Energy Management Systems in Microgrid Operations
Microgrids are a promising technology that can increase the reliability and economics of energy supply to end consumers.Microgrid development is shifting from prototype demonstration and pilot projects to full-scale commercial deployment.Microgrid energy management systems are critical components that can help microgrids come 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 the 1930s.The aging and overburdened power grid has experiencedfive massive blackouts in the past40years (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,a microgrid 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 engineer intern at ABB’s U.S.Corporate Research Center.He is currently working toward a Ph.D.degree in the Department of Electrical and Computer Engineering at North Carolina State University.His specialties and research interests include Smart Grid,microgrid,renewable
energy,grid integration of plug-in electric vehicles,computational intelligence,and power system modeling
and simulation.
Jianhui Wang is an energy system engineer at Argonne National Laboratory.
He is also an affiliate professor of the Department of Industrial and Systems Engineering at Auburn University.He is the chair of the IEEE Power&Energy Society(PES)Power System Operation Methods Subcommittee and co-chair of an IEEE task force on integrating wind and solar power into power system operations. He has authored/co-authored more than 100journal and conference publications. He is an editor of the IEEE Transactions on Smart Grid,and an editorial board member of Applied Energy.He is also a guest editor of a special issue of the IEEE Power and Energy Magazine on Electrification of Transportation,which won an APEX Grand Award,a guest
editor of a special issue of Applied
Energy on Smart Grids,Renewable Energy Integration,and Climate Change Mitigation–Future Electric Energy Systems,and is a guest editor of four special issues of the IEEE Transactions on Smart Grid on communication systems,demand response,storage,and forecasting.He is the technical program chair of the IEEE Innovative Smart Grid
Technologies conference2012. This work was supported by the U.S.
Department of Energy,Basic Energy Sciences,Office of Science,under contract No.DE-AC02-06CH11357.
(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 characteristics and 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 an
interconnected 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 to safety 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.The interconnection switch that a microgrid has is compatible with islanding and resynchronization under a variety of operating conditions.
A
microgrid EMS is control software 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 islanded modes based on the real-time operating conditions of microgrid components and the system status.Figure 1shows 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 microgrid components and the corresponding control/management schemes are discussed as follows.
Figure 1:Control Hierarchy in Microgrid
B.Microgrid components
Although there is not a universal 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. Table1summarizes the major components associated with microgrid EMSs and their functionalities.
1.Distributed generator
It is usually defined as a ,kilowatts)electric power generator which is directly connected to the distribution 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 heat
and 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 best35percent
efficient with a significant loss of
primary energy.A CHP system
can potentially reaches an
efficiency of up to80percent to85
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 approximately28
percent of the energy-related
carbon emissions of the country
(Marnay et al.,2008),the U.S.
Department of Energy(DOE)sets
up an aggressive goal of having
CHP plants comprise20percent
of ation capacity by the
year2030(Shipley et al.,2008).
The United States would see a
5,300trillion British thermal unit
(Btu)annual energy
consumption reduction,an848
million metric ton(MMT)annual
CO2reduction,and a231MMT
annual carbon reduction(DOE,
2012).
Some types of non-fuel-based
,wind turbines,
photovoltaic[PV]panels)are non-
dispatchable,and their output
depends on uncertain and
variable energy sources.Fuel-
based ,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 worth
mentioning 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 various
DGs,advanced power
electronic devices are applied to
smoothly convert energy of
Table1: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.
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 in order to ensure DGs can be integrated into the microgrid.In addition,optimal energy management for microgrids with a 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 the necessary interoperability among the diverse DGs.The International Electrotechnical Commission (IEC)61850-7-420 Communications Standard for Distributed Energy Resources (DERs)has been widely used (Clevel
and,2008)to address this issue.It is an international standard that defines the communication 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 is cheap 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 the
disturbance on end-users and
maintain the system reliability.In
an interconnected mode,DES is
mainly responsible for
maintaining the stable power
output of DGs and storing low-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.Aflywheel also has
high energy density and can
interact with certain types of DGs
to provide energy for a prolongededitor at large
period of time.In the long run,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 the
concepts 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 this
type 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/PEVs
to feed energy directly back to
the distribution network,which
creates a reverseflow and
complicates EMS operations.
4.Critical load
A typical microgrid consists
of both critical load and
controllable load.In the normal In a conventional
distribution
system,consumers
have littleflexibility
to fully participate
in electricity
markets.
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 be supplied.In order to improve the availability and reliability of power supply for critical l
oads, some of , controllable)load may have
to be disconnected or shed accordingly.
5.Point of common coupling PCC is the point at which the power production,distribution network,and customer interface meet.In the most common
configuration,DGs,DESs,and loads are tied together on their own feeders,which are then linked to the utility grid at a single PCC.II.Functionalities of
Microgrid EMS
A microgrid is a small portion
of a power distribution system
which is tied with the rest of the
distribution system via an
interconnection 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 to
monitor 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.Figure2
shows the role of EMS in a
microgrid associated with policy,
electricity market,load/DER
forecast,customers,utility,loads,
DGs,and DES.The microgrid
EMS receives the load and
energy resource forecasting data,
customer information/
preference,policy,and electricity
market information to determine
the best available controls on
powerflow,utility power
purchases,load dispatch,and
DG/DES scheduling.
T here are a number of EMS
software programs available
in practice.Table2summarizes
an incomplete list of vendors for
microgrid EMS systems.Each
EMS has different features that
can be customized for a specific
microgrid.
Figure2:An Illustrative Microgrid EMS

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