systemengineerat Argonne NationalLaboratory.
He is also an affiliate professor of the
Department of Industrial and SystemsEngineering at Auburn University. He is
the chair of the IEEE Power & EnergySociety (PES) Power System Operation
Methods Subcommittee and co-chair of an IEEE task force on integrating wind andsolar power into power system operations. He has authored/co-authored more than100 journal and conference publications.
He is an editor of the IEEE Transactionson Smart Grid, and an editorial board
member of Applied Energy. He is also a
guest editor of a special issue of the IEEEPower and Energy Magazine onElectrification of Transportation, which
won an APEX Grand Award, a guesteditor of a special issue of Applied
Energy on Smart Grids, RenewableEnergy Integration, and Climate Change
Mitigation – Future Electric EnergySystems, and is a guest editor of four
special issues of the IEEE Transactionson Smart Grid on communication
systems, demand response, storage, and forecasting. He is the technical program
chair of the IEEE Innovative Smart GridTechnologies conference 2012.
This work was supported by the U.S.Department of Energy, Basic Energy
Sciences, Office of Science, undercontract No. DE-AC02-06CH11357.
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(DES), and controllable loads. The
unique characteristics and
dynamics of a microgrid’s
components present a unique
challenge with regard to grid
control and operation.
Depending on the characteristicsand penetration of distributed
energy resources (DERs) and DES
nodes within a particular
microgrid, the desired energy
management scheme can be
significantly different from a
conventional power system. A
typical microgrid runs in two
operational modes (Asmus, 2010;
Lasseter, 2002): in aninterconnected mode linked to the
main grid through the
distribution substation
transformer and in an islanded
(autonomous) mode when it is
isolated from the main grid
during a blackout or brownout.
In the islanded mode, the
microgrid remains operational
and functional as an autonomous
entity. In a conventional
power distribution system, the
islanding process is prohibited
for practical operation, due tosafety concerns and hardware
limitations. Nowadays,advanced
power electronic devices (i.e.,
solid-state transformers)
consolidate the two-way
communication, switching
functions, protective relaying,
metering, digital data processing,
two-way power flow, and high
computational capability. Theinterconnection switch that a
microgrid has is compatible with
islanding and resynchronization
under a variety of operating
conditions.
A microgrid EMS is controlsoftware that can optimally
allocate the power output among
the DG units, economically serve
the load, and automatically
enable the system
resynchronization response to the
operating transition between
interconnected and islandedmodes based on the real-time
operating conditions of microgrid
components and the system
status. Figure 1 shows a typical
control hierarchy of a microgrid.
In general, a sophisticated
microgrid EMS has to operate and
coordinate a variety of DGs, DESs,
and loads in order to provide
high-quality, reliable, sustainable,and environmentally friendly
energy in a cost-effective way.
T he most common microgridcomponents and thecorresponding control/
management schemes are
discussed as follows.
[
Figure 1: Control Hierarchy in Microgrid
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B. Microgrid components
Althoughthereisnotauniversal
definition of what constitutes a
microgrid, it can be generally
stated that a microgrid is
composed of several major
components which normally do
not exist in traditional power
systems. High penetration of these
components increases the
complexity of the microgrid EMS.Table 1 summarizes the major
components associated with
microgrid EMSs and their
functionalities.
1. Distributed generator
It is usually defined as a small-
scale (e.g., kilowatts) electric
power generator which is
directly connected to thedistribution system at or near the
load feeder. In contrast,
conventional power plants
supply electricity through high-
voltage transmission lines with a
capacity of hundreds of
megawatts. Since DGs are
normally onsite or close to the
end-users, some types of DGs
(e.g., micro gas turbine), or more
broadly speaking combined heatand power (CHP) plants, can
simultaneously generate both
electric power and usable heat,
which can be a great benefit of
installing a microgrid. CHP
plants will likely be at the heart of
microgrid economics (Lasseter
et al., 2002). Traditional large
generators are at best 35 percent
efficient with a significant loss of primary energy. A CHP system
can potentially reaches an
efficiency of up to 80 percent to 85
percent. Without CHP systems,
microgrids may be less efficient
than the traditional power grid.
Moreover, since the waste heat
emitted from U.S. power plants
accounts for approximately 28
percent of the energy-relatedcarbon emissions of the country
(Marnay et al., 2008), the U.S.
Department of Energy (DOE) sets
up an aggressive goal of having
CHP plants comprise 20 percent
of U.S. generation capacity by the
year 2030 (Shipley et al., 2008).
The United States would see a
5,300 trillion British thermal unit
(Btu) annual energy
consumption reduction, an 848million metric ton (MMT) annual
CO2 reduction, and a 231 MMT
annual carbon reduction (DOE,
2012).
Some types of non-fuel-based
DGs (e.g., wind turbines,
photovoltaic [PV] panels) are non-
dispatchable, and their output
depends on uncertain and
variable energy sources. Fuel- based DGs (e.g., micro gas
turbines, diesel generators) can be
dispatched according to their
operating cost. An effective
microgrid EMS needs to
determine the optimal energy
scheduling of all DGs depending
on fuel costs, heat/energy
requirements, and customer
preferences. It is worthmentioning that the heat and
electricity demand may not
occur at the same time, which
places an additional constraint on
the microgrid’s control algorithm.
D ue to the nature of variousDGs, advanced powerelectronic devices are applied to
smoothly convert energy of
Table 1: Microgrid Components Controlled by Energy Management System.
Components Examples Functionalities
DG Reciprocating internal combustion engines with
generators, fuel cells, microturbines, small-scale wind
turbines, and photovoltaic arrays
Generate electricity and useful heat to local users and
utilize a variety of energy resources.
DES Battery banks, flywheels, super-capacitors, compressed
air energy storage
Store excess energy at off-peak time and operate as an
additional generator at peak time.
Controllable load Heating, ventilation, and air conditioning (HVAC) system,
plug-in hybrid electric vehicle (PHEV), plug-in electric
vehicle (PEV), and commercial and residential buildings
Dispatch the load to minimize the disturbance to power
grids and maximize customer preference.
Critical load School, hospital Serve as base load.
Need power quality support for critical loads.
PCC Static switch Switch between islanded and interconnected modes.
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variable frequency into the grid-
compatible alternating current
(AC) or direct current (DC) power.
The local regulator embedded in a
DG is mainly responsible for
voltage/frequency control and
real/reactive power control inorder to ensure DGs can be
integrated into the microgrid. In
addition, optimal energy
management for microgrids witha
significant DG penetration
requires the monitoring control of
DGs through free information
flow. It is critical to maintain the
compatibility of communication
technologies and provide thenecessary interoperability among
the diverse DGs. The International
Electrotechnical Commission
(IEC) 61850-7-420
Communications Standard for
Distributed Energy Resources
(DERs) has been widely used
(Cleveland, 2008) to address this
issue. It is an international
standard that defines thecommunication and control
interfaces for all DER devices, in
particular when DERs are
interconnected with the electric
utility grid.
2. Distributed energy storage
DES can make microgrids more
cost-effective by storing energy
when energy from the main grid ischeap or there is excessive
generation from the local DGs.
DES can also be operated as an
additional generator during peak
demand periods. The detailed
operations on DES are performed
by the embedded local regulators
within DES while the microgrid-
level EMS will control when to
dispatch the stored energy and
how much. The overall energy
management objective for DES
varies depending on the microgrid
operational modes. In an islanded
microgrid mode, DES can return
electric energy to minimize thedisturbance on end-users and
maintain the system reliability. In
an interconnected mode, DES is
mainly responsible for
maintaining the stable power
output of DGsandstoringlow-cost
electricity when it is available. In
general, some forms of DES are
coupled with DGs according to
their power/energy density. For
instance, a supercapacitor with
high power density is an excellent
candidate for short-term
balancing. A flywheel also has
high energy density and can
interact with certain types of DGsto provide energy for a prolonged
periodoftime.Inthelongrun,DES
can also provide a reasonable
amount of reserve capacity to main
the reliability of the microgrid.
3. Controllable loads
Controllable loads refer to the
loads that can adjust their own
electric energy usage based on
real-time set points. In a
conventional distribution
system, consumers have little
flexibility to fully participate in
electricity markets. Controllable
loads are usually tied with theconcepts of demand-side
management (DSM) or demand
response (DR). For example, the
load of buildings can be
controlled by adjusting the
HVAC system and temperature
to save energy cost while not
sacrificing the customer’s
comfort level. More and more
buildings equipped with thistype of control can be easily
interfaced with microgrid EMS.
Another controllable load
example is residential/
commercial lighting control,
which has been proven
successful (Dounis and
Caraiscos, 2009). Plug-in hybrid
electric vehicles (PHEVs) and
plug-in electric vehicles (PEVs)can be another special class of
controllable load. Unlike other
controllable loads, these
vehicles can be connected to the
outlets anywhere and at any
time, bringing more spatial and
temporal diversity and
uncertainty to the grid. Also
vehicle-to-grid (V2G)
technologies allow PHEVs/PEVsto feed energy directly back to
the distribution network, which
creates a reverse flow and
complicates EMS operations.
4. Critical load
A typical microgrid consists
of both critical load and
controllable load. In the normal
In a conventionaldistribution
system, consumershave little flexibility
to fully participatein electricity
markets.
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operational mode, the DG and
DES nodes can be utilized to
support as many critical loads as
possible. Once a microgrid is
disconnected from the main
utility grid, not all of the load
within a microgrid can besupplied. In order to improve the
availability and reliability of
power supply for critical loads,
some of non-critical (i.e.,
controllable) load may have
to be disconnected or shed
accordingly.
5. Point of common coupling
PCC is the point at which thepower production, distribution
network, and customer interface
meet. In the most common
configuration, DGs, DESs, and
loads are tied together on their
own feeders,which arethenlinked
to the utility grid at a single PCC.
II. Functionalities ofMicrogrid EMS
A microgrid is a small portion
of a power distribution system
which is tied with the rest of the
distribution system via aninterconnection switch. From the
system point of view, a microgrid
can freely route the energy
among the utility grid, local
renewable energy generators,
controllable loads, and DES
devices, opening up a new
paradigm of ‘‘Internet for
Energy’’ (Huang et al., 2011). The
microgrid EMS is expected tomonitor the operational
conditions and optimally
dispatch power from DERs and
DES nodes to supply the
controllable and critical loads.
Controllable loads can also be
dispatched accordingly to
maintain system reliability and
other critical loads. Figure 2
shows the role of EMS in a
microgrid associated with policy,
electricity market, load/DER
forecast, customers, utility, loads,
DGs, and DES. The microgridEMS receives the load and
energy resource forecasting data,
customer information/
preference, policy, and electricity
market information to determine
the best available controls on
power flow, utility power
purchases, load dispatch, and
DG/DES scheduling.
T here are a number of EMSsoftware programs availablein practice. Table 2 summarizes
an incomplete list of vendors for
microgrid EMS systems. Each
EMS has different features that
can be customized for a specific
microgrid.
[
Figure 2: An Illustrative Microgrid EMS
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Table 2: Major Vendors for Energy Management System.
Vendor Product Feature
Tridium, Inc
(www.tridium.com)
Vykon A comprehensive set of applications that synchronize, manage, and control
major building system functions required in a facility, such as HVAC systems,
energy, lighting, security, fire, safety, and unitary devices.
Encorp (www.encorp.com) Microgrid System
ControllerTM
Microgrid
SecureTM
Virtual
Maintenance
MonitorTM
A controller to remotely connect existing onsite generators with the latest clean-
and-green energy assets, such as PV systems and microturbines, and then
monitor and control the resulting microgrid. The software suite allows the
user to control and aggregate multiple energy systems remotely and provides
the user with site-specific generator metering, monitoring, and control.
Sutron (www.sutron.com) GenCom A wireless remote generator control system, in addition to many other
monitoring and control systems.
Invensys Energy Solutions
(www.ies.invensys.com/ )
Local Area
Power Control
Barber-Colman DYNA
It provides integrated, reliable, cost-effective power delivery system control and
management solutions for onsite power generation.
Wonderware
(www.wonderware.com/ )
Wonderware1 A real-time operation management software. Wonderware software delivers
significant cost reductions associated with designing, building, deploying,
and maintaining secure and standardized applications for manufacturing and
infrastructure operations.
GE
(www.ge-ip.com/products/ )
iPower An open, standards-based supervisory control and data acquisition (SCADA)
solution. Typical application solutions include: substation management
solutions; rural and municipal utility SCADA solutions, and in-plant
distribution solutions.
ABB (www.abb.com) MicroSCADA Pro
Network
Manager
SCADA
Manages the entire distribution network in utility and in industry environment
within the same system. It offers immediate access to real-time information
as well as easy connectivity to other systems. Provides a complete set of
advanced power system application functions, all proven under a wide
variety of field conditions.
Siemens
(www.energy.siemens.com)
Spectrum
PowerTMSpectrum Power offers a comprehensive range of SCADA functions for
requirements in energy generation, transmission/distribution network
operations, energy data management, and extensive communications
options with communication protocols.
Viridity Energy
(www.viridityenergy.com)
VPowerTM Provide the ability to communicate with and control equipment so that
executing the optimal energy strategy is as easy as pushing a button.
Power Analysis
(www.poweranalytics.com/ )
Paladin1
SmartGridTM A software platform designed specifically for the on-line management and
control of next-generation ‘‘hybrid’’ power infrastructure incorporating both
traditional utility power and on-premise power generation (e.g., solar power,
wind turbines, battery storage, etc.).
Green Energy Corp.
(www.greenenergycorp.com)
GreenBus1 GreenBus enables packaged applications such as SCADA, AMI/MDM, OMS,
CIS, IVR, and AVL to share data over a standard application programming
interface (API) and industry standard interfaces like MultiSpeak 1.
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http://www.awea.org/learnabout/publications/reports/upload/AWEA_First_Quarter_2012_Market_Report_Public.pdfhttp://www1.eere.energy.gov/manufacturing/distributedenergy/chp_benefits.htmlhttp://energy.gov/sites/prod/files/Microgrid%20Workshop%20Report%20August%202011.pdfhttp://www.epia.org/publications/epiapublications/global-market-outlook-for-photovoltaics-until-2015.htmlhttp://www.smartgrid.epri.com/Demo.aspxhttp://www.smartgrid.com/wp-content/uploads/2011/09/12___Richard.pdfhttp://grouper.ieee.org/groups/scc21/dr_shared/http://www.electricenergyonline.com/%3Fpage=show_article%26mag=63%26article=491http://www.pikeresearch.com/research/microgrid-deployment-tracker-4q11http://www.poweranalytics.com/http://www.greenenergycorp.com/http://dx.doi.org/10.1016/j.tej.2012.09.010http://dx.doi.org/10.1016/j.tej.2012.09.010http://www.greenenergycorp.com/http://www.poweranalytics.com/http://www.pikeresearch.com/research/microgrid-deployment-tracker-4q11http://www.electricenergyonline.com/%3Fpage=show_article%26mag=63%26article=491http://grouper.ieee.org/groups/scc21/dr_shared/http://www.smartgrid.com/wp-content/uploads/2011/09/12___Richard.pdfhttp://www.smartgrid.epri.com/Demo.aspxhttp://www.epia.org/publications/epiapublications/global-market-outlook-for-photovoltaics-until-2015.htmlhttp://energy.gov/sites/prod/files/Microgrid%20Workshop%20Report%20August%202011.pdfhttp://www1.eere.energy.gov/manufacturing/distributedenergy/chp_benefits.htmlhttp://www.awea.org/learnabout/publications/reports/upload/AWEA_First_Quarter_2012_Market_Report_Public.pdf
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III. Microgrid EMSArchitecture and ControlPhilosophy
A. Centralized microgrid
EMS
From the energy management
and control perspective, a
microgrid consists of three
hierarchical levels (Katiraei et al.,
2008): distribution network
operator (DNO) and market
operator (MO); microgrid central
controller (MGCC); and local
controllers (LCs) associated with
each DER/DES/load unit. TheMO is responsible for exchanging
information between the
microgrid and the electricity
market. DNO is a high-level
management system that
aggregates real-time information
and operating commands among
multiple microgrids and utility
grids. MGCC serves as a gateway
between the DNO/MO and LCswithin the microgrid. Ideally, a
microgrid EMS is an information
and control center embedded in
an MGCC.
A n MGCC is engineered fortwo major functions forupstream/downstream
distribution systems,
respectively. Firstly, an MGCC
has a two-way conversationchannel with the DNO and MO to
meet the utility requirements
(e.g., supply of electricity and
provision of ancillary services)
and participate in the energy
market (e.g., bidding). The MGCC
monitors the system operational
conditions, responds to any
disturbance, and switches/
resynchronizes the microgrid
operational modes (i.e.,
interconnected or islanded).
Secondly, the MGCC receives
information and requests from
multiple LGs within a microgrid.
Given the system set point sentfrom the DNO and MO, an MGCC
makes a decision to appropriately
allocate the power output among
DER/DES units according to a
certain objective function (e.g.,
loss or cost minimization, or profitmaximization). Then the MGCC
will send back the control signals
and power scheduling references
to the corresponding DGs. The
entire scheduling process is
subject to certain constraints
including reserve requirements,
renewable generation
uncertainties, and physical
constraints of DG and DES units.
I n a centralized operationregime, an MGCC is expectedto be computationally powerful in
order to process a large amount of
real-time data from all DER/DES
and loads in a timely manner. A
reliable two-way communication
infrastructure also needs to be in
place. The centralized microgrid
EMS design holds a number of
advantages such as easy
implementation, standardized
procedure, and high expansion
cost. However, as the number of
control devices rapidly increases,
high requirements oncommunication network capacity
and computational ability become
a major bottleneck for this type of
centralized design.
B. Real-world examples of
centralized microgrid EMS
In the early adoption of
microgrids, utilities largelyfocused on the system reliability
and security issues, based on the
assumption that microgrids are
less likely to completely replace
the existing grid structures.
Figure 3 illustrates a centralized
control scheme of microgrid EMS
(EPRI, 2011).
The energy supply for a
centralized microgrid design maycome from various sources. For
example, Northern Power
Systems has installed and
operated a customer-designed,
utility-connected microgrid
within the area known as Mad
River in Waitsfield, Vt. (Barnes
et al., 2007). A central controller
monitors the system states and
sends out real-time control signalsto multiple generators through a
reliable, secure, and high-speed
communications network.
In comparison, in some other
examples, a single large-capacity
generator or energy storage unit is
installed onsite. For instance, the
BC Hydro Boston Bar (Kroposki
et al., 2008) project is a microgrid
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that is interconnected to 69 kV
feeders through a 69/25 kV
substation. It allows a 3 MW peak
load and 8.6 MVA of
hydroelectric generation in the
islanded mode. Since there is no
storage device involved in thismicrogrid, the central controller
effectively manages a single large
hydro generator by sending
control signals using a leased
telephone line.
Table 3 lists some ongoing and
existing microgrid projects and
testbeds. As can be seen in the
table, the radical (centralized)
microgrid structure is still apopular design for microgrid
installations.
C. Decentralized microgrid
EMS
In contrast to centralized
control, distributed control in a
decentralized microgrid EMS
constitutes a framework where
each microgrid component is
regulated by one or more local
controllers rather than being
governed by a central master
controller. Every local control
monitors and communicateswith the other local controllers
through the communication
network. The local controllers
have the intelligence to make
operational decisions on their
own, without receiving the
control signals from a ‘‘master’’
control in the centralized EMS.
The local controllers then
exchange the information amongneighbors to reach consensus.
Figure 4 illustrates a
decentralized control scheme in
microgrid operations.
B ecause the local regulatorsonly need to communicatewith neighboring devices, the
amount of information
transferred is much less than
what is needed in the centralized
scheme. The computational
burden is also distributed on local
agents because the local
controllers only need to make a
decision locally. Local controllers
are no longer subject to a MGCCto determine the optimal power
output in such a distributed
system. Hence, this kind of
structure significantly reduces the
computational need and releases
the stress on the communication
network of the entire microgrid
system. But it should be
mentioned that a MGCC and its
associated EMS still play animportant role in even this
decentralized framework. For
example, an EMS in a
decentralized microgrid
exchanges energy price
information with the DNO and
MO and is able to take over the
control of the local regulator from
the system level in the event of
[
Figure 3: Centralized Microgrid EMS
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Table 3: Summary of Microgrid Projects and Testbeds.
Region Microgrid DG DES Load Control
North America British Columbia Boston Bar Hydro N/A Residential Centralized
Boralex, Canada Diesel Generator N/A Residential Centralized
CERTS, US Diesel Generator Battery Static,
Induction Motor
Decentralized
University of
Wisconsin-Madison, US
Diesel Generator, PV N/A Static Centralized
Mad River, Waitsfield,
Vermont
Biodiesel Genset,
Microturbine,
Propane Genset
N/A Industrial,
Commercial
Centralized
Asia Shimizu, Japan Gas turbine Battery,
Supercapacitor
Residential Centralized
Hachinohe, Japan PV, Wind,
Diesel Genset, CHP
Battery Industrial,
Commercial
Centralized
Kyoto Eco-Energy, Japan PV, Wind, Fuel Cell,
Biogas
Battery Residential Centralized
Aichi, Japan PV, Fuel Cell Battery Industrial,
Commercial
Centralized
Sendai, Japan PV, Fuel Cell,
Gas turbine
Battery Residential,
Industrial,
Commercial
Centralized
Hsingchiang, China PV, Diesel Genset Battery Residential,
Commercial
Centralized
Hefei University of
Technology, China
PV, Wind, Diesel
Genset, Hydro
Battery,
Supercapacitor
Static, Motor Centralized
Europe Kythnos, Greece PV, Diesel Genset Battery Residential Centralized
Labein Experimental Centre Wind, PV, Microturbine,
Diesel Genset,
Battery,
Supercapacitor,
Flywheel,
Static Centralized and
Decentralized
CESI, Italy, PV, Wind, CHP,
Diesel Genset
Battery,
Supercapacitor,
Flywheel,
Static Centralized
Lab-scale Testbed, University
of Leuven, Belgium
PV, CHP Battery Static Decentralized
Continuon Holiday Park,Netherlands
PV Battery Residential Centralized
Demotec, Germany PV, Wind, CHP,
Diesel Genset
Battery Residential,
Commercial
Centralized
Am Steinweg, Germany PV, CHP Battery Residential Centralized
Lab-scale testbed,
National Technical
University of Athens,
Greece
PV, Wind Battery Static Centralized
Source : Lidula and Rajapakse (2011) and Barnes et al. (2007).
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serious contingencies and
equipment failure.
D ecentralized control isone potential solutionto many challenging controland energy management
problems in microgrids
(Liu et al., 2007). For instance,
as mentioned, the computational
requirement for the MGCC is
much more limited. Also if the
MGCC fails, the rest of the system
can still survive. It is a modular
system to ensure the plug-and-
play flexibility. However, becausethe local controllers have more
authority in this setting, the
inherent security issues make
decentralized microgrids more
vulnerable to cyber and physical
attacks, which can be more
difficult to detect and
troubleshoot. Smooth operations
are highly dependent on
successful communications
among the local agents and their
neighbors. The desired
communication topology
needs to be carefully investigated.In the legacy power system,
the communication channel is
mostly dedicated to the EMS
(master) and local agents
(slave). The local agents have
little flexibility to exchange
information with their neighbors
through the exiting
communication network. Utilities
are seeking a practical way toupgrade/replace an aging power
grid, which cannot be achieved by
a one-step-at-a-time approach.
The expensive initial upgrading
cost on control and
communication facilities are
another bottleneck of the
decentralized EMS in microgrid
operations.
D. Real-world examples of
decentralized microgrid EMS
and control
Figure 5 illustrates the
schematics of the AEP/CERTS
microgrid (Barnes et al., 2007). TheCERTS microgrid is intended to
act as a single self-contained and
autonomous entity. Under the
peer-to-peer concept, this CERTS
microgrid does not require a
single ‘‘master’’ controller. It
operates in a distributed
(decentralized) fashion. The local
controllers have a certain level of
intelligence in order to respond tothe system dynamics (e.g., voltage
magnitude and frequency) by
using droop control and
proportional–integral (PI) control
loops. However, it is important to
mention that a low-dynamic
central controller may still be
needed in this kind of microgrid
to broadcast the steady-state set
points. The dynamic control isperformed by the local controller/
regulator. An example of a
‘‘pure’’ distributed microgrid
operation can be found in
(Brabandere et al., 2007), as shown
in Figure 6. The primary droop
control is responsible for
maintaining the frequency and
voltage at their set points to
ensure reliable operation evenwhen communication fails. A
gossip-based secondary control is
used to minimize the average of
all voltage and frequency
deviations. Then a gossip-based
economic optimization is
performed to determine the cost-
effective energy scheduling by
finding a unique optimal
[
Figure 4: Decentralized Microgrid EMS
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marginal cost. The entire process
is fully distributed without a
central controller.
A s discussed above, twomicrogrid EMS
architectures are available:
centralized and decentralized.
Each of these two control options
has its advantages and drawbacks.Table 4 shows the comparison
between centralized and
decentralized microgrid EMS.
Centralized control is widely
deployed in various SCADA
systems, as shown in Table 3.
Because the system operator has
direct control over the entirepower system in a centralized
control environment, system-wide
optimization can be achieved in a
timely fashion. However, a
microgrid is a complex and
heterogeneous system with
diverse controllable devices. A
centralized microgrid EMS
requires a reliable, high-speed
communication network betweenthe central controller and local
regulators. In addition, the current
centralized control structure is not
fully compatible with the plug-
and-playfunctionality which is the
key feature of microgrids. The
decentralized control option is
being advocated as well. It is
believed that microgrid operations
rely on the intelligence of localcontrollers/regulators. Microgrid
[
Figure 6: Overview of the Proposed Primary, Secondary and Tertiary ControlSource: Brabandere et al. (2007).
[
Figure 5: Schematics of AEP/CERTS MicrogridSource: Lidula and Rajapakse (2011).
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devices (e.g., DG, DES, and load)
operate autonomously (e.g.,
frequency droop control, Volt-
VAR control) based on local
information only. Without the
information exchange between the
master controller and local
regulators, decentralized control
can greatly reduce the need of
high-bandwidth communication.Since it is fairly immune to a single
point of failure, decentralized
control is a good candidate for
small-scalemicrogrids withhigher
priorities on system reliability.
However, due to the nature of
decentralized control, there is no
direct link to broadcast global
information to each controllable
device (e.g., generator, load, andbattery storage). The local
regulator negotiates with
neighboring local regulators to
reach consensus (e.g., nominal
operating set-points) iteratively
among communication networks,
which causes the additional cost of
time synchronization. It is
challenging to achieve global
optimization in microgrid
operations (e.g., energy/power
dispatch) in form of a ‘‘pure’’
decentralized control.
IV. Challenges andOpportunities for Microgrid EMS
A. Dynamic energy supply
In comparison with the
topology of the bulk power
system, which is relatively static,
a microgrid can have a highly
dynamic topology and a number
of heterogeneous devices. The
ability of microgrid components
to plug-and-play is one salientfeature of the microgrid. Plug-
and-play allows any energy
source or storage device to be
connected with the microgrid,
anywhere and anytime. Since
most DGs and DESs in a
microgrid are locally owned and
operated, consumers can become
independent of the conventional
electricity supplier to a certain
extent. In other words, consumers
can operate their DGs and DESs
optimally to supply their own
load or provide ancillary services
(A/S) to the utility grid,
depending on the electricity and
A/S prices on the utility grid. The
plug-and-play functionality is the
key to equipping the microgridwith such flexibility. Essentially,
microgrids are capable of fast
reconfiguration without
redesigning the energy
management scheme.
C ontrollable loads are alsoplaying a very importantrole in microgrid operations. The
ability to shift or curtail certain
load can help improve thereliability of electricity supply to
the critical load. In addition, the
rapid deployment of electric
vehicles can further contribute to
the magnitude of controllable
loads. Ideally, customers may
charge the vehicles at any time.
But, with central/coordinated
signals from the microgrid EMS,
Table 4: Comparisons between Centralized and Decentralized Microgrid EMS.
Pros Cons
Centralized control Simple to implement. Computational burden.
Easy to maintain. Requires high-bandwidth links.
Relatively low cost. Single point of failure.
Widely used and operated. Not easy to expand.
Wide control over the entire system. Weak plug-and-play functionality.
Decentralized control Easier plug-and-play (easy to expand). Need synchronization.
Low computational cost. May be more time-consuming for local agents to reach consensus.
Avoid single point of failure. Convergence rates may be affected by the communication
network topology.
Suitable for large-scale,
complex, heterogeneous systems.
Upgrading cost on the existing
control and communication facility.
Needs new communication structure.
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electric vehicles also have the
potential to shift the electric
demand for charging from peak
times to off-peak times or provide
A/S to the microgrid or the utility
grid.
M oreover, the dynamicinteractions of variousmicrogrid devices may require acomplete reconfiguration of the
microgrid network topology at
certain times, just as how
reconfiguration can be done in a
distribution system. Installation of
reliable breakers/switches is
needed to implement such actions.
Also the existing algorithms fornetwork configuration are based
on certain kind of heuristics, as the
resulting non-linear optimization
problem is of large scale and it is
difficulttofindanoptimalsolution
in real time.
B. Renewable energy
intermittency
Microgrids can be an
immediate solution to better
utilize renewable energy
resources. The United States had
2,820 MW of cumulative installed
capacity of photovoltaic in 2010,
and that figure almost doubled in
2011 (EPIA, 2012). The U.S. wind
industry installed 52 percent
more MW during the first quarterof 2012 than the first quarter of
2011. During the first quarter of
2012, the U.S. wind industry
installed 1,695 MW across 17
states. This brings cumulative
U.S. wind power capacity
installations to 48,611 MW
through the end of March 2012
(AWEA, 2012). Although a large
portion of renewable energy such
as wind power directly sends
power to the bulk power grid on
the transmission level, distributed
renewable energy sources have
been playing an increasing role in
the distribution systems.Normally, DGs using renewable
energy resource are considered as
non-dispatchable units. From a
long-term operation point of
view, the operational cost of those
renewable energy-based DGs isneglectable. The inherent
intermittency and variability of a
renewable energy resource (e.g.,
wind and solar) has complicated
implications for microgrid
operations (Wang et al., 2011a).
These renewable energy
resources tend to fluctuate
dramatically depending on the
time of day and time of year. Suchvariability and uncertainty need
to be carefully taken into account
in microgrid EMS design.
C. Other uncertainties
With increasing control loads,
the accurate load forecasting is
becoming more and more
challenging. In general, the load
profile varies with time and
season. However, the uniqueness
of microgrid controllable loads
can be present in both temporal
and spatial dimensions. For
example, unlike other traditionalpower loads, PHEVs/PEVs can
be connected to power grids
anywhere and anytime, which
brings more spatial and temporal
diversity and uncertainty (Su
et al., in press). Under the
‘‘primary’’ and ‘‘standard’’
charging level (Level 2), the
hourly charging load of a typical
battery on an electric vehicle, 6.7/7.7 kW, is approximately
equivalent to the average
household power consumption at
peak time. At this level, multiple
charging loads connected to one
feeder at peak time may cause
serious transformer overloading
during a short period of time.
Also, depending on user
preference and interest, thevehicle owner may charge a
PHEV or PEV at any charging
location (e.g., public parking deck
or home garage) and at any time.
Therefore, a well-designed
microgrid EMS has to incorporate
both spatial and temporal scales.
D. Communication
requirements
In general, the communications
network can be categorized as:
wide area network (WAN), field
area network (FAN), and home
area network (HAN). The needed
microgrid communications
network architecture falls in the
categories of FAN and HAN. A
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FAN, which can be either field-
based or customer-based with
different critical requirements
(Wang et al., 2011b), is normally
implemented on the distribution
system. A HAN is usually
implemented for residentialconsumers to enable Smart Grid
functionalities such as DSM and
advanced metering infrastructure
(AMI). A reliable and compatible
communication network is
required to monitor and
effectively manage a variety of
microgrid components (e.g., DG,
DES, and load).
Two-way communication:Unlike most of existing control
and communication systems in
today’s unidirectional power
systems, two-way power flow
and information flow is the
backbone of a microgrid.
Reliability: A successful
microgrid EMS relies heavily on
the communication infrastructure
to send control signals and receivefeedback on device status.
Communication reliability is
affected by a number of possible
failures such as time-out failures,
network failures, and resource
failures (Wang et al., 2011b).
Compatibility: A number of
communication protocols (e.g.,
HomePlug, ZigBee, Cellular
Network, WiFi, and Bluetooth)can be good candidates for
achieving reliable, secure, and
two-way communication. Since
local communication nodes will
talk to each other and
communicate with the microgrid
EMS, the compatibility of various
communication technologies and
protocols is critical.
Network latency: Just as with
any communication network,
network latency may present a
problem to microgrid EMS
implementation.
Time synchronization: From
the control perspective, some of microgrid devices need to be
synchronized in real time to
achieve accurate real-time energy
management.
Cyber security: In addition to
the standard requirements for acommunications network such as
bandwidth and reliability,
security is another critical aspect
in implementing microgrid EMS.
Traditional power grid
communications mainly rely on a
dedicated wired communication
network to support reliable
monitoring and control. In a
microgrid, since more wirelesstechnologies are extensively
deployed, a unique security threat
exists due to the shared and
accessible nature of medium.
Some recent work (Ericsson, 2010)
identified the threats and
vulnerabilities of wireless
technologies and summarized
their security performance.
V. Future Trends ofMicrogrid EMS andControl
Figure 7 illustrates the history
of the power system EMS.
The future research topicsregarding a microgrid EMS can be
summarized as:
Openness: An open
communication peripheral highly
compatible and standardized
microgrid EMS enables utilities to
move from legacy operation
systems to micro-scale energy
management applications in a
highly scalable architecture.There are numerous related
standards (e.g., IEC 61850, DNP3,
C22.12) that have been recently
published or are currently being
reviewed. Since various third-
party end-user software packages
are available in the market, non-
proprietary open
communications protocols are
highly recommended. EMS architecture: The choice
of EMS architecture—centralized
or decentralized—is ultimately a
philosophical question. There are
pros and cons for both
approaches. Such a choice may
not have to be an either/or
proposition. To achieve a cost-
effective microgrid EMS with
better control performance,microgrid operators have to make
a tradeoff between those two
approaches. Those two control
strategies are not mutually
exclusive and they can operate in
harmony. A mix of microgrid
EMS architectures can combine
many of the advantages of
centralized and decentralized
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approaches. A mix of microgrid
EMS architectures would be able
to facilitate the successful rollout
of microgrid deployment in the
near term. Communication gateway: A
communication gateway must be
developed to allow a microgrid to
supply ancillary services to the
main grid. A low-cost, reliable,
standardized communication
gateway should be developed to
meet both the needs of the utility/
independent system operator and
the needs of the microgrid EMS. Reliability and cyber security:
The exchanged information
among microgrid components
might be of interest to cyber
attackers. A robust microgrid
EMS needs to sustain cyber
attacks such as protocol and
routing attacks, installation of
worms/spyware/malware and
denial-of-service (DoS). Requiredcyber security features include
intrusion detection, server
firewalls, access control, and data
encryption.
Human-man interface (HMI):
On-demand microgrid
monitoring and control is
required in a microgrid EMS
design to collect system
information in real time through a
two-way communication
network. The next-generation
monitoring and control functions
should provide system operatorswith useful information rather
than raw data (Zhang et al., 2010).
The HMI should be capable of
visualizing and archiving the
collected data and processing
commands and additional
information, which moves
microgrid operations from being
data-intensive to information-
directed. On the customer side,HMI allows customers to more
actively interact with the
microgrid EMS.
Standards and protocols:
Currently, there is no such a well-
defined universal end-to-end
microgrid communications and
control standard that links all
microgrid devices with
standardized componentcapabilities. The existing IEC
61850 communications and
control standards do not fully
satisfy the need of microgrid
operations. The Institute of
Electrical and Electronics
Engineers (IEEE) has made many
efforts to develop a guideline for
the deployment of microgrids.
According the recently published
DOE microgrid report (DOE,
2011), IEEE 1547.4 was
acknowledged as the benchmark
milestone and baseline standardfor microgrids. IEEE P1547.8
would further support the IEEE
1547 standard to develop a
national standard for microgrids.
The IEEE 2030 Smart Grid
Interoperability Series of
Standards (IEEE Standards
Association, 2012) were also
considered an important standard
with respect to interoperability inmicrogrids. It addresses the
interoperability of energy
technology and information
technology operation with electric
power systems and end-use
applications and loads.
VI. Conclusion
In summary, microgrids are one
promising technology that can
increase the reliability and
economics of energy supply to end
consumers. According to Pike
Research (Pike Research, 2011),
microgrid development is shifting
from prototype demonstration
and pilot projects to full-scale
[
Figure 7: History of General EMSSource: H. Lee Smith (2010) and IBM (2010).
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commercial deployment.
Microgrid energy management
systems are critical components
that can help microgrids come to
fruition.&
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