A Review on Protection Schemes and Coordination Techniques in Microgrid System
Ahmad Razani Haron,
A protection scheme ensures that short-circuit event is detected and fault is cleared by the protective devices and protective coordination is to ensure that the correct device operates to isolate the faulty parts. Combination of these algorithms affirms the successful operation of protective devices in a distribution system. Significant changes occur in a distribution system when there is a high proliferation of distributed generation in the system. This results in the development of different approaches for tackling the protection issues in a microgrid distribution system. This study presents a review on the available protection schemes and coordination techniques used for addressing the protection issues in a microgrid distribution system. The various protection schemes and coordination techniques applied for microgrid protection are discussed in terms of implementation methods, modes of operation, types of distributed generations, availability of communication links, and lastly their advantages and disadvantages.
Received: August 04, 2011;
Accepted: November 21, 2011;
Published: January 18, 2012
Microgrid is a new approach of power generation and delivery system that considers
Distributed Generations (DG) and loads as a small controllable subsystem of
a distribution system. The subsystem has characteristics such as the ability
to operate in parallel or in isolation from the macrogrid, the possibilities
to improve service and power quality, reliability and operational optimality
(Zamani et al., 2011). It can be operated either
in interconnected to or islanded from the main grid depending on factors like
planned disconnection, grid outages or economical convenience (Brucoli
et al., 2007). Due to the need to capitalize benefits of DG from
its high proliferation in distribution systems, the concept has gained interest
by many researchers as an alternative and effective solution to the supply and
demand related problems. Four classes of microgrid architectures are reported
by the Navigant Consulting based on the load sizes of the subsystems. They are
single facility microgrid, multiple facility microgrid, feeder microgrid and
substation microgrid (Bose et al., 2007). A typical
configuration of a microgrid is shown in Fig. 1. It consists
of a group of radial feeders, a point of common coupling, critical and non-critical
loads and micro-sources (Lasseter, 2007). This subsystem
is capable to operate in dual modes, i.e., normal operation in which the microgrid
is connected to the main grid and islanded operation in which it is separated
from the main grid.
During a fault in the main grid, the static switch will ensure fast separation
of the microgrid from the main grid to island in less than one cycle (Feero
et al., 2002). This action is to ensure that the downstream part
from the point of common coupling can operate continuously with the supply from
Many technical and economical benefits have been reported as a result of microgrid
implementation (El-Saadany et al., 2007; Colson
and Nehrir, 2009; Basu et al., 2011; Agrawal
and Mittal, 2011). The benefits are: (i) increase in reliability and stability
of power supply, (ii) reduction of green-house gases emission as a result from
utilization of renewable energy sources, (iii) reduction of line losses and
deferral of investment in new construction and upgradation of infrastructures
and (iv) provide excess power and assist voltage support to the utility grid
during interconnected mode of operation. Nonetheless, a microgrid does not only
tender its benefits but also has adverse effects due to the interconnection
of DGs. The increasing penetration of DGs in power systems will deeply affect
the distribution network topology, amplitude and distribution of short-circuit
current in the existing distribution network and consequently affect the operation
and control of distribution networks (El-Saadany et al.,
2007; Wang et al., 2008). Several protection
issues related to the effects of DG have been addressed in Feero
et al. ( 2002), Baghzouz (2005), Driesen
et al. (2007), Salam et al. (2008),
Butler-Purry and Funmilayo (2009) and Conti
et al. (2009). These issues are related to relay selectivity, overcurrent
and earth-fault protection, protective disconnection of generators, islanded
operation and neutral grounding. Studies related to impacts of DG on the protection
of distribution systems have been widely researched to enable its implementation
into the distribution grids. It is a fact that addition of DGs to the distribution
grid increases the complexity of protecting the distribution system due to the
change in magnitude, duration and direction of fault current (Dugan
and Rizy, 1984; Hadjsaid et al., 1999). The
severity of the problem increase significantly when there is increased proliferation
of DG into the distribution system. The DG impacts to the distribution protection
include loss of protection coordination, unnecessary tripping and reduction
of reach (Girgis and Brahma, 2001; Salman
and Rida, 2001; Doyle, 2002; Emhemed
et al., 2007; Colson and Nehrir, 2009). Chilvers
et al. (2003) proposed the use of microprocessor-based relay with
incorporated distance and directional elements but without a communication link
to protect the grid with DG from phase and earth fault currents. Vermeyen
et al. (2004) conducted a detailed simulation study on the effects
of the DG to the distribution grid protection for different types of faults
and different combination of DG types. Brahma and Girgis
(2004) introduced a zonal protection scheme to overcome the effect of high
DG penetration into the distribution grid. In this scheme, the grid is divided
into zones that have reasonable balance of load and DG, with DG capacity a little
higher than the load. Zeng et al. (2004) on the
other hand proposed a decentralized multi-agent based protection with capability
of high impedance fault detection. The method uses digital relay as an agent-based
current differential relay which is capable of searching for information from
other relay agents, interact with other relay agents and perform tasks of protection
with autonomy and cooperation. Multi agent relays cooperate to locate faulty
section and shed loads in the isolated DG system.
One of the techniques used to protect the distribution system with DG is known
as the adaptive overcurrent protection scheme (Javadian
et al., 2008). In this scheme, an existing distribution network is
divided into two categories of zones, namely, zones without DG and zones with
DG. System protection is carried out through a computer-based relay which is
installed in sub-transmission substation. The relay determines the systems
status after it receives the required network data, and in case of fault occurrence,
it diagnoses its type and its location and finally issues a proper command for
protection devices to clear the fault and to restore the network. Offline calculation
gives the power flow and short-circuit current for all types of faults at all
points in the network. Characteristics of minimum melting and total clearance
of all fuses in the network are stored in the relay. Zayandehroodi
et al. (2010) presented an automated protection scheme for distribution
networks with high penetration of DGs using Radial Basis Function Neural Network
(RBFNN). Here, zoning of the distribution system with dedicated circuit breaker
was done and three staged RBFNNs have been developed to detect different types
of faults at different fault points. The first RBFNN identifies the distance
of fault point from each power source, the second RBFNN determines the faulty
lines and the third RBFNN decides which circuit breakers must be triggered to
isolate a fault.
Efforts for studying protection scheme for distribution system with microgrid
have started since 2002 when a group of researchers from Consortium for Electric
Reliability Technology Solutions (CERTS) studied the protection problems that
must dealt with to successfully operate a microgrid when the utility is experiencing
abnormal conditions. Chen et al. (2011) have
categorized the microgrid protection schemes according to (i) improved current
protection schemes, (ii) protection schemes based on fault current limiter,
(iii) wide area protection schemes, (iv) abc-dq transformation protection schemes
and (v) THD protection schemes. This paper presents a review of microgrid protection
schemes and the coordination techniques that are available in the literature.
A critical analysis of the existing schemes and techniques are made in an attempt
to suggest a viable protection scheme and coordination technique for a microgrid
PROTECTION OF MICROGRID
In designing a protection system of any power system, the protection requirements
that must be considered are reliability, speed, selectivity and cost (Gers
and Holmes, 2005). The main function of a protection system is to quickly
remove from service any component of the system that starts to operate in an
abnormal manner. Other functions are to ensure the safety of personnel, to safeguard
the entire system, to ensure the continuity of supply, to minimize damage and
to reduce resultant repair cost. When a fault occurs, a protection system is
required to disrupt as few section of the system as possible. Selective operation
of the protective devices will ensure maximum continuity of service with minimum
Conventional distribution system is designed to operate radially, where only
a single source is present. The current flows in one direction from the higher
voltage levels at the substation, through the distribution feeders and laterals
at lower voltage levels, to the customers loads. Based on this characteristic,
the system relies on a simple and low-cost protection schemes that consist of
fuses, reclosers, circuit breakers and overcurrent relays (Girgis
and Brahma, 2001; Gers and Holmes, 2005; Javadian
et al., 2009). Example of a conventional distribution system with
the protective devices installed is shown in Fig. 2. Breakers
and reclosers are normally installed at the main feeder to allow clearance of
temporary faults before lateral fuses blow. In normal condition, breakers and
reclosers are equipped with inverse time overcurrent relays which normally installed
at the substation where the feeder originates (Aslinezhad
et al., 2011). The protective devices are coordinated to operate
according to criteria of selectivity based on current or time so as to ensure
the device nearest to a fault will operate first. The basic criteria that should
be employed when coordinating time or current devices in distribution systems
are (Gers and Holmes, 2005):
||The main protection should clear a permanent or temporary
fault before the backup protection operates, or continue to operate until
the circuit is disconnected. However, if the main protection is a fuse and
the back-up protection is a recloser, it is normally acceptable to coordinate
the fast operating curve or curves of the recloser to operate first, followed
by the fuse, if the fault is not cleared
||Loss of supply caused by permanent faults should be restricted to the
smallest part of the system for the shortest time possible
In a traditional distribution system, the protection systems are designed assuming
unidirectional power flow and are usually based on overcurrent relays with discriminating
capabilities. According to IEEE (2003) for any fault
situation, DG sources connected to the system are tripped off. In other words,
islanded operation of DG sources is not allowed. When a microgrid is created
in a distribution system, the configuration becomes a complex multi-source power
system. The protection philosophy of the microgrid is to assure safe and secure
operation of the subsystem in both modes operation, i.e., during interconnected
mode and islanded mode. However, the two operating modes pose new challenges
in protecting the microgrid. Therefore, two sets of protection settings is the
most probable solution to the dual modes of operation. During grid-connected
operation, the mains supply large fault currents to the fault point. This makes
possible the employment of the existing protection devices in the distribution
system. Yet, the protection coordination may be compromised or even entirely
lost in some cases due to the installation of DGs. On the other hand, such large
fault current contribution from the microgrid cannot always be expected, especially
when it is dominated by electronically-coupled DGs (Zamani
et al., 2011). Thus, the use of conventional overcurrent protection
in the microgrid is no longer valid due to this low short circuit current contribution
from the micro-sources.
|| Typical distribution system with protection devices
|| Different types of protection schemes for microgrid operation
It is obvious that alternate means of detecting an event within an isolated
microgrid must be studied and new protection schemes have to be devised.
In order to design a microgrid protection system that suits both grid-connected
and islanded modes of operation, the following features must be considered (Jenkins
et al., 2005):
||The system must be able to respond both to distribution system
and microgrid faults
||For a fault on the main grid, isolate the microgrid as quickly as possible
||For a fault within the microgrid, isolate the smallest possible section
of the radial feeder carrying the fault to get rid of the fault
||The protection scheme must ensure an effective operation of customers
Microgrid protection schemes: The protection of a microgrid must respond to both main network and microgrid faults. If a fault occurs on the main grid, the desired response is to isolate the microgrid from the main network as rapidly as necessary to protect the microgrid loads. This caused islanding of the microgrid operation. If a fault occurs within the microgrid, the protection system is required to isolate the smallest possible faulted section of the microgrid to eliminate the fault. Various possible microgrid protection schemes and coordination techniques that are available from the literature are summarized as shown in Fig. 3. The protection schemes can be divided into overcurrent-based, voltage-based, current component-based, harmonic content-based, fault current limiter-based and current traveling wave-based. As for protection coordination techniques, time-current grading and optimization algorithms are used to ensure selectivity of the protective devices. The functionality and reliability of different types of protection schemes have been studied to fulfill different mode of operations, different types of DGs that exist in a microgrid, different issues that it has to tackle like bi-directionality of power flow and location of DG placement.
Overcurrent scheme: Jenkins et al. (2005)
simulated a simple microgrid system containing 200 kW flywheel storage system,
a number of micro sources and a group of residential consumers, in order to
investigate possible protection schemes for a microgrid. Two scenarios are studied,
i.e., when microgrid is connected to the main network and when the microgrid
is islanded from the main network. For a fault on the main distribution network,
overcurrent relay protection and balanced earth fault protection were installed
at the grid side of the circuits between the main distribution network and the
microgrid, with the capability of inter-tripping the microgrid. Investigation
was also made to the test system for different location of faults. For fault
on the microgrid, overcurrent relay protection and Residual Current Device (RCD)
are used to protect the feeder from the fault. During the fault, the flywheel
supplies a high fault current (e.g., 3 p.u based on its rating or above) if
the microgrid is operated in islanded mode. After the fault, the protection
disconnects the faulty feeder from the microgrid and has the capability of inter-tripping
all the micro sources on the feeder at the same time. For a fault at the residential
consumer, a Short Circuit Protective Device (SCPD) like Miniature Circuit Breaker
(MCB) or fuses and a RCD can be installed at the grid side of the residential
consumer. The SCPD is used to protect the residential consumer against the phase
to phase and phase-to-neutral faults while the RCD is used for phase-to-ground
or earth fault. However, the use of conventional overcurrent protection scheme
gives rise to problems such as less sensitivity, refusing to operate and mal-operation
(Chen et al., 2011).
Improved overcurrent with directional element: The deficiency of the
traditional overcurrent to protect the microgrid due to bi-directional flow
of current has led to the use of directional element in the overcurrent devices.
Hadzi-Kostova and Styczynski (2006) and Oudalov
and Fidigatti (2009) suggest the use of directional overcurrent relay with
communication for microgrid protection. The relays are capable of detecting
and isolating the faults external and internal to the microgrid. However, this
type of protection scheme requires a relatively high investment cost in comparison
to the conventional protection system using fuses. Chunguang
et al. (2009) proposed the use of directional comparison and current
comparison pilot protection based on two terminals information to distinguish
internal faults from external faults without coordination with other protection
devices. A master-slave concept of the protection devices has been used to protect
the microgrid where through communication the fault direction and its position
could be identified. Zamani et al. (2011) presented
a new scheme for protecting low voltage grid-connected and islanded microgrid
using programmable microprocessor-based relays with directional elements. This
scheme does not rely on communication link and fairly independent of fault current
magnitudes and mode of operation. The improved overcurrent with directional
element protection scheme still has some limitations and can only suit the requirements
to some extent due to the continuously addition of DGs and uncertainty of connecting
and disconnecting of DGs from the main grid, the distribution network topology
and protection setting values that change frequently (Chen
et al., 2011).
Voltage-based scheme: The protection scheme based on DG output voltage
measurement was used to detect and clear faults (Al-Nasseri
et al., 2006; Redfern and Al-Nasseri, 2008).
The output voltages were monitored and transformed into dc quantities using
the d-q reference frame. Any disturbance at the DG output due to a fault on
the network will be reflected as disturbances in the d-q values. Zones of protection
and use of communication link between relays to aid the protection scheme in
discriminating between in-zone and out of zone faults are also considered. Verification
via simulation has been done for different fault locations and different types
of faults. However, the proposed scheme is devised for islanded operation only
without considering the grid-connected mode of operation and high impedance
The use of voltage based protection scheme for islanded microgrid dominated
by inverter-based DGs was presented (Hou and Hu, 2009).
In this scheme, a new fault judgement method based on detecting the positive
sequence component of the fundamental voltage was used to judge the fault location
and fault types in a microgrid. The waveforms of the 3-phase voltages and the
voltage magnitudes under symmetrical and unsymmetrical fault conditions were
transformed into the d-q reference frame and compared with the amplitude of
the fundamental positive sequence voltages in the d-q coordinate system.
Symmetrical and differential current component scheme: Nikkhajoei
and Lasseter (2006) proposed a method for protection of microgrid based
on symmetrical and differential current components against SLG and LL faults.
In this method, a microgrid is divided into several protection zones with relays.
The differential current components are used to detect fault that occurs in
the up-stream zone of protection and the symmetrical current components (zero-and
negative-sequence current components) are used to detect SLG fault in down-stream
zone of protection and LL faults in all zones of protection. Simulations were
done at different location of faults for inverter-based DG microgrid and the
results showed that the scheme is capable of protecting the microgrid in islanded
mode of operation.
Zeineldin et al. (2006a) applied a differential
current protection scheme for inverter-based DG in grid-connected and islanded
microgrid. Current sensors at both ends of a line will determine whether a relay
should send a tripping signal to the breakers or not. The scheme has been tested
for faults inside the microgrid. Conti et al. (2009)
also applied the differential protection scheme for a test microgrid consisting
of synchronous-based and inverter-based DGs. Various solutions using zero-sequence
directional current for differential protection was presented.
Sortomme et al. (2010) presented a protection
scheme using differential protection and communication-assisted digital relays.
The synchronized phasor measurements and microprocessor based relays were used
to detect all types of fault conditions including high impedance faults. Here,
the primary protection for each feeder relies on instantaneous differential
protection. If absolute values of two samples are found to be above the trip
threshold, a tripping signal is sent to the switching device.
Overcurrent protection based on fault current limiter: Connection of
DGs to a distribution network increases the fault current closer to the rating
of the protection devices and disturbs overcurrent protection coordination (Kumara
et al., 2006; El-Khattam and Sidhu, 2008;
Ustun et al., 2011). The existing protective
devices can be replaced with higher rating devices but this choice is costly.
One way to overcome overrating of protective devices is to limit the fault current
to an acceptable level. The fault current limiter device is connected in series
with power lines to limit the fault current contribution from the DGs while
contributing a small impedance and power loss under normal operations.
Harmonic content based scheme: Another method for protecting microgrid
has been presented by Al-Nasseri and Redfern (2008)
based on harmonic content of inverter output voltage. When the type of DG embedded
in the distribution system is of inverter-based type, the fault current contribution
from the DG during islanded operation is limited to about twice the inverters
rated output. The inverter-based DG is a good source of harmonics that are injected
into the network during a fault. Using the harmonic content based protection
scheme, the protection relay continuously monitors the Total Harmonic Distortion
(THD) of the inverter terminal voltage and shut down the inverter if the THD
exceeds a threshold value during a fault.
Distance protection scheme: Distance protection scheme is another solution
for converter-controlled microgrid as investigated by Dewadasa
et al. (2008). Distance relays having Mho characteristic with two
zones of protection were used in the study. Zone settings were chosen such that
Zone-1 covers 80% of the protected line and Zone-2 covers the whole protected
line, plus 50% of the adjacent line. In this method, the fault currents in the
faulted phases were limited by reducing the converter output voltage. Next,
by analyzing fault characteristics, the sequence currents and voltages at the
relay locations are calculated. Simulations were done for grid-connected and
islanded modes of operation for different types of faults at different locations
with changes in fault resistance and load conditions. However, the effectiveness
of this scheme is still not proven.
Uthitsunthorn and Kulworawanichpong (2010) suggested
the use of distance relaying for multisource systems so as to reduce complication
in impedance-based setting of distance relays. Three zones of protection have
been considered to protect the lines. A comparison was made between the use
of distance protection scheme and directional overcurrent protection scheme.
Adaptive scheme: Brahma and Girgis (2004) introduces
a Global Positioning System (GPS) based adaptive protection scheme for a distribution
network with high penetration of DG. The distribution network was divided into
a number of zones by the circuit breakers according to a reasonable balance
of the DG and local loads. The main relay at the substation has the capability
of storing and analyzing large amounts of data and communicating with the zone
breakers and the DG. The measurements of the GPS based adaptive protection were
the synchronized current vectors of all three phases from every DG and the main
source and the current directions of the zone breakers. The synchronized vectors
were obtained using a global positioning system based phase measurement unit.
In normal operating conditions, the sum of all these current phasors would be
equal to the total load in the network. In case of fault condition, the sum
will be significantly larger than the total load.
Oudalov and Fidigatti (2009) presented a novel adaptive
protection scheme using digital relaying and advanced communication technique
which is based on a centralized architecture where protection settings are updated
periodically by the microgrid central controller with regards to the microgrid
operating states. The scheme was realized using numerical directional relay
with directional interlock capability to selectively protect the microgrid.
Han et al. (2010) presented an adaptive fault
current protection algorithm for inverter interfaced DG based microgrid. By
calculating the system impedance, the method adaptively changes the setting
value of the protection to adapt to the grid-connected or islanded modes of
operation. As the operating modes of a microgrid affect the fault component,
a comparison is made between the system impedance and the microgrid side impedance
such that current instantaneous protection can automatically adjust the settings.
In a more recent study, Voima et al. (2011)
presented a novel adaptive protection scheme based on telecommunication using
Intelligent Electronics Device (IED). The method depends on information available
through measurement of current flow direction and voltage from multiple measurement
locations. Implementation of this method was done in two phases involving detection
of fault condition based on undervoltage and detection of faulted zone based
on current flow information from the IEDs.
Current traveling waves: Shi et al. (2010)
introduced the current traveling waves protection based on local information
without using communication to determine an event caused by fault of switching
operation. The scheme uses busbar voltages to determine whether a fault occurs
and current traveling waves to identify the faulted feeder. When a fault occurs
in one feeder, the power frequency voltages in the busbar connected to the faulted
feeder will change according to the fault types. Current traveling waves are
measured by current transformers in lines and wavelet multi-resolution analysis
is used to decompose the traveling wave signals. The initial traveling waves
are compared with each other in terms of magnitude and polarity to determine
the faulted feeder. The protection is independent of microgrid operation modes
and immune to power flow, fault current, unbalanced load and plug-and-play generators.
PROTECTION COORDINATION TECHNIQUES IN MICROGRID
Protective device coordination or selectivity is the process of applying and
setting the protective relays that overreach other relays such that they operate
as fast as possible within their primary zone but have delayed operation in
their backup zone (IEEE, 2001; Blackburn
and Domin, 2006). The goal is to ensure maximum service continuity with
minimum system disruption. Historically, protective device coordination was
done on translucent log-log paper but modern methods normally include detailed
computer based analysis and reporting. A properly coordinated protection maximizes
power system selectivity by isolating faults to the nearest protective device,
as well as avoiding nuisance operations that are due to transformer inrush or
motor starting operations. System protection is said to be selective if only
the protection device closest to the fault is triggered to remove or isolate
a fault. If this takes too long, the protection at a higher level takes over.
This rule of thumb allows disconnection of those components that are faulty.
For a distribution system without DG, power flows in one direction during normal
operation as well as when a fault occurs. This allows for the use of a selective
system by applying time grading to overcurrent relays. When DG is installed,
this system becomes inadequate. A possible scenario is disconnection of a healthy
feeder by its own protective relay because DG contributes to short-circuit current
flowing through a fault in a neighboring feeder. On the other hand, if a fault
occurs on the connection between the supplying grid and a local network, disconnection
of feeders or generators should take place. In an islanded microgrid, the voltage
drop caused by a fault is almost the same across the entire network, due to
limited geographical span. Therefore, it is almost impossible to coordinate
protective devices based on voltage profile (Zamani et
Coordination using time-current discrimination: Nikkhajoei
and Lasseter (2006) presented time delay discrimination for coordinating
all relays in a microgrid. Here, the SLG faults and LL faults are detected based
on symmetrical and differential current components. A threshold value is selected
for all relays to detect downstream SLG fault based on a zero-sequence current
component and another threshold value is set for relays to detect upstream SLG
faults based on differential current component. For detecting a LL fault, another
threshold value is selected based on negative-sequence current component..
To ensure coordination of protective devices during islanded mode of operation,
Zamani et al. (2011) proposed the use of microprocessor-based
overcurrent relay with directional element, in conjunction with fault detection
module. The relays are discriminated based on the definite-time scheme, starting
at the load side of secondary main and ending at the microgrid interface point.
This will cause a longer fault clearance time at the generation side of the
grading path but will not damage the microgrid equipment. The upper limits for
the definite time delays at the generation side are determined based on constraints
such as the sensitivity of the critical loads to voltage disturbances, the duration
over which electronically-coupled DGs can contribute to the fault current and
the stability of the rotating-machine based DGs.
Coordination using particle swarm optimization algorithm: Zeineldin
et al. (2006b) presented a protection coordination algorithm using
directional overcurrent relays for protecting microgrid consisting of synchronous-based
DGs. The relay coordination problem was formulated as a Mixed Integer Nonlinear
Programming (MINLP) problem and was solved using the Particle Swarm Optimization
(PSO) technique, which was one of the heuristic techniques capable of solving
constrained optimization problems. The main advantage of MINLP formulation is
that it takes into account the discrete pickup current (Ip) thus
providing better results than linear programming formulation. However, the complexity
of the problem increases as the number of available discrete current settings,
IP increase for each relay. For this reason heuristic techniques
such as PSO and genetic algorithm became the best solution to solve such complex
problems. The scheme also used central protection unit via communication link
to change the settings of the relays based on the system configuration.
Coordination using modified particle swarm optimization algorithm: Qu
et al. (2010) presented the Modified Particle Swarm Optimization
(MPSO) technique derived from Zeineldin et al. (2006a)
to coordinate the directional relays in a microgrid. The relay coordination
problem was solved by improving the initialization and weight of PSO algorithm
and choosing suitable pickup current. The optimal settings of the relays were
determined using the PSO algorithm. The improvement over the initial PSO algorithm
is in the phase initialization part and phase up dating part of the PSO algorithm.
In the initialization phase, the space for feasible solutions is defined as
xmax = (xmax-xmin) x rand + xmax),
vmax = (vmax-vmin) x rand + vmin,whereas
in the update phase, all particle positions in the D-dimensional space are not
moved simultaneously but only one in each dimensional space at a time. The particless
position in the PSO algorithm is updated based on feasibility, fitness value
and acceptance ratio. The study showed that the PSO algorithm is capable of
obtaining a close to optimal solution.
CRITICAL ANALYSIS AND SUGGESTION
As mentioned in Gers and Holmes (2005), the attributes
of protection scheme are reliability, selectivity, speed, cost and yet simple.
However, it is impossible to have all the attributes in a single protection
scheme due to many contributing factors like topology change, bi-directionality
and relays characteristic. Thus, each protection scheme is designed uniquely
for a specific test system and DG technology. Table 1 shows
the comparison of the available protection schemes based on their application
in a distribution system with microgrid.
As shown in the table, the scheme for protecting a microgrid differs from each
other depending on the type of relay used, microgrid operating mode, type of
DG connected in the microgrid, and availability of communication link. For a
certain microgrid configuration, the selection of protection scheme also depends
on the availability of high fault current and cost (Jenkins
et al., 2005). It is obvious that a single protection scheme cannot
fully protect a distribution system with microgrid. Therefore, a new protection
scheme is required for a distribution system connected to a microgrid. The use
of communication for microgrid protection is useful to ensure fast and reliable
operation of protective devices but adverse effect due to breakdown of communication
As for protection coordination, the grading of the relays is either based on time or current discrimination. By maintaining a specific coordination time interval between relays, the selectivity can be assured. The optimum relay settings can be obtained through optimization techniques like PSO. As the system configurations and operating conditions often change, a review of the existing settings should be done when the available short-circuit current to a plant changes or when significant changes in plant loading occur.
|| Comparison of protection schemes in a distribution system
This paper has presented an introspective review of the protection schemes and coordination techniques that have been developed or presented in the literatures. All the presented protection schemes and coordination techniques have their own merits and demerits that draw interest to their application in the microgrid operation. A good protection should be capable of fulfilling the requirements of reliability, selectivity, speed and cost. For accurate selection of a protection scheme, a thorough study has to be done on the network characteristic and suitability of the scheme to be adopted based on the network itself. For microgrid protection, it is always better to use a scheme that utilizes communication links as it will ensure fast operation of the protective devices. As for existing distribution system connected to a microgrid, maximizing the existing infrastructure without much investment will be the most economical decision although not always the most desirable.
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