Recent developments in microgrids and example cases around the world—A review

Abstract

Climate change concerns due to the rising amounts of the carbon gas in the atmosphere have in the last decade or so initiated a fast pace of technological advances in the renewable energy industry. Such developments in technology and the move towards cleaner sources of energy have made distributed generation (DG) from renewable resources more desirable. However, it is a known fact that rising penetrations of DG can have adverse impacts on the grid structure and its operation. The microgrid concept is a solution proposed to control the impact of DG and make conventional grids more suitable for large scale deployments of DG. Covering many aspects of the power systems and power electronics fields, microgrids have become a very popular research field. This paper reviews the background and the concept of a microgrid, the current status of the literature, on-going research projects, and the relevant standards. It also presents a review of the microgrid pilot projects around the world in further detail and discusses the potential avenues for further research.

Full text

Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
Recent developments in microgrids and example cases around
the world—A review
Taha Selim Ustun∗, Cagil Ozansoy, Aladin Zayegh
School of Engineering and Science, Victoria University, Melbourne, Australia
article info
Article history:
Received 7 January 2011
Accepted 5 July 2011
Available online 21 August 2011
Keywords:
Distributed generation
IEEE Standard 1547
Microgrid control
Microgrid management system
Protection
Renewable energy
abstract
Climate change concerns due to the rising amounts of the carbon gas in the atmosphere have in the
last decade or so initiated a fast pace of technological advances in the renewable energy industry. Such
developments in technology and the move towards cleaner sources of energy have made distributed
generation (DG) from renewable resources more desirable. However, it is a known fact that rising pene-
trations of DG can have adverse impacts on the grid structure and its operation. The microgrid concept is
a solution proposed to control the impact of DG and make conventional grids more suitable for large scale
deployments of DG. Covering many aspects of the power systems and power electronics fields, micro-
grids have become a very popular research field. This paper reviews the background and the concept of
a microgrid, the current status of the literature, on-going research projects, and the relevant standards.
It also presents a review of the microgrid pilot projects around the world in further detail and discusses
the potential avenues for further research.
Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
Contents
1. Introduction ........................................................................................................................................ 4030
2. The microgrid concept ............................................................................................................................. 4031
3. Current status of literature and ongoing research ................................................................................................ 4032
3.1. Control ...................................................................................................................................... 4032
3.2. Protection................................................................................................................................... 4034
3.2.1. Islanding .......................................................................................................................... 4034
3.3. Fault current protection .................................................................................................................... 4034
3.4. Microgrid energy management system (MEMS) .......................................................................................... 4036
4. Examples around the world ....................................................................................................................... 4036
4.1. European union (EU) ....................................................................................................................... 4036
4.2. Japan ........................................................................................................................................ 4037
4.3. Korea ....................................................................................................................................... 4037
4.4. North America .............................................................................................................................. 4037
4.5. Australia .................................................................................................................................... 4037
5. Standards and universalization .................................................................................................................... 4038
6. Future work and possible research areas .......................................................................................................... 4039
7. Conclusion ......................................................................................................................................... 4040
References ......................................................................................................................................... 4040
1. Introduction
The prominence of generating electric power in very large
steam-powered central power stations seems to have ended. The
∗Corresponding author. Tel.: +61 3 9919 5046; fax: +61 3 9919 4908.
E-mail address: tahaselim.ustun@live.vu.edu.au (T.S. Ustun).
increased concerns for the environmental impacts of centralized
coal-fired generation, most importantly those that relate to high
CO2emissions, are the main factors driving the transition towards
small-scale decentralized generation of power. Decentralized
(distributed) generation of electricity most favorably occurs from
renewable sources that are located on the distribution system
close to the point of consumption. Governments and industries all
1364-0321/$ – see front matter. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.rser.2011.07.033

T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041 4031
around the world are increasingly looking for ways to reduce the
greenhouse emissions from their operations with a major focus on
the use and installation of sustainable distributed energy systems
[1].
The need for far more efficient electricity management sys-
tems has given rise to the development of innovative technologies
and groundbreaking ideas in power generation and transmis-
sion. The trend is to increase the share of DG in the electricity
supply. DG may also comprise renewable energy (RE) systems
such as solar, wind and wave, which are promising cleaner tech-
nologies leading to reductions in greenhouse gas emissions and
in effect aiding in the remedy of the global warming problem
[2]. Consequently, governments and energy regulation authori-
ties worldwide are encouraging more deployments of RE based
distributed generator systems (DGS). However, higher penetra-
tion of micro-sources, i.e. small scale PV panels, wind turbines,
and diesel generators into the grid changes the traditional “radial”
structure of the grid. This revolutionary change in the structure
triggers many problems which were previously unknown to the
grid operators and power engineers [3]. There are now various
microsources at different penetration levels in the grid and this
new structure invalidates the traditional power flow control meth-
ods. Moreover, DGS also make contributions to the fault currents
around the network. Hence, in case of a fault, the transient char-
acteristics of the network become completely different [4]. These
are only a few of the issues that have arisen in relation with the
revolutionary changes occurring in the grids and the way they are
operated.
There are still many technical challenges that must be overcome
so that DGS can be cost-effectively, efficiently and reliably inte-
grated into existing electric power systems. Existing distribution
systems are not designed for significant penetration of DG. Distri-
bution systems were traditionally designed with the assumption of
a passive network. The interconnection of decentralized renewable
energy generation systems to such networks inevitably changes the
characteristics of the system and presents key technical challenges
such as circuit protection coordination, power quality, reliability,
and stability issues that must be overcome. Controlling a huge num-
ber of geographically dispersed DGS in a large network is a daunting
challenge for the safe, reliable, and effective operation of the net-
work.
The search for alternative energy sources and more efficient
utilization of the energy as a means of tackling the global warm-
ing concerns will require fundamental changes in the electrical
engineering (EE) field explicitly in relation to the matters asso-
ciated with the Transmission and Distribution (T&D) of this
renewable electricity. Although T&D grids have been around for
many decades, DG and RE concepts have recently become irre-
versibly popular. As a result, many research and development
needs have evolved as a necessity to enable the scaling up of the
implementation and uptake of renewable energy systems giving
them recognition and equal status in energy sector investment
processes.
Microgrids, which are small entities in a power system network,
are capable of coordinating and managing DGS in a more decentral-
ized way thus reducing the need for the centralized coordination
and management of such systems [5]. This is highly recommended
in [5], where it is claimed that such a scheme would permit DGS
to provide their full benefits. Yet, there are still many research and
development needs associated with the microgrids. [5,6]
This paper presents a detailed review of the literature with
regard to microgrids by outlining the existing knowledge as
well as the problems and challenges being encountered. It
also provides an overview of the current research and devel-
opment work being carried out all over the world. It gives
an insight into some real-life implementations of microgrids.
Finally, it focuses on knowledge gaps yet to be addressed and
possible future work in this field of power/electrical engineer-
ing.
2. The microgrid concept
A microgrid is a new concept which refers to a small-scale power
system with a cluster of loads and distributed generators oper-
ating together with energy management, control and protection
devices and associated software. Such devices include the flexi-
ble AC transmission system (FACTS) control devices such as power
flow controllers and voltage regulators as well as protective relays
and circuit breakers [7]. In other words, a microgrid is a collection
of loads and microgenerators along with some local storage and
behaves just like a model-citizen from grid side thanks to intelligent
control [8]. This means, although a microgrid is itself composed of
many generators and loads, it appears as a net load or a net gen-
erator to the broader grid with well-behaved characteristics [9].A
sample microgrid architecture is shown in Fig. 1.
As shown in Fig. 1, the microgrid is a very versatile concept as
it can accommodate various types of the micro generators (wind
turbine, photovoltaic (PV) array, diesel generator, and wave gen-
erator), local storage elements (capacitors, flywheel) and loads. A
distributed generator might be a diesel generator (DG4) which can
be coupled to the grid directly, or a PV array which needs direct cur-
rent (DC)/alternating current (AC) inverter interface (DG2, DG3) or
an asynchronous wind turbine (DG1) which requires AC–DC–AC
inversion for proper grid connection. Similarly, the storage devices
used in the system may or may not require an inverter interface as
in the case of capacitor banks and flywheel, respectively.
A microgrid can be a DC [10], AC or even a high frequency AC
grid [11]. It can be a single or a three phase system or it may
be connected to low voltage or medium power distribution net-
works [7]. Furthermore, a microgrid could be operating in either
grid connected or islanded operation mode. For each operating
mode operational requirements are different and distinct control
schemes are required.
The groundbreaking feature of a microgrid is its ability to oper-
ate “autonomously” when there is a power outage in the main grid.
This operation mode is called islanded operation since the micro-
grid disconnects from the grid and becomes an island with local
generators and loads. In this way, the consumers may receive con-
tinuous service even when there is power outage in the grid due to a
fault or maintenance. Moreover, if there are voltage sags, frequency
drops, or faults in the main grid then the microgrid can be easily
disconnected, i.e. islanded from the rest of the grid and the users
can be isolated from those problems. In this way, microgrids not
only help in providing uninterrupted service but also contribute to
the maintaining service quality.
The motivation behind using microgrids is to divide the enor-
mous conventional utility network into smaller and more easily
operable grids. These smaller electrical networks will manage dis-
tributed generators, loads, storage and protection devices in their
own grid. Provided that each microgrid is operating as a model cit-
izen, i.e. either as a load receiving power with acceptable electrical
characteristics or as a power supply supplying power with accept-
able electrical characteristics, then the overall utility grid can be
operated properly. It is a well-known fact that higher penetration
levels of distributed generators, especially those that require power
electronics (PE) interface, alter the grid structure and jeopardize
safe and reliable operation. The microgrid concept is introduced to
manage these generators in smaller quantities rather than trying
to tackle the whole network in a holistic manner. In this way, more
distributed generators can be employed in the grid and side-effects
on the grid operation can be eliminated.

4032 T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041
Fig. 1. A sample microgrid architecture.
3. Current status of literature and ongoing research
Distributed generators and storage devices almost always
require a PE interface for a proper connection to the grid. Accord-
ingly, all research on PE devices such as inverter control and
storage can therefore be linked to the microgrid research field.
For better operation and control, PE devices such as insulated
gate bipolar transistors (IGBTs) or thyristors with higher cur-
rent/voltage ratings are required. References [12,13] discuss the
research projects that are being carried out to accomplish these
objectives. Energy storage is an indispensible part of microgrids for
storing excess energy, supplying power in case of shortages, and
for preventing voltage sags. For a better operation; energy stor-
age devices with higher efficiencies, longer lifetimes along with
faster charging and slower discharging characteristics are desired.
Projects with a focus on energy storage matters are outlined in
[14].
The most promising one of the new generation batteries is the
NAS (sodium–sulfur) battery. Although it needs to be maintained
at high temperatures, it has advantages when compared with other
types of batteries. Table 1 [15] shows that a NAS battery, when com-
pared with other battery types, has substantially a higher energy
density, a fair efficiency and no self discharge. These characteris-
tics of NAS make it very appealing and hence it is being deployed
by companies operating in the RE sector in Japan and in the United
States (U.S.).
Another innovative energy storage system is being developed
by the RAPS Pty Ltd. which is based on high purity graphite blocks.
These blocks are heated up with energy received from solar panels,
wind generators or the grid for later use. This energy may then
be used to produce steam through embedded heat exchangers and
converted back to electricity with steam turbine generators [16].
According to the inventor company, the storage capacity of high
purity graphite ranges from around 300 kWh (thermal) per ton at
a storage temperature of 750 ◦C to around 1000kWh (thermal) per
ton at 1800 ◦C[17]. There are pilot deployments in Silverwater and
King Island, Australia.
The research and development (R&D) work being undertaken
at the device level is very comprehensive and the literature can
be referred to. The main focus of this article will be three main
sub-topics of microgrid research: control, protection and microgrid
management systems.
3.1. Control
The controller capabilities of a microgrid are one of the most cru-
cial elements in determining the introduction of this new concept
to the utility and its wide acceptance. Depending on the type and
depth of the penetration of distributed energy resource (DER) units,
load characteristics and power quality constraints of a microgrid
can be significantly and even conceptually different than those of
the conventional power systems [18]. Market participation strate-
gies, the required control and operational strategies can also be
added to this list. This is due to the fact that the characteristics of
DGS are much different than conventional synchronized motors.
They rely on intermittent resources in the system which cannot be
controlled or estimated. Hence, a microgrid is a dynamic entity and
DGS might connect/disconnect while the microgrid is in operation.
The main objective of control systems in microgrids is to con-
tinuously supply power to the loads despite the changes in the
system. A microgrid may be operating in grid-connected mode
and gets islanded due to a fault. One or more DG units may con-
nect to/disconnect from the grid, or there might even be significant
changes in the amount of power demanded by the loads. Under all
these circumstances, the microgrid control shall ensure that power
is supplied to the loads with acceptable voltage and frequency
characteristics. Since there are two distinct operation modes of a
microgrid, control schemes are designed for two distinct phases.
Table 2 [18] shows the control strategies of DG in a microgrid. When
there is grid connection, grid-following controls are employed. On
the other hand, in islanded operation, when a grid needs to be
formed, grid-forming controls are used.
In the grid-following mode, the frequency and voltage values
are dictated by the utility grid and DG units are operated to follow
these set values. The non-interactive control method strives to har-
vest the maximum power available through maximum power point
tracking (MPPT), or a predetermined amount of power whereas the
interactive control method is used for real and reactive power sup-
port depending on the system and the loads. The former method is
suitable for solar panels and wind generators where the energy
resources are inconsistent and unreliable. The latter method is
implemented in DGs such as diesel generators or distributed stor-
age devices which can supply energy continuously. A microgrid
may incorporate both of these control methods for different types
of DGs.

T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041 4033
Table 1
Batteries used for distributed energy storage.
Unit NAS Redox flow Lead ZincBr
Voltage V 2.08 1.4 2 1.8
Ideal energy density Wh/kg 780 100 110 430
Wh/l 1000 120 220 600
Efficiency % DC 85 80 85 80
Temperature ◦C 280–350 40–80 5–50 20–50
Auxiliaries – Heater Pump Water Pump
Self Discharge – No Yes Yes Yes
Table 2
Control strategies for DG coupling inverters in MG.
Grid-following Grid-forming
Non-interactive control
methods
Power export (may be
with MPPT)
V–fcontrol
Interactive control
methods
P and Q support Load sharing
Fig. 4. Control levels of the microgrid [70].
When microgrid operates in islanded mode, a similar two-way
approach is used. Some large DGs are operated with voltage and fre-
quency (V–f) control to keep these values constant in the islanded
microgrid. If this is achieved with a number of DGs, then the rest of
them will be operated in the load sharing mode to share the loads
in the system. In this fashion, the microgrid operates within accept-
able voltage/frequency limits and the loads are supplied with the
required power.
The most popular control scheme employed for load sharing is
droop control since it realizes automatic load sharing in a micro-
grid without any central control mechanism or communication
between DGs [19,20]. This feature shines when the microgrid is
working in islanded mode where droop control solves the control
problem [21]. In fact, it has been demonstrated that by imitating
the generator-turbine-governor units, drooping characteristics can
be successfully applied to inverters working in an isolated AC sys-
tem. [22]. Of the two possible generation modes, V–fand, real and
reactive power (P–Q) droops, the latter is more preferable by pro-
ducers whereas the former is needed to form a grid, especially in
islanded microgrids [23]. This is yet another challenge for micro-
grid operation where there is poor market organization. To better
understand the theory behind it, please consider the power flow
equations given in (1) and (2):
Urec ×sin ı=X×P−R×Q
Usen
(1)
Usen −Urec ×cos ı=R×P+X×Q
Usen
(2)
where X, line reactance; R, line resistance; P, real power; Q, reactive
power; Usen, sending end voltage magnitude; Urec , receiving end
voltage magnitude and ı, phase difference between sending and
receiving ends.
For high voltage lines XR,Rmay be neglected, and ıis very
small. So assumptions can be made that sin ı=ıand cosı= 1. The
equations can then be rewritten as follows:
ı=X×P
Usen ×Urec
(3)
Usen −Urec =X×Q
Usen
(4)
It can be seen from (3) and (4) that the power angle mostly depends
on the real power whereas the voltage difference mostly depends
on reactive power. In other words, by controlling Pand Q, the fre-
quency and the voltage of the grid might be set.
f−f0=−kp(P−P0) (5)
Usen −U0=−kq(Q−Q0) (6)
where f, frequency; f0, rated frequency; U0, rated grid voltage; P0,
Q0, set real and reactive power of the inverter.
The infamous droop regulation Eqs. (5) and (6) are obtained by
associating a permissible error on the voltage and frequency values
with real and reactive power values. When there is a variation in the
frequency or the voltage value, the DGs adjust their real and reac-
tive output values accordingly. The adjustment should be done and
the power mismatch should be recovered immediately to maintain
the system frequency. Consequently, storage devices are needed
for a successful droop control implementation [21,24]. The power
sharing of the generators are mostly indirectly proportional to their
capacities. The load sharing coefficient mPis based on an equitable
load share in the form:
mP1×P1=mP2×P2=mPi ×Pi=constant (k),(7)
where Piis the rated output power of ith DG [25]. It is worthy
to note that this scheduling scheme does not take technical or
financial aspects into account. That is to say, DG’s ability to pro-
vide sufficient level of reserves or the economical outcome is not
considered in determining new droop operating points. In tackling
this issue, [26] acknowledges the economic importance of selective
sharing amongst DGs and proposes using four different droop coef-
ficients based on production cost or available reserves. Although
this improves the droop performance, it has stability concerns since
the increase of droop coefficients beyond a limit triggers instability
[27].
Another shortcoming of droop control presents itself when
implemented in low voltage networks. For low voltage wires, XR
assumption does not hold [28] and classical droop equations are
insufficient. The control system proposed in [29] considers the
effect of R and uses improved droop equations. Another improve-
ment presented in [37] is controlling not only the fundamental
component of the voltage but also its harmonics. Systems proposed
in [25,29,30] have droop control for harmonic components and thus
can feed non-linear loads. In order to address the versatile nature
of microgrids, the central controller in [31] calculates droop lines
for every different condition, shown in Fig. 2, and updates them.
This gives the controller the flexibility it needs to respond varying
operation modes.
The conventional droop method employs low-pass filters to cal-
culate average Pand Qvalues, hence it has a slow dynamic response
[32]. A wireless controller was proposed in [33] to enhance the
dynamic response with integral–derivative term addition. Further-
more, the power sharing is degraded when the sum of output and
line impedances are not balanced. This might be solved with inter-
face inductors or with control loops that emulate lossless resistors
and reactors as in [34].
There are alternative drooping schemes available in the liter-
ature. For example, [35] proposes a control scheme that droops
microgrid output voltage with real power (P) and system fre-
quency (f) with reactive power (Q). This is in stark contrast with
conventional load sharing schemes [36]. However, this system
can only be used for only one voltage source inverter (VSI) and

4034 T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041
Fig. 2. Different droop lines and operating points in [31].
another scheme called voltage-power droop/frequency-reactive
power boost (VPD/FQB) control is proposed in [37]. This scheme
also droops the voltage reference against real power output and
boosts the frequency with reactive power output. In addition to
that, it can be run in parallel with other VSIs and the utility grid
itself.
Other controls include controlling resistive output impedance of
the inverters and realizing load-sharing with wireless load-sharing
controllers [38,39] or running all inverters in grid-following mode
even if the microgrid is islanded [40]. The system in [24] has two
level control design which implements droop control for system
level multiple DG coordination controllers and L1 control theory
for device-level inverter controllers. This realizes a more reliable,
robust and stable microgrid control.
3.2. Protection
3.2.1. Islanding
Islanding is a situation that occurs when part of a network is
disconnected from the utility grid but is still energized by one or
more DGs [41]. In conventional distribution networks, in the case
of a fault in the transmission system, the distribution network does
not receive any power. However, with the introduction of DGs,
this presumption is not valid any more [42]. This phenomenon of
unintentional islanding may cause some of the following issues:
•Safety issues since a portion of the system remains energized
when it is not expected;
•Loss of control over system frequency and voltage levels;
•Insufficient grounding of the islanded network over DG intercon-
nection;
•Out of phase re-closure problems which may damage the equip-
ment [43]
Because of these issues, a DG unit should pass either one of the
two anti-islanding standard tests, UL 1741 [44] or IEEE 1547 [45]
before it can be installed. Moreover, almost all utilities require DG
units to be disconnected from the grid as soon as possible in case
of islanding. IEEE 929-1988 standard [46] requires the disconnec-
tion of DG units once the microgrid is islanded. The IEEE 1547-2003
standard on the other hand requires all DGs to be shut down after a
maximum delay of 2 s once islanding is detected. In order to achieve
this, there must be a fast and reliable islanding detection method.
There are various kinds of islanding detection methods in the lit-
erature which are studied under two sub-headings; remote and
local techniques. The local techniques include three types which
are: passive, active and hybrid detection methods.
Passive detection methods measure some local parameters such
as voltage, frequency, total harmonic distortion [42], rate of change
of power signal [47], rate of change of frequency over power. By
comparing these values with pre-determined thresholds, island-
ing is detected. Passive detection methods detect islanding very
fast and without disturbing the system. However, should the mis-
match be small they become unreliable. The challenge of setting
suitable thresholds and the large non-detection zone are the major
drawbacks [10] of passive methods.
Active detection methods try to address the shortcomings of
passive methods by intentionally introducing perturbations into
the system and detecting islanding according to the response of
the system [48]. In this way, it is possible to detect islanding even
if the power mismatch is very small hence a much smaller non-
detection zone. The downside of active methods is that they are
not as fast as passive methods and they degrade the power quality
with the injected perturbations.
Hybrid detection methods are the combination of passive and
active methods. The system is constantly monitored with passive
methods and if islanding is suspected by the passive methods then
the active methods are implemented. This combines smaller non-
detection of active methods whereas unnecessary disturbance in
the system is prevented by passive methods [42]. The drawback is
long detection time since both of the methods are implemented.
There are communication based approaches to islanding detec-
tion with an effort to overcome the problems posed by active
and passive detection methods. These methods are based on a
direct communication between the utility and DGs in a micro-
grid [49]. Islanding is caused by opening of a line circuit breaker
so it is proposed to use this event to detect islanding and exe-
cute necessary protection schemes. Some of the methods that
incorporate this principle are [50]: power line carrier (PLC) com-
munications, supervisory control and data acquisition (SCADA),
intertripping/disconnection signal. These methods do not suffer
from non-detection zones and they do not affect the power qual-
ity by de-stabilizing the system. The only set back is the additional
cost of communication systems and their reliability. According to
[51], PLC communication is a practical, reliable and cost-effective
method to realize islanding detection. In this method, a signal is
injected to bus bars at substations and DGs operate as long as
they receive the signal. Should a tripping occur and the micro-
grid become islanded, the signal vanishes and DGs are shut down
automatically [49].
3.3. Fault current protection
Integration of DGs to the grid and the increasing penetration
level changes fault current level and direction in networks [52]. Tra-
ditional protection schemes shall be re-designed in order to meet
these fundamental changes. Also microgrids have dynamic struc-
tures, i.e. several DGs and loads connect/disconnect at any instant,
and various operating modes. Fault current levels may vary for all
these situations and current protection designs are not sufficient
to tackle these issues. Some of the prominent protections issues
are: short circuit power, fault current level and direction, device
discrimination, reduction in reach of over-current relays, nuisance
tripping, protection blinding etc. [53,54].
In conventional power networks, the power flows from higher
voltage levels to lower voltage levels. In case of a fault, the short
circuit current decreases as the distance from the source increases.
However, DGs change these concepts as there may be power flow
from the microgrid to the utility if the local generation exceeds
local consumption. As Fig. 3 shows the fault current may grow
downstream with the contribution of fault currents from DGs.
Fault contribution from a single DG may not be large; however
the combined effect of several DGs can reach significant levels [55].

T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041 4035
Fig. 3. Grid and DG fault current contribution in MG.
Estimating this contribution is not an easy task as it highly depends
on the type of the DG. Another problem is that DGs with PE inter-
face do not supply short circuit currents sufficient enough to trigger
protective devices [56]. If the fault current settings of the relays are
decreased so that they sense fault currents from inverter-interfaced
DGs, then undesired trippings and nuisance trippings will occur due
to transients in the system [57]. The position of a fault with respect
to DG also affects the operation of protection system. It may be that
the relay is only measuring some part of the actual fault current as
there may be several fault current paths towards the fault [58].
Moreover, fault currents are different for grid-connected and
islanded modes of operation. In the former case, the utility grid
contributes to the fault current whereas the latter only includes the
fault currents from DGs. In an islanded grid, the storage devices,
intermittency of DGs such as solar panels or wind turbines, load
types and their power demand affect the fault levels. The new pro-
tection system should be dynamic and respond to those changes in
the operation conditions. [59]
Furthermore, low voltage systems may have single phase loads
which will alter the balanced system parameters. DC microgrids
which have simpler connection and more efficient PE interfaces
need special attention [60]. Protection of microgrids against over
voltages [61] or utility voltage sags [62] are also amongst popular
research fields.
There are different solutions proposed in the literature for the
issues stated above. The use of anti-islanding frequency relays is
proposed in [63]. However, it is not realistic to assume that all relays
can be replaced [54], so the method in [64] can be implemented
where operating points of relays are calculated with modified par-
ticle swarm optimization. In another approach, instead of using
relays and circuit breakers blindly, some smart algorithms are
implemented to selectively operate relays and isolate the fault [65].
There are more ground-breaking systems which employ a devel-
oped communication system to follow the system parameters and
carry out necessary calculations [53,59]. Standard communication
protocols such as the IEC 61850 might be used in these systems as
in [66], however it is of concern that how the new fault currents
and fault levels shall be calculated for any change occurring in the
system. Current systems use some sort of database or event table
to search the current status and take pre-determined precautions

4036 T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041
Fig. 4. Control levels of the microgrid [70].
[53,64]. However, since microgrids are designed to accommodate
new generators and loads, these schemes are not practical. Some
sort of algorithm which manages to adjust the protection scheme to
the new state of microgrid is direly needed. Machine learning, arti-
ficial intelligence or fuzzy logic algorithms are the trivial candidates
as a foundation for such adaptive-automatic microgrid protection
schemes. It is also discussed in the literature that having extensive
communication networks would make the microgrid more expen-
sive and less reliable. Moreover, in a distributed energy system,
components may be located far away which makes communication
difficult [27].
3.4. Microgrid energy management system (MEMS)
In order to execute the duties described so far, a microgrid
utilizes a microgrid management system. This system ensures
that different components of the microgrid are managed to serve
towards a certain objective [67]. It typically comprises of three
hierarchical levels of control as shown in Fig. 4.
Microgrid central controller (MGCC) acts as an interface
between the microgrid and the outside world. It communicates
with distribution network operator (DNO) and market operator
(MO) and optimizes microgrid operation through local controllers
(LCs). It ensures that in a network where more than one microgrid
exists, microgrids work in harmony to sustain a reliable and safe
operation. LCs are responsible to control components of a microgrid
such as distributed generators, storage devices, loads or protec-
tion equipment. MGCC manages LCs and updates their operation
modes and points in parallel with the events occurring in the net-
work and/or the microgrid. Once the updates are received, LCs
mostly behave autonomously until a new instruction is received
from MGCC [68]. Based on the decision making scheme, the control
systems (also known as supervisory systems [18]) are categorized
as centralized and decentralized microgrid control [69].
Centralized control strives to maximize the local production
according to market prices. For this to occur, there is a two-way
communication between the MGCC and each LC [18]. Starting from
this point there is a new concept called virtual power plants (VPPs)
where distinct entities inside a microgrid are controlled by a central
unit to act as a power plant [71].
Conversely, in decentralized control, the larger part of the
decision making is in the hands of LCs. They act in a smart fash-
ion and communicate with each other to increase the revenue
and the performance of the microgrid. The most popular way to
design this intelligent system, which is composed of smaller less
intelligent components, is by using multi-agent systems [70].
Multi-agent systems are already proposed for the protection, sta-
bilization, restoration of large power systems.
There are on-going research projects being carried out to inves-
tigate alternative control systems. A control system based on ‘Game
Theory’ is proposed in [72]. In this approach, power electronics
based inverters are treated as variable impedance loads and every
generator in the microgrid is taken as a player. The stabilization
and the control of the system may be achieved through real-time
or turn-based games. For different scenarios, different games such
as “Maximum Power Game” or “Microgrid Stability Game” can be
played. Depending on the level of communication, games might be
cooperative and non-cooperative.
In an another study, the “Ant Colony Optimization” algorithm
is proposed to solve constraint satisfaction problem for microgrids
[73]. Ant colony optimization is a nature-inspired artificial intel-
ligence technique where several ant colonies cooperate to solve a
problem. In microgrid applications, distributed generators, storage
devices and loads are represented by a variable. The domain of each
variable represents the quantized operating points at which the
generator or load could be commanded to operate [73]. By stipu-
lating constrains, ants walk on the construction graphs and each
variable will be assigned an operating point. The touring of the
colonies continue until the solution is attained for each and every
variable.
By implementing a central control unit in microgrids, different
objectives can be realized. Minimizing system disturbances, max-
imizing efficiency, meeting power demands from local resources,
stabilizing the system are to name a few. This formulation outlines
the difficulty of the control task. With competing objectives and
highly variable parameters, a real-time power management system
which is both robust and flexible is needed.
4. Examples around the world
There are various microgrid implementations or active exper-
iments worldwide to understand the operation of microgrids in a
better sense. Different technologies and topologies have been stud-
ied for different purposes. Some of the experiments are run for
purely R&D purposes whereas others are deployed on islands or
isolated/distant grids. Since the microgrid concept is very versa-
tile, the experiment conditions and the objectives have a very wide
span.
4.1. European union (EU)
The level of climate change awareness in the EU is very high and
there are certain targets that need to be achieved by the member
states by 2020. There are various directives passed by the European
Parliament such as 2001/77/EC, 2003/30/EC and 2006/32/EC. These
directives stipulate that the carbon emissions shall be reduced by
certain amounts, the share of renewable energies in the energy
market shall be increased and the energy consumption shall be
reduced by increasing energy efficiency [74]. Accordingly, there are
incentives from the EU and several on-going projects in member
states.
The first project funded by the EU was the “Microgrids Project”
and it was undertaken by a consortium led by National Technical
University of Athens (NTUA). The objective was to investigate the
dynamics of DGs in microgrids and develop strategies for a num-
ber of issues such as control algorithms, protection schemes, black
start strategies as well as definition of DG interface response and
intelligence requirements [5]. A pilot installation was realized on
the Kythnos Island, Greece. A comprehensive study on microgrid
control methods was performed in ISET microgrid, Germany. The

T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041 4037
continuation of the project was “More Microgrids Project” again
undertaken by a consortium led by NTUA. This project was executed
to study alternative methods, strategies along with universalization
and plug-and-play concepts. The demonstration site is an ecological
estate in Mannheim–Wallstadt, Germany [75].
Other implementations with smaller scales include the Labein
Microgrid in Spain, Frielas Feeder in Portugal, CESI Microgrid in
Italy, Continuon Holiday Camp Microgrid in the Netherlands, Am
Steinwag Settlement in Germany [5,8,75].
4.2. Japan
Japan is committed to utilize RE systems, however this puts
the country’s well-earned high power quality reputation in jeop-
ardy. The RE systems used in Japan are mostly wind turbines and
PV systems, intermittent nature of which is an additional set-
back. Microgrid’s ability to address these problems motivated the
projects in Japan and hence the country has the most microgrid
implementation projects worldwide [76]. Most of the projects are
funded by the New Energy and Industrial Technology Development
Organization (NEDO). Three demonstrations sites were installed in
2003 under NEDO’s Power grid with renewable Energy Resources
Project.
The first project started operation in 2005 World Exposition in
Aichi although it is removed to Tokoname City near Nagoya in 2006.
This system uses fuel cells, PV panels and NaS battery storage sys-
tem. This microgrid is used to feed some major pavilions and it
was put to test twice for independent operation in 2005 and 2007.
Although the first test revealed some deficiencies in controlling the
voltage and frequency, the second experiment was more successful
[76]. The second demonstration site is in Kyotango where a biogas
plant is connected to two PV systems and a small wind turbine. This
network operates as a VPP and interestingly instead of the latest
technology the communication is realized over conventional infor-
mation networks such as ISDN and ADSL [5,8]. The third project in
Hachinohe is being undertaken by the Mitsubishi Research Institute
and Mitsubishi Electric [77]. This system has its private distribu-
tion line and consists of PV systems, wind turbines, gas engines
and storage. The management scheme developed here ensures sta-
bility and meets building demands. An additional project has been
started by NEDO in Sendai city where four levels of customer power
will be studied. The system will have power quality backup system
in order to reduce interruptions and voltage drops. This project is
aimed at studying the possibility of supplying different service lev-
els to customers in the same area. The system has enhanced the
power quality since it was put into action in 2007 [6].
There are several private microgrid research projects. For exam-
ple, the Shimizsu Microgrid is being developed by the Shimuzu
Corporation with the cooperation of the University of Tokyo to
develop an optimum operation and control system. Tokyo Gas on
the other hand, again partnering with the University of Tokyo, is try-
ing to develop an integrated DG control through simulations and
experiments at its facility in Yokohama [6]. Crossing boundaries,
Mitsubishi Corporation has installed a small grid in Hsinchiang,
China and it can be supplied by distribution network, PV systems,
battery storage and genset operation [78].
4.3. Korea
Korea’s first and only pilot project is being developed by the
Korean Energy Research Institute (KERI). The test system is very
comprehensive as it includes several types of DGs such as PV sim-
ulator, fuel cells, diesel generators, wind turbine simulator along
with significant and non-significant loads. The network is equipped
with storage and power quality devices. An energy management
system is being implemented which even takes weather conditions
Fig. 5. CERTS microgrid [8].
into account and communicates with the components through a
gateway. Being equipped with rich mixture of components, the
KERI microgrid is aimed at testing and studying almost all aspects
of microgrids. The whole project was implemented in two phases
where in the first phase, the microgrid was kept as a 100 kW class
plant and in the second phase it was extended for further studies
[79].
Jeju Island is receiving increasing attention due to its immense
potential for RE resources. The total wind power energy in Jeju
was only 19 MW in 2006 and it has increased to 230 MW in 2009
whereas several fuel cell plants are either constructed or planned
on the island [80]. Jeju Island and similar Korean islands are prime
candidates for microgrid implementations in Korea in the future
[81].
4.4. North America
CERTS (the Consortium for Electric Reliability Technology Solu-
tions), shown in Fig. 5 [8], is the most well-known of U.S. microgrids.
It is a collaboration between AEP, TECOGEN, Northern Power
Systems, S&C Electric Co, Sandia National Laboratories and the Uni-
versity of Wisconsin [8]. It consists of several DGs and a thyristor
based switch to allow isolation from the grid. The main objective of
this research was to facilitate easy connection of small distributed
generators to the network. As a result, three advanced concepts,
also referred to as the CERTS microgrid concept, have been devel-
oped and demonstrated to decrease the field engineering work on
microgrids. These concepts can be listed as a method to ensure
automatic and seamless transition between grid connected and
islanded modes, a protection method inside the microgrids which
does not depend on high fault currents and a microgrid control
scheme to stabilize system frequency and voltage without utilizing
high speed communication [82].
Also, two software tools, which are required for microgrid
deployment, are being developed in relation with CERTS project.
These are ␮grid Analysis tool (␮Grid) developed by the Georgia
Institute of Technology and the Distributed Energy Resources Cus-
tomer Adoption Model (DER-CAM) in use at the Berkeley Lab [5].
There are other implementations going on in Mad River Wait-
sfield by Northern Power Systems, British Colombia Institute of
Technology Microgrid and the General Electric Microgrid [8]. These
systems are currently at R&D stage and the objective is to design
control and protection strategies for different types of microgrids.
4.5. Australia
Currently, there are no microgrid pilot projects in Australia
but there is a large potential, and with the Government’s incen-
tives, extensive research on distributed energy and microgrids has
recently started. Australia is a very vast continent-country which

4038 T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041
Fig. 6. IEEE SCC21 1547 series of interconnection standards.
has many isolated communities. It is a new trend in the Australian
Electricity Market to utilize RE resources for local power genera-
tion and local consumption. Some of these communities are not
only far away but also lack access roads to the weather conditions.
Utilization of DGs and implementation of microgrids for that pur-
pose will ease the transmission and distribution burden on service
providers. The ultimate goal is to realize the design and provision
of power station infrastructure which will be optimized to suit the
unmanned status of the power stations. The Yungngora and Kalum-
buru communities in Western Australia [83], and the Windorah
community in Queensland [84] are examples of such distant com-
munities which are candidates for microgrid operations. In addition
to this, some energy companies are trying to operate microgrids on
islands such as Thursday Island in Queensland [84] and King Island
in Tasmania [16].
5. Standards and universalization
DG offers numerous potential benefits including reduced elec-
tric line losses, reduced transmission and distribution congestion.
Improved grid asset utilization, improved grid reliability, cleaner
energy, voltage support, uninterrupted service can further be added
to the list. However, this needs proper integration of DGs to the
electric power systems since these systems are not designed to
accommodate active generation and storage at the distribution
level [85]. The evolution of DG was not pioneered by a single orga-
nization or a company rather every institution runs its own R&D
project. As a result, there are many different types of DGs, intercon-
nections, electronics interfaces. This makes it incredibly difficult
to draft a set of rules/guidelines for DGs interconnection and uti-
lization. Since microgrids are formed with DGs, the very concept
of a microgrid and its wide acceptance are also paralyzed by this
fact. In an effort to tackle this issue, there are several standard-
ization and universalization works performed by several bodies.
The ultimate objective is to standardize certain aspects of DGs
and microgrids while there is no technology or design constraint
stipulated to hinder the versatility of these concepts. Some of these
standards, shown in Fig. 5, are in force while others are still in
drafting phase.
The first standard prepared was about the rules governing the
connection of DGs to the electric power system. For a DG to be
connected to the grid, it has to conform with requirements of IEEE
Standard 1547.1 [45]. Only for this purpose, an alternative standard
UL 1741 [44] can also be used. Both standards require that in the
case of islanding, all DGs shall be disconnected from the islanded
microgrid. This does not take full advantage of DGs nor is it possible
to conform with this requirement if islanded operation is desired
[69]. For this reason, another part to the IEEE Standard 1547, namely
1547.4, is being drafted which focuses on integration of islanded
systems with the utility. The part 1547.4 is being treated as one
of the fundamental standards to play a key role for microgrid stan-
dardization as it covers vital planning and operating aspects such as
impacts of voltage, frequency, power quality, protection schemes
and modifications, the characteristics of the DER, reserve margins,
and load shedding. The part 1547.3 is about monitoring and com-
munication of DGs. Its purpose is to facilitate the interoperability
of DGs in an interconnected system.
IEEE Standard Coordinating Committee 21 (SCC21) is well aware
of the developments in the electricity sector and it constantly
supports the development of new standard drafts. Initially, IEEE
1547 standard covered DGs with capacity less than 10 MVA [85].
However, with the recent developments in technology, there are
systems with larger capacities. The draft standard 1547.5 is aimed
at preparing guidelines for such systems which are not covered by
the IEEE 1547.1. The draft standard 1547.6 considers interconnec-
tion of distribution secondary network system types of area electric
power systems (Area EPS) with DG. 1547.7 is a very significant
step towards standardization and universalization in microgrids
and DGs. It covers the necessary methodology, testing steps and
aspects to assess the impact of a DG on the system. This study will
be helpful for network operators, contractors, and regulatory bod-
ies to understand the impact of a particular DG on the network after
connection.
In a larger scale than that of the 1547.7 draft, a “Microgrid Cit-
izenship Tool” is proposed in [1] to evaluate how a microgrid will
appear to the grid. This tool assesses a microgrid’s good citizenship
level based on the three key characteristics: generation capacity,
installed storage and load. A microgrid is classified as a good citi-
zen if it contributes to the performance of the grid with a consistent
profile whereas a microgrid with high transient and unpredictable
variations will represent a bad citizen. Such a method may be uti-
lized in electricity market as a useful tool to estimate the impact
of a microgrid on the network. If a standard methodology is devel-
oped, such a concept will be an indispensible part of standardized
microgrid assessment and planning process.
National Renewable Energy Laboratory (NREL) conducted
research on interconnection, grid effects and tariff design for DERs
and one of the areas was “Advanced Universal Interconnection
Technology”. It is believed that universalized interconnection DGs
will facilitate the connection with EPS [67]. The objective is to
design a modular interface device which will respond the power
electronics requirements of any DG system and provide intercon-
nection interface in a safe, reliable and cost effective manner. A
prototype was developed by the Northern Power Systems (NPS)
which manages power management, conditioning and relaying
functions with a DSP-based architecture. It was designed to be
compatible with different circuit breakers, switching technologies,
RE based DGs and conventional generators [86]. The research out-
comes were promising and a new design with cost reduction as the
primary objective is planned.
As outlined in the preceding sections, there is a growing interest
in extensive communication for network management, control and

T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041 4039
Fig. 7. A centralized protection scheme with communication.
protection purposes. However, there is no consensus in the liter-
ature about which communication protocol shall be used in these
systems. It is known that communication devices and systems will
add to the complexity of microgrids and this may constitute some
problems. For this reason, worldwide collaboration is required in
identifying a universal or standardized communication protocol
that shall be used in microgrids for DGs, storage and protection
devices to tackle the arising problems. IEC 61850 was released
in 2003 for the first time for a communication within a substa-
tion automation system, yet it has been used for other purposes
[87]. With a vision of controlling DGs it has been extended. The
first release, IEC 61400-25 was about the communication in wind
power [66]. Two more extensions IEC 61850-7-410 on hydroelec-
tric power plants and IEC 61850-7-420 on DERs logical nodes have
also been published. These two extensions might be used in design-
ing the communication system of DGs in detail.
6. Future work and possible research areas
Microgrid is a very exciting research field in the power engi-
neering and it has many different aspects which are considered as
individual research fields in their own. Furthermore, microgrids are
just like an intersection zone of several concepts such as network
operations, protection, power electronics, distributed generation,
renewable energy etc. Consequently, it is not a surprise to see
more researchers focusing on microgrids and more publications
appearing in the literature. It is safe to assume that this field will
continue to expand in the future. When the literature is sifted, it
is noticed that considerable amount of attention has been given
to some aspects of the microgrid, while others are left untouched
either because they simply did not exist before or gained impor-
tance recently. Whatever the reason, the possible future research
areas are mostly under these topics.
Microgrid energy management systems (MEMS) which are
aimed at controlling the microgrid in a holistic sense are fairly
new in the literature. Although the very concept of MEMS is pro-
posed some time ago, real design and implementations of MEMS
are crucial to get more knowledge and experience in the field. Con-
ceptual designs look appealing but implementations or simulations
on models shall be carried out to see the real side of the picture.
Protection of microgrids against fault currents and design of
new protection schemes are also promising research fields. Sim-
ilar to above, there are conceptual designs or proposed opinions
in the literature while it is hard to find a new protection design
in a microgrid which responds to the needs of microgrid opera-
tion modes and components. There are proposals to change the
relay types used or update their operating currents regularly or re-
designing the protection techniques from scratch. Fig. 6 shows a
new protection system proposed by the authors.
This protection scheme employs communication between a
central protection unit and protection devices and DGs. According
to the conceptual design, operating points of relays are continu-
ously updated in parallel with the changes occurring in the system
(Fig. 7).
Despite of these several proposals, there is lack of implemen-
tation in the literature. All of these alternative protection schemes
proposed in the literature shall be implemented/modeled to see the
feasibility and the performance of the proposals.
Unlike traditional utility networks, it is highly probable that
microgrids will incorporate high speed communication between
the components, operators, equipments etc. However, there is
obscurity on how to realize the communication, what type of an

4040 T.S. Ustun et al. / Renewable and Sustainable Energy Reviews 15 (2011) 4030–4041
infrastructure is needed, what types of protocols shall be used.
Doubtlessly, some additional work is needed to clarify these details
as well.
For microgrids to be embraced rapidly and implemented easily,
there is a need for systematic standardization and universalization
in all aspects of this field. This would not only help in bringing
different organizations together but also encourage more peo-
ple to accept transition to microgrid. If standard procedures are
implemented and universalized components/interfaces are utilized
instead of re-inventing the wheel for every single microgrid project,
past experiences can easily be put into practice.
7. Conclusion
The world we live in today is being troubled by the concerns
on global warming, pollution and CO2emissions. RE systems offer
means of generating cleaner and sustainable energy. However,
there are lots of challenges that must be tackled so that RE resources
could be utilized to their full potential. RE resources are mostly
dispersed and different generation approaches should be used to
harvest the maximum potential out of those sources. This is con-
tradictory to the traditional concept of central generation and
distribution over large distances. For this reason, existing grids are
not entirely compatible for excessive integration of DG units. On top
of that, micro-scale implementation of known generation plants
such as micro hydroelectric power plants, diesel generators and
etc., have similar aspects since they are also distributed and their
generation capacities are much smaller than their traditional giant
counterparts.
In order to achieve a cleaner, reliable, and secure power genera-
tion, transmission and distribution system, the various challenges
brought about by this new grid structure and management system
shall be tackled with similar research projects. The outputs of stud-
ies on microgrids will aid in the development of secure, reliable, and
stable real-life networks with greater penetration of RE sources.
This will aid in achieving a more reliable, secure and cleaner energy
without compromising from environment protection and similar
concepts.
This paper has presented the current status of the literature on
microgrid related research. It has described the microgrid concept
and the motivations behind its utilization then outlined the dif-
ferent research fields under this heading. The undergoing research
work was summarized to give an overall insight about the current
level of the knowledge. Finally, possible research areas have been
proposed which are essential for future development.
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Taha Selim USTUN received his B.E. degree in Electrical
and Electronics Engineering from Middle East Technical
University, Turkey in 2007 and the Master of Engineer-
ing Science degree from University of Malaya, Malaysia
in 2009. He is currently a PhD student in Victoria Univer-
sity, Melbourne. His research interests are power systems,
power electronics, distributed generation and microgrids.
Cagil Ozansoy received his B.Eng. degree in Electrical
and Electronic Engineering (Hons.) from Victoria Univer-
sity, Melbourne, Australia in 2002. In 2006, he completed
his PhD research degree in the area of power system
communications. He is now working as a lecturer and
researcher in the School of Engineering and Science, Vic-
toria University. His major teaching and research focus is
on electrical engineering, renewable energy technologies,
energy storage, and distributed generation. He has suc-
cessfully carried out and supervised many sustainability
related studies in collaboration with local governments in
the past. These include the assessment of solar and wind
electric energy potential in Melbourne’s west. He has over
25 publications detailing his work and contributions to knowledge.
Aladin Zayegh received his BE degree in Electrical Engi-
neering from Aleppo University in 1970 and PhD degree
from Claude Bernard University, Lyon, France in 1979.
He has held lecturing position at several Universities and
since 1991 he has been at Victoria University, Melbourne,
Australia. He has been head of School and research direc-
tor where he has conducted research, supervised several
PhD students and published more than 250 Papers in peer
reviewed International Conferences and Journals. He is
currently Associate Professor at the School of Engineering
and Science, Faculty of Health, Engineering and Science
at Victoria University, Melbourne, Australia. His research
interest includes Renewable energy, Embedded Systems,
instrumentation, data acquisition and interfacing, Sensors and Microelectronics for
Biomedical applications.