Abstract-- Microgrids are miniature versions of traditional grids. They comprise of distributed energy resources like wind turbine, Photovoltaic, Storage devices like batteries and renewable resources of power systems. They can operate in two modes, viz. Connected mode, where they are connected to the Main grid; and island mode, where they are isolated from Main grid. While microgrids offer advantages like improved reliability, stability,and efficiency; their implementation poses different technical challenges specifically for protection of microgrid. It is essential to protect a micro grid in both the grid-connected and the island mode of operation against all different types of faults. This paper gives literature review for different challenges and protection of techniques for microgrid protection.

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Abstract-- Microgrids are miniature versions of traditional grids.
They comprise of distributed energy resources like wind turbine,
Photovoltaic, Storage devices like batteries and renewable
resources of power systems. They can operate in two modes, viz.
Connected mode, where they are connected to the Main grid; and
island mode, where they are isolated from Main grid. While
microgrids offer advantages like improved reliability, stability,
and efficiency; their implementation poses different technical
challenges specifically for protection of microgrid. It is essential
to protect a micro grid in both the grid-connected and the island
mode of operation against all different types of faults. This paper
gives literature review for different challenges and protection of
techniques for microgrid protection.
Index Terms-- Distributed energy resources; Distributed
generation; Microgrids; Micro Grid Central Control; Renewable
energy sources; Power Electronics
I. INTRODUCTION
ITH the development of renewable energy sources,
energy storage devices and distributed generation (DG)
the microgrid has attracted a lot attention. A micro grid
consists of a low to medium voltage network of small load
clusters with DG sources and controllable loads. Microgrids
can operate in an island mode or can be connected to the main
grid system. If a micro grid is connected to the main grid, it is
seen as a single aggregate load or source; when connected in
island mode it caters to considerably smaller loads and energy
sources. One of the potential advantages of a microgrid is that
it could provide a more reliable supply to customers by is
landing from the system in the event of a major disturbance.
As a distributed generator placed/situated close to the load, it
has the advantage of reducing transmission losses and
preventing network congestions [1,2]. Thus, distributed
energy resources can enhance system reliability and stability.
Micro grids offer advantages, such as: Improved energy
efficiency, improve power quality and reliability, minimized
overall energy consumption, reduced green- house gases and
pollutant emissions and cost efficient electricity infrastructure
replacement [1-4].
The paper is organized as follows. A complete review of
protection challenges for microgrids in Section II. In Section
III, Different protection schemes are discussed in depth which
also discusses limitations of protection techniques. The
experiment test bed setups are discussed in Section IV and
future development in the area of microgrid protection in
Section V. Finally, conclusions are drawn in Section VI.
II. KEY PROTECTION CHALLENGES FOR MICROGRID SYSTEM
A. Fault current level modification
Connection of a single large DER unit or a large number of
small DER units that use synchronous or induction generators
will alter the fault current level (FCL) as both types of
generator contribute to it. This change in FCL can disturb
coordination. A different scenario results when DER units are
connected to the Microgrid; due to the PE interface of these
interfaces (i.e. Inverters), the fault current is limited
electronically to typically twice the load current or even less
and hence an independent relay will not be able to distinguish
between normal operation and a fault condition. The severity
is more when there is hardly any increase in phase current in
case of a fault or failure. As fault current is not clearly
distinguishable from the operational current, some of the over
current (OC) relays will not trip; others that might respond
would take many seconds instead of responding in a fraction
of a second. The undetected fault situation can lead to high
voltages despite lower fault currents. Moreover, if the fault
remains undetected for long, it can spread out in the system
and cause damage to equipment [2].
The position of a fault point relative to a DG unit and a MV
system transformer also affects the operation of the protection
system. When a fault occurs downstream of the point of
common coupling (PCC), both the MV distribution system
and the DER will contribute to the fault current as shown in
Figure1. But the relay situated upstream of the DG will only
measure the fault current supplied by the upstream source. As
this is only one part of the actual fault current, the relays,
especially the ones with inverse time characteristics, may not
function properly resulting in coordination problems. When
the fault is between the MV bus and the Microgrid, then the
fault current from MV bus would not change significantly as,
generally a DER unit is contribution is quite small.
B. Device discrimination
In traditional power systems that have a generation source at
one end of the network, as the distance of the fault point from
the source increases, the fault current decreases [9]. This is
due to the increase of the impedance in proportion to the
distance from the source. This phenomenon is used for
discrimination of devices that use fault current magnitude. But
in case of an islanded microgrid with DER units, as the
maximum fault current is limited, so the fault level at
locations along the feeder will be almost constant. Hence, the
traditional current based discrimination strategies would not
Comprehensive Review of Protection Strategies
for Microgrids
H.M.Parikh, Student Member, IEEE, and W. L. Lee, Fellow, IEEE
W

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work. New device discrimination strategies are therefore
required to protect the system effectively.
C. Reduction in reach of impedance relays
The reach of an impedance relay is the maximum fault
distance that causes the relay to trigger in a certain impedance
zone, or in a certain amount of time. This maximum distance
corresponds to maximum fault impedance or a minimum fault
current that is detected [5]. In case of a fault that occurs
downstream of the bus where DER units are connected to the
utility network, the impedance measured by an upstream relay
will be higher than the real fault impedance (as seen from the
relay). This is equivalent to an apparently increased fault
distance and is due to increased voltage resulting from an
additional in feed at the common bus. As a consequence, this
will affect the grading of the relays and will cause delayed
triggering or no triggering at all. This phenomenon is defined
as under reaching of relay.
D. Bi-directionality and Reserve power flow
The power flow changes its direction in case of when local
generation exceeds the local consumption [2]. The reverse
power flow may hinder the working of directional relays as,
traditionally; radial distribution networks are designed for
unidirectional power flow. Moreover, reverse power flow also
means a reverse voltage gradient along a radial feeder. This
can cause power quality problems, result in violation of
voltage limits, and cause increased equipment voltage stress.
E. Sympathetic tripping
This phenomenon can occur due to unnecessary operation of a
protective device for faults in an outside zone, i.e., a zone that
is outside its jurisdiction of operation [3-6]. An unexpected
contribution from a DER can lead to a situation when a
bidirectional relay operates along with another relay, which
actually sees the fault, thus resulting in malfunctioning of the
protection scheme.
F. Islanding
The DER can create severe problems when a part of the DN is
islanded. This phenomenon is described as Loss of Mains
(LOM) or Loss of Grid (LOG). In case of LOM, the utility
supply neither controls the voltage nor the frequency. In most
of the cases, islanding is due to a fault in the network. If the
embedded generator continues supplying power despite the
disconnection of the utility, the fault might persist as the fault
will be fed by the DG [13]. The voltage magnitude gets out of
control in an islanded network as most of small embedded
generators and grid interfaces are not equipped with voltage
control. This can lead to unexpected voltage levels in case of
island operation. Frequency instability may be another result
of the lack of voltage control that poses a risk to electric
machines and drives.
III. REVIEW OF PROTECTION SCHEMES FOR MICROGRIDS
One of the major challenges of micro grid protection system
is that it must respond to both island and grid connected faults
[1,2]. In the first case the protection system should isolate the
smallest part of the micro grid when the fault occurs inside
(F3 and F4 in Fig.1) microgrid. In the second case the
protection system should isolate the micro grid as rapidly as
necessary to protect the micro grid loads from the main grid
when fault occur on MV side of distribution system (F1 and
F2 in Fig.1).
Fig.1. Different fault scenarios in microgrid
When different type of distributed resource connected to
micro grid and Utility grid, the DER contribute fault current to
the system and the contribution level depends on distribution
resource type. To ensure safe operation of micro grid the
protection equipment should be updated accordingly. It also
point out the importance of the “3S” (sensitivity, selectivity
and speed) requirements for different cases, which provides a
basis for the design criteria for the microgrid protection
system.
A. Intelligent communication protection scheme
Conventional protection systems cannot give reliable
protection for DER units, because there is limited capacity to
supply fault current. The solution can be achieved by using
DER units which have high fault current capability. This
scheme requires use of faster communication system which
has higher cost to benefit ratio. [5]
B. Differential protection scheme
The conventional differential protection cannot give the
reliable protection. The protection scheme for micro grid with
PE interfaced DER units cannot differentiate between fault
current and an overload current, which results in nuisance
tripping when system is overloaded. For proper clearing of
fault in an islanded microgrid and to ensure selectivity, it is
important that different distributed generators should
effectively communicate with each other. Use of evolving
distribution system version of pilot wire line differential
scheme is required for protection [5, 6].
C. Inverter control design
A protection scheme for an islanded microgrid is heavily
dependent on the type of the DER units. The PE interface of
DER actively limits the available fault current from units. This
has been demonstrated in [6,7] where two different controllers
i.e. one using 'dqo' coordinates and the other using three-phase
(abc) coordinates, are employed to control a standalone four
leg inverter supplying a microgrid. In both cases, the fault
current is quite small but its magnitude is different. Thus,
selection of a controller can be helpful for the protection
system.

3. 3
D. Voltage based detection techniques
A protection scheme that combines conventional over current
characteristics and under voltage initiated directional fault
detection with definite time delays is proposed in [8]. A large
depression in network voltage cannot be used alone for
detecting low levels of fault current in a microgrid as voltage
depression would not have sufficient gradient to discriminate
the protection devices. So measurement of some other
parameters is recommended. It is mentioned in [8] that simple
device discrimination can be achieved by current direction
along with definite time delays. The duration of delays is
proposed to be set on the basis of sensitivity of loads or
generation to under voltage. For setting up adequate
discrimination paths, selecting different delays for forward
and reverse direction flow of the fault current is
recommended. This scheme looks sound but the use of
communication channels for coordinating protection with
control and automation schemes can complicate the things.
The authors of [9] propose various voltage detection methods
to protect networks with a low fault current. One of the
suggested methods make use of the Clarke and Park
transformations to transform a set of instantaneous three
phase utility voltages into a synchronously rotating two axis
coordinate system. The resultant voltage is compared with a
reference value to detect the presence of the disturbance. In
case of an unsymmetrical fault, the utility voltage 'dq'
components have a ripple on top of the DC term[10]. Fault
detection in case of low fault current networks can be
achieved by making use of voltage source components. It is
possible to calculate the values of voltage source components
for different types of faults since the theory of the
interconnection of equivalent sequence networks in the event
of a fault.
E. Protection based on symmetrical and differential current
components.
An islanded microgrid can be protected against Single Line-
to-Ground (SLG) and Line-to-Line (LL) faults with a
protection strategy that makes use of symmetrical current
components [11]. Based on these facts, a symmetric approach
for protection of a microgrid is proposed in [11]. This scheme
makes use of differential and zero-sequence current
components as a primary protection for SLG faults and
negative-sequence current component as a primary protection
against LL faults. It is recommended that a threshold should
be assigned to each of the symmetrical current components to
prevent the microgrid protection from operating under
unbalanced load conditions. This threshold should be selected
carefully to avoid any mal-operation of relays.
F. Adaptive protection scheme
The dynamic structure of micro grid and their various
operating conditions require the development of adaptive
protection strategies. Adaptive protection is as “an online
activity that modifies the preferred protective response to a
change in system conditions or requirements in a timely
manner by means of externally generated signals or control
action”.
Fig.2. Centralized adaptive protection system for microgrid
There is a microgrid central controller (MCC) and
communication system in addition to elements shown in
Figure 1. Communication electronics make each CB with an
integrated directional OC electronic trip unit (relay) capable of
exchanging information with MCC. By polling individual
relays the MCC can read data (electrical values, status) from
CBs and if necessary modify a subset of the relay settings
(tripping characteristics) on the fly without any resetting
protection needs.
Adaptive protection schemes are presented as a solution for
microgrid protection both in grid connected as well as in
islanded mode. In an islanded microgrid, the adaptive
protection strategy can be used by assigning different trip
settings for different levels of fault current, which in turn are
linked to different magnitudes of system voltage drops
resulting from disturbance in the system.
Fig.3. Base case trip curve and tripping sequence with directional over
current protection
Fig.4. Base case and modified trip curves and a tripping sequence

4. 4
As discussed earlier, in an islanded microgrid with DER units,
the fault in the system can result in severe voltage depression
in the entire network (due to low impedances within the
network). In such a scenario, selectivity can't be assured using
voltage measurement alone. A possible solution could be the
use of a voltage restrained over current technique as proposed
in [12]. The scheme is shown in Figure 4. A large depression
in voltage (which happens mostly in case of short circuit as
opposed to overload) will result in the selection of a lower
current threshold. This would effectively move the time-
current characteristic down and thus the tripping time would
be reduced. In contrast to this, tripping times would be longer
during overloads, as small voltage depression would not be
able to switch the scheme to the lower setting. Thus the
system would retain the longer time setting corresponding
with long-term characteristics.
This scheme although appears to be sound, it has its
drawbacks. It is not clear how the scheme will perform if there
is a little difference in magnitude of voltage depression
resulting from short circuit and overload conditions. The
scheme seems to make use of the principle of relays with
inverse time characteristics, i.e., the larger is the fault current,
the smaller would be the response time. Also it will suffer
from a long clearing time in case of faults that cause small
voltage drop, thus posing the risk of fault current spreading in
the entire network.
IV. EXPERIMENTAL MICROGRIDS AND MICROGRID TEST-BEDS
Microgrids are likely to play a key role in the evolution of
smart grids. They could become prototypes for smart-grid
sites of the future. There are many variations in adopting the
microgrid architecture and design. However, they are
implemented with common perspectives such as reliability
and optimal integration of DGs. Recent developments are
discussed in [13].
 The CERTS test bed – United States
 UW microgrid – United States
 Bronsbergen Holiday Park microgrid – Netherland
 The Residential Microgrid of Am Steinweg in Stutensee
German
 CESI RICERCA DER test microgrid – Italy
 Kythnos island microgrid – Greece
 University of Manchester microgrid energy storage
 Laboratory prototype – UK
 Laboratory scale microgrid – China
V. FUTURE WORK
 Identifying the dependable control strategies and utilizing
them accordingly to further improve the system reliability.
 It is important to research in more reliable, fast responding
islanding detection method for integrating to a microgrid
with MV grid.
 Critical transition period (from grid connected mode to
islanded operation mode) for stable operation of a
microgrid.
 The areas of transient stability performance, protection and
control strategies require implementation of new
technologies.
 Implementation of common standard design protocols and
operating guidelines for microgrids.
VI. CONCLUSION
An extensive review of all major microgrids protection
schemes are presented in this paper comprehensively. All
main characteristics of adaptive protection scheme are
discussed. The covered protection methods are: Intelligent
communication protection scheme, The differential protection
scheme, Inverter control design, Voltage based detection
techniques, Protection based on symmetrical and differential
current components, adaptive protection scheme. Among all
discussed schemes, a recently proposed adaptive protection
scheme seems to be more reliable as it covers wide variety of
considerations. All other reviewed techniques are either
economically infeasible, or they do not satisfy all the
protection requirements. The paper successfully reviews range
of challenges, solutions and future developments in the field
of microgrid systems.
VII. REFERENCES
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