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Chapter

Microgrid Protection Systems

Mylavarapu Ramamoorty and
Suraparaju Venkata Naga Lakshmi Lalitha

Abstract

Micro grids are miniature version of conventional large power grids functioning
either autonomously or with inter connection to the main grid. Primary function
of micro grid is to serve power at distribution level. Distributed energy resources
(DERs) connected to the micro grid enables reliable and efficient operation of
micro grid. Protection of micro grids assumed importance due to increased penetra-
tion of distributed energy resources. Most of the distribution systems in earlier days
are radial in nature and protection systems are designed for that. These protection
systems pose serious challenges when applied to present day distribution systems
which are mesh connected and fed by the distributed energy resources. Limitation
of the conventional protection scheme demands new insights and methodologies
for micro grid protection. Due to intermediate current injection from DERs the
conventional coordination of over current (O/C) relays is not possible. Further
in meshed systems the fault current flow is bidirectional. Hence the protection
of micro grid systems with DERs require different approach to ensure faults are
cleared in less time and minimal number of consumers connected to the system are
affected. A comprehensive analysis of the suitable techniques applicable for micro
grid protection is presented in this chapter.

Keywords: renewable energy sources, distributed energy resources, micro grid,

distribution systems, protection, over current relay, distance relay, differential relay

1. Introduction

Protection is a vital aspect of power system which needs lot of attention every-
where. Majority of the existing protection techniques for distribution systems
are developed for radial distribution lines. These techniques will not be directly
applicable to the micro grids with meshed network in the presence of distributed
energy resources (DER). A CIGRE definition of micro grid is given as Microgrids are
electricity distribution systems containing loads and distributed energy resources, (such
as distributed generators, storage devices, or controllable loads) that can be operated in a
controlled, coordinated way either while connected to the main power network or while
islanded [1]. A typical CIGRE benchmark LV micro grid is shown in Figure 1 [2].
The role of DERs in the present and future distribution systems is inevitable.
Deployment of distributed generators (DGs) proved to be very effective means of
meeting the ever increasing energy needs and concerns for Environment pollution
and the depletion of fossil fuels. Employability of proper protection schemes to suit
the micro grid environment fed by the renewable energy resources has assumed
lot of importance. Protection of micro grids poses several challenges for the utility

1
Micro-Grids - Applications, Operation, Control and Protection

Figure 1.
CIGRE LV benchmark microgrid.

engineers. Protection of micro grids opened the doors for various investigations by
the researchers across the globe. Some important aspects related to the protection
issues of micro grids are presented in this article.
General protection methods applied to the distribution network are designed for
radial systems having unidirectional power flow. With DGs power flow is no longer
unidirectional and it causes a serious threat when conventional protection methods
are used for the micro grid with DGs. Another concern is that the micro grid is
expected to operate safely in grid connected or islanded mode. The intermittent
nature of the output power from a DG makes the selection of the operating char-
acteristics of the relays to be complicated. Further, most of the DGs are connected
to the grid through converters which have independent control strategies. Limited
fault current of the inverter based DGs and maintenance of Fault ride through
capability should be given due consideration in protection. Locating the fault and
proper isolation of the fault are also important [3, 4].
Time graded and current graded over current protective schemes have been in
use for the radial distribution systems. Distance and differential protection schemes
are also employed. Voltage based protection and THD (total harmonic distortion)
based protection are found to be suitable for protection of micro grids with DGs.

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Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

Adaptability is the need of any protection method used for micro grid. The continu-
ous change in the network configuration due to the addition of DGs and/or future
expansions necessitates that the protection equipment must be adaptable as per
the requirement [5]. Communication is another aspect of the protection of the
micro grid. IEDs (intelligent electronic devices) are being used for this purpose in
the grid. Suitable communication protocols are developed and IEC 61850 is being
followed. Protection plays a key role in the development of micro grids due to the
increase in the number of DGs, IEDs, storage systems and the requirement of a
suitable communication medium [6].
This review article covers the need for changes to be made to the conventional
protection systems when applied to micro grids in general and discusses recent
advances made in the field of micro grid protection. Brief and critical review of the
recent papers published on this subject is included. It is expected that this review
article will provide a bird’s eye view of the status of protection systems adapted for
the micro grids with DERs.
This article comprises of six sections with the introduction as first section.
Section 2 discusses conventional distribution system protection along with the
deficiencies of the conventional O/C protection systems as applied to typical micro
grids. Third section deals with brief description of the renewable energy sources
(RES) and the need to replace the conventional generation systems considering
environmental considerations. Configuration of micro grids with DERs is explained
in Section 4. Problems of interfacing micro grids with conventional grid will be
discussed in this section. Section 5 presents critical review of the recent papers
dealing with the protection of micro grids. Section 6 concludes the article.

2. Conventional distribution system protection

Any protection system must be simple, fast, reliable and consistent apart from
being selective and sensitive to the faults. Any protection system should not operate
under normal conditions and must operate under abnormal conditions ensuring
security and dependability of the protective system. These are the two important
reliability indices which need to be optimized always. The two main classes are the
radial distribution system and the meshed system.
For a radial feeder, fault current flows in one direction only as there is a single
source of power. Relay setting in this case is relatively easy. This makes designing
of strategies for protection become very straightforward for distribution systems
typically. Simple devices such as reclosers, fuses and over current relays are used
for protection. As a thumb rule fuses are set to operate for permanent fault and
reclosers are set for temporary fault clearance. This is done as a part of fuse to
recloser coordination with the intention of saving the fuse and also allow for the
temporary faults to clear themselves with fast recloser action. Fuse to fuse coordina-
tion, relay to relay coordination and relay to fuse coordination are also required to
be done. This is to ensure that minimum number of consumers connected to the
distribution system are affected. Generally the fuse to fuse coordination is done
from characteristic curves or selectivity tables supplied by manufacturer. In relay
to relay coordination, time graded/current graded/combination of time and cur-
rent grading is employed. Definite time, inverse time O/C relays are used. Inverse
definite minimum time relays allow the protection engineer for flexible settings of
the relay. Discrimination time of 0.5–0.3 s is possible with the fast acting relays and
circuit breakers. In relay to fuse coordination, time margin is computed by taking
into consideration, the operating time of the upper fuse for proper relay setting. It is
essential that for proper coordination, fault current flowing through the protective

3
Micro-Grids - Applications, Operation, Control and Protection

devices must be between the set minimum and maximum fault current that is
possible and the fault current through all protective devices are almost equal. It is
important to note that in case of a radial feeder, ensuring continuity of supply to
maximum possible number of consumers after clearance of sustained fault is not
possible. Ring main distribution system is an alternative [7].
In ring main system, each load can be supplied power from two different paths.
In case of a fault in one feeder, the other feeder continues to supply whole or a
percentage of total loads. Directional O/C relays are used along with non-directional
O/C relays to minimize the number of consumers affected. Grading of the direc-
tional O/C relays starts from the load end to the source in the ascending order of
the time, whereas for the non-directional O/C relays time discrimination is from
the source side to the load side. In case the ring main is supplied by more than one
source, coordination among the relays is not that easy. If two sources are present,
the ring is opened at one end usually at one of the sources and the grading is done
by presuming the other source as a single source. Employing differential protection
for the section between the two sources is another practice. In this case, the rest of
the system is treated as being fed from a single source. If more than two sources are
present, then the design of protection system becomes more involved [8].
Most of the conventional protective systems are designed for the radial distribu-
tion systems where there is only one directional power flow. Design of a proper
protection system that can be adapted for radial or ring main system with more
than two sources has challenges posed due to bidirectional power flows that are
encountered. Dependability of the usage of conventional protection scheme which
is suitable only for radial system is very low and is therefore not recommended for
the modern meshed distribution systems with DERs. It would be economical if the
existing protection system can be modified or upgraded to match the protection
needs of the modern system rather than discarding the old systems and going for
an altogether new protection system. It is very expensive and is not advisable. Ways
and means of using the existing protection system without losing the important
aspects of protection is highly desirable [9]. Lot of research is focused in this direc-
tion to find out effective utilization of the existing infrastructural facilities of the
convention distribution protection systems.

3. Need of renewable energy sources (RES)

In order to meet the increased demand and to reduce the transmission/distribu-

tion losses, generation at the load points is being done using the RES. There is a
pressing need to redesign the conventional protection systems incorporating the
RES. Another point worth mentioning is that the conventional protection systems are
designed for radial systems expecting large values of fault currents. With the intro-
duction of the RES, there are two possible modes of operation namely grid connected
mode and islanded mode. In case of grid connected operation, there is a possibility of
large magnitude fault currents but may not be always true in islanded operation and
it poses serious concerns related to protection. Inclusion of a DER will causes bidirec-
tional power flows in the distribution system. It reduces the possible upper and lower
limits of fault current along with reduction in the fault current through protective
devices posing serious threat to the conventional protection coordination [10].
DERs and their associated control, communication and protection devices have
become an integral part of modern distribution systems. In any distribution system,
if the penetration of these DERs is more in any area then that geographical area
is being referred as a micro grid. Micro grid is a part of the main distribution grid
and it operates independently to some extent. Major element of a micro grid is a

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Microgrid Protection Systems
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DER. DER can be a PV cell, fuel cell, wind turbine, diesel generator, energy stor-
age system mainly based on Battery, etc. There are many advantages of DERs like
reduction of transmission and distribution losses, eco-friendly power generation
reducing the carbon emission, possible reduction in congestion in the networks,
enhance the energy efficiency by proper utilization of the solar and wind energy.
Major differences between a conventional distribution system and micro grid
can be categorized into three parts namely interfacing of the inverter fed DGs, grid
connected and islanded operation and bi directional power flows. A micro grid is
expected to operate successfully and independently even when there is a distur-
bance in the main grid. The main challenges posed in the protection of micro grids
when compared to conventional system are listed below.

1. Sensitivity and selectivity of the protective relays gets affected due to the local
generation by the DG. Settings should be done in such a way that protection is
ensured even in islanded mode of operation.

2. Due to the presence of DGs interfaced to the grid through inverters, fault
current seen by the relays is reduced during islanded operation. It affects the
protective action by the relays in terms of, either delay in the protective action
or non-detection of the fault.

3. DGs will also affect the maximum and minimum fault currents through a
feeder and it results in serious coordination problems between recloser, fuse
and O/C relays.

4. Undesired tripping of the non-directional O/C relay in a healthy feeder when

the DG feeds a fault outside the healthy feeder. It happens because the DG tries
to feed the fault through the healthy feeder.

5. An auto recloser clears a temporary fault by fast opening and reclosing of

the circuit. If a synchronous DG is present in the micro grid, during this auto
recloser action, it might experience a slight shift in synchronism. In that case,
recloser will be connecting two systems which are not synchronous causing
a serious threat to the entire system. Also the DG will be trying to maintain
the system voltage and in turn the arc at the fault location. It might make the
temporary fault to appear as a permanent fault.

4. Micro grid and DER integration

Integration of DER is an important aspect of micro grid operation. There are

different control strategies applied in micro grid operation. Basically these can be
classified as overall network control and DER control. Supervisory control of the
network is done in centralized and decentralized mode using distribution manage-
ment systems (DMS). DER control is normally chosen depending on the circum-
stances considering the network operation scenarios and the interaction with other
DERs. In grid connected mode real and reactive power control is adopted where as
in islanded mode frequency and voltage control is used [11].
In a micro grid, apart from DER, there are many other types of equipment such
as data interfaces, monitoring devices, communication protocols, protective devices
etc. Communication is another important element of modern distribution systems.
Effective communication protocols have been established and standardized for use
in substations. IEC 61850 is a global standard communication protocol which plays

5
Micro-Grids - Applications, Operation, Control and Protection

a significant role in all aspects of distribution system viz. control, metering and
protection. If the state of the micro grid is subjected to frequent changes due to inter-
mittent nature of DGs and changes in load profile, operation strategies of different
equipment need to be adjusted accordingly. Thus the system integration efficiency
depends on the equipment integration. Further, the conversion of the operating
mode of the micro grid from grid connected mode to islanded mode or vice versa
also demands the adjustments in operation strategies of different equipment. IEC
61850 provides a flexible architecture, service and service essential for interoperabil-
ity and upgrading required for various needs of modern distribution systems.
IEDs (intelligent electronic devices) are required as the devices are expected to
be intelligent enough for data acquisition, transmission to control centres as well
as decision making whenever necessary. These devices are being used extensively
and are having the latest technology for sensing. It allows for two way communi-
cation and greater awareness on the situation in the power distribution system.
These devices can be controlled remotely thus allowing efficient operation during
disturbances. Another feature of the IEDs is that they can communicate with other
devices present in the system allowing effective fault identification and restoration.
With the application of FPGA technology, IEDs are becoming more effective [12].
As the micro grid is interconnected to the main grid, it is essential that the protec-
tive system must ensure the safety for faults in micro grid as well as for the faults in
main grid. In case of a fault in main grid, micro grid should be isolated such that the
consumers supplied by micro grid are not affected. If the fault is in the micro grid
itself, then smallest possible percentage of consumers must be disconnected. Under
these two circumstances, many challenges are there in the protective system design
[13]. Some points to be considered while designing the protective system are (i)
intermittent nature of the power generation by DGs due to changes in solar power,
wind power, etc., (ii) variations in the load (iii) number of DGs, (iv) type of DGs
such as inverter fed DG or synchronous DG, etc., and (v) topology of the network.
In the grid connected mode, islanding may result accidentally or incidentally due to
faults/human error/intentional opening for servicing/faulty operation of protective
devices/natural disasters/and equipment failure. IEDs are employed for control and
protection in modern distribution systems. Active management of the network and
adaptive protection is possible through IEDs [14]. Inverter based DERs are expected
not to get disconnected following a fault or contingency immediately. They should
possess the ability to remain connected to the Grid for some time. It is called Fault
ride through (FRT) capability. It is necessary to have sufficient fault current for the
relays to sense the fault and to maintain the voltage during any contingency. Unlike a
synchronously connected DER, inverter based DERs do not possess the FRT capabil-
ity inherently [3]. FRT requirements in micro grids can be easily accomplished with
IEDs by employing suitable controllers for inverters. To change over the protection
strategies when the micro grid isolates from the main grid either intentionally or
otherwise there is a need to detect quickly such isolation and secure the micro grid.
The detection techniques adapted for sensing isolation and taking appropriate action
for the controls and protection are outlined in the next section.

5. Islanding detection and recommended practices for micro grid

protection

An efficient protection scheme must ensure proper protection to the micro grid
in its both modes of operation, i.e., grid connected mode and islanded mode. It also
should ensure proper functionality during the transition from one mode to another
depending on the requirement. The topological network changes due to the transition

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Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

from one mode to other demands for the changes in the settings of the protective
relays. Before proceeding further, one must understand the nature of the fault currents
during grid connected mode and islanded mode. There are several factors that need
to be taken into consideration such as the size of the DERs, type of DER, no. of DERs,
how they are integrated to the main grid and the islanding detection methodologies.
Many functional differences in the operation of a synchronous DER and inverter
based DER calls for alternative protection strategies for them. Initially the effect of
micro grid operation on the fault currents is discussed in this section. Later, general
categories of O/C protection, distance protection, differential protection along with
voltage based methods applied to suit the requirements of micro grid in grid con-
nected and islanded mode are discussed. Adaptive protection is the main ingredient of
micro grid protection.

5.1 Effect of micro grid integration with main grid

Micro grid is integrated with the main grid with an interfacing switch. As per the
IEEE standard 1547-2003, a DG should be immediately get disconnected for any type
of fault occurrence in the grid [15]. If a fault occurs anywhere in the main grid or
micro grid, the static switch connecting the two gets opened and thus the micro grid
goes into the islanded mode of operation. Along with opening the static switch, loca-
tion of the fault also should be detected simultaneously. If incase, the faults happens
to be within the micro grid, a suitable protective system should be brought into opera-
tion immediately. It should be done online, i.e., detection of the fault location and the
initiation of protective action within the micro grid [16]. For faults in the main grid
the static switch opens and islands the micro grid so that the DGs do not contribute to
the fault current. Then the control system for islanded operation comes into play. For
faults in the micro grid the static switch opens to remove the fault current contribu-
tion from main grid and the protection system of the micro grid comes into play and
clears the fault. Therefore, one should recognize the importance of islanding detec-
tion preceding the protective action. In time and accurate detection of islanding is
essential for fulfilling adequate protection requirements in micro grid operation.
Few points worth mentioning are as follows:

1. In inverter based DER, fault current gets affected by the limitation of 2 pu

rated current of the interfacing inverter [17].

2. If the DER is connected to a single phase load, it might result in considerable

unbalance between the phases in a three phase system.

3. Intermittent nature of the power output of a DER throws serious challenges in

assessing the possible fault currents and relay settings.

4. Short circuit level of the main grid is considerably increased when micro grid
is connected if the size of the DER is large enough.

5. Impact of appreciable amount of load being met by voltage sourced converters

makes the fault currents to be significantly different.

5.2 Over current protection

Over current protection that has been in use for conventional distribution
system protection requires some modifications to be made so that it can be used for
the protection of mesh connected micro grid with DERs.

7
Micro-Grids - Applications, Operation, Control and Protection

In order to understand the proper functioning of the overcurrent protection,

let us consider a simple structure of a micro grid shown in Figure 1. In general this
can be divided into four zones namely MV feeder and busbar protection zone (Z1),
transformer protection zone (Z2), LV feeder protection zone (Z3) and micro grid
protection zone (Z4) as shown in Figure 2.
Based on the location of fault with respect to DER, they can be classified into
external (in Z1 and Z2) and internal faults (in Z3 and Z4). If the CB1 is open the
micro grid is in islanded mode and if it is closed it is in grid connected mode [18].

5.2.1 Fault outside micro grid

If the fault is in zone Z1 or Z2 then main grid protection system will clear the
fault. As per the requirements of IEEE Standard 1547-2003, the micro grid has to
be islanded by opening the CB1. If there are inverter based DERs in the micro grid,
then the fault current will be limited by them. If conventional over current relays

Figure 2.
Typical micro grid showing the zones.

8
Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

are used for tripping CB1, then the fault current will not be sufficient to trip the
breaker. By employing a directional over current relay at LV bus, protection can be
ensured. Alternatively changes in frequency or voltage can also be taken as useful
indicators for detection of Islanding to initiate the desired protective action. The
current setting of this relay should be the cumulative weighted sum of fault current
contribution by all the DERs present in the micro grid governed by (Eq. (1)). The
weighting factor varies from 1.1 (for inverter based DG) to 5 (for synchronous DG)
depending on the number and type of DERs.

n
Ikmin = ∑ kDER ∗ IrDER (1)
1

Here the Ikmin is the required adaptive relay current setting, kDER and IrDER are
the weighting factor and rated current of the DER [19]. Based on the permissible
voltage sag considerations, if sensitive loads are present in micro grid, CB1 should
be opened in 70 ms [20].

5.2.2 Fault inside the micro grid

If the fault occurs on the LV feeder or the consumer end, i.e., Z3 or Z4 then the
protective system should isolate the faulty section ensuring that minimum number
of consumers get affected.
Here again the two cases of grid connected and islanded modes of operation
must be considered. Also the presence of inverter based DERs and synchronous
based DERs should be given due consideration. Following are the key points to be
considered.

• If there is a fault in Z3 or Z4 in grid connected mode, main grid will supply
sufficient fault current and faulty section will be isolated.

• If a large synchronous DER is present, then the fault current seen by the relay
will be smaller than the fault current without DER causing protection blind-
ing in case of a fault in Z3. It may also lead to delay in tripping the breaker if
inverse definite minimum time (IDMT) over current relays are employed for
protection. It is due to the fact that the IDMT relay characteristic has inverse
characteristic for low magnitude portion of the fault current against the
definite time characteristic for higher fault currents.

• A low power diesel generator has low inertia. If there is a delay in the tripping, it
might lead to unwanted tripping of the synchronous DER if the power rating is
low. To avoid this, a proper adaptive coordination among the relays is essential.

• In islanded mode, if there is fault on Z3 and if there are inverter based DGs,
they will limit the fault current as described in the case of faults outside the
micro grid earlier.

• In islanded mode, if there is a fault in Z4, it can be isolated by proper relay

setting based on the possible fault current supplied by the inverter based DERs
without any selectivity problem

In a nutshell, the major challenge in over current protection is the potential

difference in the fault currents due to the presence of DERs in grid connected
and islanded mode. This calls for adaptive schemes which demand expensive and

9
Micro-Grids - Applications, Operation, Control and Protection

complex communication infrastructure. The decision of disconnecting/keep it

connected/shut down the micro grid depends on several factors such as reliability,
cost and the number of customers that get affected. [18] Lot of research is focused
on the application of the adaptive over current protection which demands effective
communication infrastructure and the IEDs.

5.3 Distance protection

Based on the challenges of relay settings and coordination of the over current
relays due the large difference in the fault currents in grid connected and islanded
mode, research has been diverted towards the application of distance protection
to micro grid in which the tripping decision is based on the impedance seen by
the relay and not on the current magnitude [21, 22]. The DER output may result
in under reach and power drawn by the loads may cause over reach of the distance
relays. By employing more number of distance relays, these issues can be addressed.
The impedance seen by the distance relay gets affected by the fault current limiting
nature of the inverter based DERs. In case of induction motor generator based DGs
employing SCIM (squirrel cage induction motor), when the machine starts absorb-
ing reactive power, the line current leads the voltage. It poses the over reach prob-
lem to the connected distance relay which measures it. In case of a DFIM (doubly
fed induction motor) based DG, the power factor of the DG unit is controlled by
the control system of DFIM during fault conditions. If an unbalanced fault occurs
and the fault currents are not large, then the control system can easily maintain the
power factor of DFIM. It may lead to protection problems similar to that encoun-
tered in case of an inverter based DG [3]. This hinders the application of distance
relays for protection of micro grid.

5.4 Differential protection

Difference between the measurements made at different points located in a

micro grid (preferably at the two ends of a feeder section) is considered as an
actuating quantity for this type of protection. Employing symmetrical components
(zero sequence) a differential protection applying the directional features of the
difference current can be used in three different ways as shown below [23].
In the first method (shown in Figure 3), in order to protect the micro grid and
main grid a master micro grid control center (MGCC) is used. Using MGCC it is
possible to integrate all protective schemes. Based on the information received from
monitoring relays it is expected to protect the main grid and micro grid. However,
this method is found to be costly and unreliable to protect either micro grid or main
grid alone due to the complex communication infrastructure and the associated
data analysis to be carried out.
Second method (shown in Figure 4) logic employed only local controllers. Every
relay communicates with its neighboring relay directly and monitors the current
direction. In this there is no master control center. Whenever a reversal of current is
sensed, the faulted section is isolated.
Third method (refer to Figure 5) is an improvised version of second method.
Each feeder has two monitoring relays. In this method, magnitude of the fault
current also is considered in addition to direction unlike the previous two
methods. With this the problem of low magnitude fault currents can be handled
successfully.
Out of the three methods, second method is more cost effective. In the first
method there would be a time delay as the data analysis has to be completed before
the protective action is initiated and hence it cannot serve the purpose of primary

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Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

Figure 3.
Method 1 with microgrid control center.

Figure 4.
Method 2 with local controllers.

Figure 5.
Method-3 improvized version with local controllers.

protection. In the third method, there is an addition of one more directional moni-
toring unit in each feeder making it expensive. In all these three methods the fault
detection and clearance are reliable and only the faulted section is isolated causing
minimum number of consumers to be affected. These schemes do not require any
change in the configuration or in the relay settings for both modes of operation of
the micro grid and are independent of the type and number of DERs connected to
the micro grid [24].
As there are no zero sequence currents in case of a phase to phase fault, nega-
tive sequence components of currents are used for fault detection [25]. Using the
positive sequence components also considering both amplitude and phase angle
the differential protection system is discussed in [26]. However, if there is unbal-
ance and negative and zero sequence currents flow is due to unbalance in the
micro grid rather than a fault, these methods need to be examined more carefully.
Challenges in this type of protection may be summarized as high cost, communi-
cation infrastructure, need for synchronized measurements, effect of unbalanced
loads etc.

11
Micro-Grids - Applications, Operation, Control and Protection

5.5 Voltage based methodologies

Extensive research has been carried out on these methods initially at University
of Bath [27]. In this method voltage is considered for the detection of fault and
subsequently for isolation. There are two methods. One is transformation method
and the other is harmonic method.

5.5.1 Transformation method

In this method, the output voltage of DER is transformed in two steps. (i)
transform voltages from abc to dq frame using Eqs. (2) and (3).

⎡Vds⎤ 1 − 1 ⁄2 1 ⁄2 Va

1 ⁄2 1 ⁄2 1 ⁄2 ][V ]
3[
2
_
⎢ ⎥ __ __
Vqs = 0 − √3 ⁄2 √3 ⁄2 Vb (2)
⎣V0 ⎦ c

(ii) From dq transform to dc values

Vdr − sin ωt Vds
[Vqr ] cos ωt ][Vqs ]
= [cos ωt (3)
sin ωt

Any fault condition will get reflected as a change in d-q values.

VDIST = Vqref − V (4)

By comparing with the reference value, it can be easily inferred which type
of fault and it can be isolated [27]. Application of transformations is an involved
process and becomes complex in certain faults detection. Even a small difference in
the voltage drop in case of a short line, shows a considerable effect on protection.
Network topology also plays a major role in the application of this method when
large numbers of DERs are present.

5.5.2 Harmonic method

In this method, when a fault occurs the total harmonic distortion (THD) of the
terminal voltage increases. By comparing the THD of the terminal voltage of the con-
verter with a predefined reference value, the type of fault can be identified. In this
method discrete Fourier transforms are employed to convert the phase voltages Va,
Vb, Vc into frequency domain. By using proper communication channel between the
relays, fault area can be located and isolated [28]. This is used as backup protection.
A correct setting for the reference value of THD is often challenging.

5.6 Adaptive protection

In this type of protection, the protection strategy must be modified in line with
the existing operating conditions in the micro grid. It is to be done online. To accom-
plish this, numerical directional O/C relays are a good choice. Existing conventional
fuses, electro mechanical and static relays settings and characteristics cannot be
changed online. It necessitates that the existing protection equipment be upgraded to
meet the requirement. Complying to IEC 61850 and installation of IEDs (Intelligent
Electronic Devices) at appropriate places can make the relays to be adaptive with the
ability to adjust their settings and characteristics accordingly on receiving the signals

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Microgrid Protection Systems
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online or following a time sequence. Thorough study of all possible topological

configurations is to be carried out offline prior to the operation. It also necessitates
conducting power flow studies and carrying out short circuit analysis for each
configuration that might occur. For adjusting the settings and characteristics, fast
and effective communication infrastructure should be in place [29].

5.7 Methods of improving protection

Considerable changes in fault current magnitudes during the grid connected and
islanded modes of operation calls for alternative measures to be taken to improve
the protection. If it is possible to modify the fault current magnitude whenever
there is a change of operating mode of the micro grid, the existing protective
systems can be used with some changes without the need of replacing them. If the
fault current can be modified suitably by deploying some additional components, it
would be very useful. These may be used either to increase or decrease the fault cur-
rent suitably to have correct protective action along with the coordination among
different protective equipment used. Response of a synchronous DER is different
from an inverter fed DER during fault conditions. In case of an inverter fed DER,
fault current need to be increased and in case of synchronous DG it should be
reduced. Usage of fault current limiters (FCL), employing an interfacing unit at the
point of micro grid interconnection with main grid to avoid the fault feeding from
main grid are some of the available options. These options demand huge investment
and maintenance. They depend on the proper functioning of islanding detection
methods employed. Fault current limiting poses challenges if the size and penetra-
tion level of the DERs is high.

5.8 Protocols and standards

Some of the relevant standards related to micro grid operation are listed here for
reference. IEEE Standard 1547 series covers Standard for Interconnecting Distributed
Resources with Electric Power Systems. Standard Conformance Test Procedures for
Equipment Interconnecting Distributed Resources with Electric Power Systems are
given by IEEE Std 1547.1, Guide for Monitoring, Information Exchange, and Control of
Distributed Resources Interconnected with Electric Power Systems is presented in IEEE
Std 1547.3. Guide for Design, Operation, and Integration of Distributed Resource Island
Systems with Electric Power Systems is IEEE Std 1547.4 and recommended Practice
for Interconnecting Distributed Resources with Electric Power Systems Distribution
Secondary Network are presented in IEEE Std 1547.6 [30]. There are reports prepared
by CIGRE Working group also for reference. WG C6.22: Micro grid Evolution Roadmap
contains the definitions and nomenclature of micro grid, WG C6.24 explains Capacity
of Distribution Feeders for Hosting DER Connection and Integration of DER [31].

5.9 Recommendations

Of all the protection methods discussed above, differential current relaying is

the most suited protection system for micro grid. This will enable fault location and
also clearing in minimal time. Either one can have pilot lines for connecting the
relays differentially (normally the feeder length in Distribution systems is not high
so the cost of pilot lines also will be low) or one can locate RTUs at the two ends of
each feeder and they will communicate the current magnitude, phase and direction
to a central station where the fault location and tripping decisions are taken. The
current measured at each end of the feeder is applied to Directional Over current

13
Micro-Grids - Applications, Operation, Control and Protection

relay with fixed operating time to provide backup protection. This system will not
require any changes either in configuration or settings for faults in the micro grid or
in the main grid. Also this is not affected by the number and location of DERs and
whether the micro grid is connected or isolated from the main grid.

5.10 Detection of islanding

It plays a major role in proper functioning of protective system. If differential relay-

ing is adapted there is no necessity to detect islanding. However for the Control of micro
grid operation and to maintain the power quality the control system for each DER has to
be changed since the reference signal for frequency and voltage which is taken from the
grid will not be available when the micro grid is isolated. There are different methods
available in the literature for islanding detection such as rate of change of frequency,
voltage, power factor, THD. Also the use of FFT or Wavelet transform of the terminal
voltage will give out different spectrum when isolation takes place. Artificial intelli-
gence techniques also have been employed for detection of islanding. Some new hybrid
techniques employing these techniques can be found in Refs. [32–35].

Year Title Ref Methodology Type of Micro grid Remarks

No. faults features
discussed

2004 Brahma [36] Here protection scheme Balanced (i) Grid (i) Applicable only
SM, Girgis is developed for micro and connected in grid connected
AA. Development grids with Synchronous unbalanced mode mode
of adaptive DERs operating in faults (ii) Radial (ii) Protection in
protection grid connected mode system the islanded mode
scheme for addressing the fuse to (iii) of operation is not
distribution fuse, fuse to recloser Synchronous included
systems with co-ordination issues based DER (iii) Works well
high penetration that arises due to large when large number
of distributed number of DERs. The of DERs are
generation. IEEE relaying strategy is connected in the
Transactions on adaptable in view of micro grid. If the
Power Delivery. temporary faults and number DERs is less,
2004;19(1):56-63 permanent faults and it poses challenges.
extension of the scheme
to additional feeders.

2005 Wan H, Li KK, [37] Protective relay Fault type (i) Grid (i) Applicable only
Wong KP. A coordination using is not connected in grid connected
multi-agent a multi agent specified mode mode
approach to communication (ii) Radial (ii) Relay
protection relay approach is presented. It system coordination is
coordination is capable of providing (iii) Both dependent on
with distributed back up protection synchronous communication
generators in in case of primary based and (iii) Capable of
industrial power protection failure inverter based providing backup
distribution in grid connected DERs protection
system. In: mode is developed.
Fortieth IAS Make use of the Java
Annual Meeting. agent Development
Conference Framework (JADE)
Record of the platform for simulation
2005 Industry of communication.
Applications
Conference, 2005.
Vol. 2. IEEE;
2005. pp. 830-836

14
Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

Year Title Ref Methodology Type of Micro grid Remarks

No. faults features
discussed

2006 Al-Nasseri H, [27] Here DER output voltage Balanced (i) Islanded (i) Protection
Redfern MA, transformation from abc and mode against high
Li F. A voltage to dq frame is performed unbalanced (ii) Radial impedance faults is
based protection and then the deviations faults system not considered
for micro-grids of these values from (iii) Inverter (ii) Effect of single
containing reference values are based DER pole tripping is not
power electronic computed. Based on the (iv) Constant explained
converters. In: difference the protective MVA load (iii) Relay
IEEE Power action is initiated. A (v) Overhead functioning
Engineering communication link line with depends on the
Society General is provided between voltage level communication link
Meeting; 2006. relays. (Voltage based 11 kV/0.48 kV between the relays
p. 7 protection schemes)

2006 Perera N, [38] Network is divided Phase to (i) Both grid- (i) Requires
Rajapakse AD, into several segments. ground connected only current
Agent-based Relay agents and phase and islanded measurements and
protection communicate through to phase mode these measurements
scheme for an asynchronous to ground (ii) D-DGs need not be time
distribution communication fault high (iii) Constant synchronized
net-works with link. Time domain impedance MVA Load (ii) Demands
distributed simulation is done fault (iv) OHL high speed
generators. In: using wavelets for radial communication for
IEEE Power fault location. Central 24.9 kV proper determination
Engineering data processing is of fault section
Society General not required as the (iii) Poses challenges
Meeting; 2006. decisions are done in a to avoid relay
p. 6 distributed manner. functioning during
switching transients

2007 Nikkhajoei [17] A static switch is placed Phase to (i) Islanded (i) Protection
H, Lasseter at the point of common ground and mode against high
RH. Microgrid coupling. Entire phase to (ii) Radial impedance faults is
protection. In: system is divided into phase faults system not considered
IEEE Power different zones. Makes (iii) Inverter (ii) Effect of single
Engineering use of symmetrical based DER pole tripping is not
Society General components and system (iv) kW load explained
Meeting; 2007. residual current is used (v) 0.48 kV (iii) Three phase faults
pp. 1-6 for protective action. distribution are not discussed
voltage

2008 Al-Nasseri [28] Protection System Balanced (i) Islanded (i) It is required to
H, Redfern is based on the and mode assess the reference
MA. Harmonics measurement of unbalanced (ii) Radial THD values for
content based amount of harmonic faults system different fault
protection content present during (iii) Inverter scenarios which
scheme for micro- the fault condition. based DER would be challenging
grids dominated For each type of fault (iv) Constant (ii) If any DER
by solid state a threshold value of MVA load supplies a harmonic
converters. In: THD is evaluated and (v) Overhead free voltage or with
12th International set as a reference. Based line with lesser harmonic
Middle-East on the measured value voltage level content, protection
Power System of harmonic content, 11 kV/0.48 kV system may fail.
Conference, required protective (iii) Variable fault
2008 (MEPCON action will be initiated. impedances, large
2008); 2008. (Voltage based dynamic load
pp. 50-56 protection schemes) switching poses
sensitivity issues
demand for proper
settings of threshold
limits of THD

15
Micro-Grids - Applications, Operation, Control and Protection

Year Title Ref Methodology Type of Micro grid Remarks

No. faults features
discussed

2009 Dewadasa [39] These relays have the Balanced (i) Both grid- (i) fundamental
M, Ghosh A, ability to operate for and connected frequency
Ledwich G. An faults in both forward unbalanced and islanded component
inverse time direction and reverse faults mode extraction may lead
admittance relay direction. Its operation (ii) Inverter to measurement
for fault detection is based on the based-DGs errors due to
in distribution measured admittance (iii) Constant harmonics and dc
networks and has an inverse MVA offset
containing time characteristic. (iv) OHL (ii) Takes more
DGs. In: 2009 The protection system radial and time of operation
IEEE Region can operate for low closed loop for high impedance
10 Conference fault currents also (v) 11 kV faults
(TENCON 2009); and thus provide (iii) Does not use
2009. pp. 1-6 protection under any communication
islanded mode also. It link
is possible to supply
the load in islanded
mode also. Network is
divided into different
zones .

2010 Sortomme E, [40] In this method, Balanced (i) Grid (i) Highly
Venkata M, Mitra digital relays are and connected expensive
J. Microgrid employed along unbalanced and islanded and time
protection using with communication faults mode synchronization is
communication- network. An additional (ii) Both not considered
assisted digital line is added in the inverter and (ii) Imbalance
relays. In: IEEE system to simulate the synchronous created between
PES General loop structure in this based DGs generation and
Meeting; paper. A new modeling (iii) Radial demand due to
Providence, RI; for high impedance and loop line removal in the
2010. p. 1 fault simulation is structure radial mode makes
presented. (iv) 18 bus the protection
system with challenging and
multiple DGs calls for effective
included communication
(v) infrastructure and
Unbalanced sensors.
load is also (iii) In case of
included communication
failure, protection
against high
impedance faults is
at stake

2010 Shi S, Jiang B, [41] This protection scheme Both grid- (i) Method is
Dong X, Bo is based on current connected independent of
Z. Protection travelling waves. Here and islanded unbalance between
of microgrid. detection of the faults mode - the load and
In: 10th IET is done using busbar - generation, level
International voltages and location - of fault current or
Conference on of the fault is found 10/0.4 kV power flow
Developments out employing current distribution (ii) Simulation
in Power System travelling waves. No voltage results are not
Protection (DPSP communication link presented
2010); Managing is used. Based on the
the Change; 2010. information available
pp. 1-4 locally, protective relay
works.

16
Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

Year Title Ref Methodology Type of Micro grid Remarks

No. faults features
discussed

2011 Voima S, [42] This is an adaptive Specific (i) Islanded (i) High
Kauhaniemi protection scheme which type of mode dependency on the
K, Laaksonen uses tele-communication fault details (ii) Radial communication
H. Novel infrastructure. Network are not system infrastructure
protection is divided into four mentioned (iii) Inverter (ii) With reliable
approach for different zones. IEDs based DER communication
MV microgrid. used have directional (iv) Constant links it can be
In: CIRED 21st over current protection MVA load made adaptable to
International function along with (v) Over head different modes of
Conference current and voltage line with operation
on Electricity measurements. voltage level (iii) Details of
Distribution; To achieve proper 20 kV simulation of
6–9 June, 2011; selectivity, interlocking different fault
Frankfurt; 2011. signal is sent along with scenarios is missing
Paper No. 0430 the direction of fault.
Applicability of distance
relay also is presented.

2012 Samantaray SR, [43] In this method, Balanced (i) Both grid- (i) Differential
Joos G, Kamwa differential energy and connected energy is used to
I. Differential applying time unbalanced and islanded recognize the fault
energy based frequency transform faults mode patters
microgrid is used to initiate the (ii) Both (ii) Makes use
protection against protective action. On inverter and of both time and
fault conditions. either end of the feeder, grid connected frequency data
In: IEEE PES amount of spectral mode where as in other
Innovative Smart energy is found out. (iii) Constant schemes only one
Grid Technologies High impedance faults MVA data is used.
(ISGT); 2012. are also considered. (iv) OHL radial (iii) Setting the
pp. 1-7 and closed loop threshold limit for
25 kV the differential
distribution energy plays crucial
voltage role

2013 Ustun TS, [44] In this method Balanced (i) Grid (i) Requires human
Ozansoy C, communication based faults connected input however it can
Ustun A. Fault coordination has been and islanded be minimized if the
current coefficient presented. Amount mode structure of the network
and time delay of fault current (ii) Inverter is obtained by running
assignment contribution by any based and an automated algorithm
for microgrid DG is represented as a synchronous (ii) Delay in the
protection system coefficient. Selectivity based DGs communication
with central of the relays is (iii) Radial depends on the type
protection unit. controlled by automatic system of protocol used
IEEE Transactions adjustment of the (iii) faults within the
on Power Systems. current setting. micro grid only are
2013;28:598-606 considered

2014 Kar S, Samantaray [45] The protection scheme Balanced (i) Both grid- (i) Results are
SR. Time- identifies the fault and connected and compared with the
frequency current patterns based unbalanced islanded mode current differential
transform-based on the S transforms. faults (ii) Inverter technique for all
differential scheme Differential energy is based and fault scenarios
for microgrid computed considering synchronous (ii) Differential
protection. IET both ends of the feeder DGs energy is less
Generation, and it is used for (iii) Constant sensitive to time
Transmission protective action. MVA load synchronization
& Distribution. (iv) OHL radial errors compared to
2014;8:310-320 and closed loop current difference
25 kV

17
Micro-Grids - Applications, Operation, Control and Protection

Year Title Ref Methodology Type of Micro grid Remarks

No. faults features
discussed

2015 Kanakasabapathy [46] Using wavelet Balanced (i) Grid (i) Fault location
P, Mohan transforms a and connected depends on the
M. Digital microprocessor based unbalanced and power signal high
protection protection scheme faults disconnected frequency details
scheme for is developed for grid mode (Phfd)
microgrids connected mode (ii) Radial (ii) Threshold value
using wavelet of the micro grid system of Phfd depends on
transform. with fault detection (iii) Both sampling frequency
In: 2015 IEEE and classification. Synchronous of the analog signal
International Cumulative sum of based and and the type of
Conference on the high frequency inverter based wavelet chosen
Electron Devices details of power signal DERs
and Solid- is computed and is
State Circuits compared against a
(EDSSC). IEEE; threshold value to send
2015. pp. 664-667 the trip signal using the
digital relay.

2016 Gururani [47] Hilbert-Huang Balanced (i) Both grid (i) Setting of
A, Mohanty transform (HHT) and connected proper threshold
SR, Mohanta has been employed unbalanced and islanded value is important
JC. Microgrid to determine the faults modes to discriminate
protection using differential energy (ii) Radial different fault
Hilbert–Huang in this method. To system conditions
transform discriminate faults (iii) Inverter (ii) When noise
based-differential in islanded mode based DERs is included in the
scheme. IET and in case of high signals protection
Generation, impedance fault an becomes challenging
Transmission appropriate setting for
& Distribution. the differential energy
2016;10(15):3707- is used as threshold
3716 value.

2017 Hooshyar [3] A comprehensive Balanced (i) Both grid (i) DG ride through
A, Iravani review of the micro and connected capabilities in
R. Microgrid grid protection unbalanced and islanded islanded mode
protection. techniques has been faults modes for different
Proceedings presented along with (ii) Radial fault scenarios is
of the IEEE. several case studies system and presented.
2017;105(7):1332- using different relays mesh system, (ii) Effect of ECDG
1353 in different modes of voltage 12.47 units on directional
operation employing kV over current relays
synchronous based (iii) Inverter and distance relays
and inverter based based and is shown to be more
DGs. The fault ride synchronous than in case of
through capability based DERs differential relays.
also is discussed. DC (iii) Frequency
microgrid protection is of fault current
also discussed briefly. is shown to be
dependent on the
slip of induction
machine.
(iv) In case of DFIG
based microgrid,
response of the
relay is shown to
be dependent on
the type of control
strategy employed

18
Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

Year Title Ref Methodology Type of Micro grid Remarks

No. faults features
discussed

2018 Aghdam TS, [48] This method discusses Balanced (i) Both grid (i) Micro grid has
Karegar HK, the stability aspect fault connected only synchronous
Zeineldin also along with fault and islanded DGs
HH. Variable clearance. A multi mode (ii) Specific type
tripping time agent approach along (ii) Modified of fault is not
differential with the zoning CIGRE mentioned
protection for principle is employed. benchmark (iii) Fuse tripping
microgrids For coordination and micro grid slower for the same
considering DG backup purposes test system fault current as the
stability. IEEE each agent has (iii) nominal current of
Transactions on three layers namely Synchronous the fuse increase.
Smart Grid primary, backup and DG (iv) Settings of
bus protection. The (iv) 20 kV. several differential
critical clearing time layers depend on
(CCT) curves of the the fuse size for
DGs employed are coordination.
analysed to establish
the mechanism
for checking the
constraints on the CCT
are developed.

Table 1.
Summary of research on protection of micro grids.

Table 1 gives a consolidated picture of ongoing efforts for protection of

Micro Grids.

6. Conclusions

A comprehensive review of various protection methods as applicable to

micro grid protection is presented. DERs are becoming an integral part of
distribution systems but the adequate changes necessary in the protection
system has not yet picked up the pace. Lot of research is going on in this area to
use the existing protective infrastructure justifiably without compromising on
the safety aspect. It is apparent that well-built communication infrastructure
is essential for meeting the requirement of micro grid protection. It is due to
the fact that there are inevitable topological changes in the network due to the
transition of micro grid operating mode from grid connected to islanded and
vice versa. Also, the intermittent nature of the DER output and the fault limit-
ing features of inverter fed DGs present several technical challenges to the micro
grid protection engineers. Making the protective system to be adaptive is the
need of the hour. But it involves lot of infrastructure development and is costly.
Many methods based on directional O/C relays, distance relays and voltage
based protection schemes have been proposed for effective implementation.
However, effective utilization of the existing protective systems with minimal
changes in the infrastructure appears to be possible with differential protec-
tion scheme. With the advancements in communication technology, micro grid
protection can be made adaptive in a cost effective manner.

19
Micro-Grids - Applications, Operation, Control and Protection

Author details

Mylavarapu Ramamoorty and Suraparaju Venkata Naga Lakshmi Lalitha*

KLEF Deemed to be University, Guntur, Andhra Pradesh, India

*Address all correspondence to: lalitha@kluniversity.in

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms
of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.

20
Microgrid Protection Systems
DOI: http://dx.doi.org/10.5772/intechopen.86431

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