Electrical Power and Energy Systems 58 (2014) 300–306

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Electrical Power and Energy Systems

journal homepage: www.elsevier .com/locate / i jepes

An analytical literature review of the available techniques for theprotection of micro-grids

http://dx.doi.org/10.1016/j.ijepes.2014.01.0320142-0615/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 129732985; fax: +60 75557005.E-mail address: dalilamatsaid@yahoo.com (D. Mat Said).

Sohrab Mirsaeidi a, Dalila Mat Said a,⇑, Mohd. Wazir Mustafa a, Mohd. Hafiz Habibuddin a, Kimia Ghaffari b

a Centre of Electrical Energy Systems (CEES), Faculty of Electrical Engineering (FKE), Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysiab Young Researchers and Elites Club, Saveh Branch, Islamic Azad University, Saveh, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 April 2013Received in revised form 16 December 2013Accepted 11 January 2014

Keywords:Micro-gridProtection schemesGrid-connected modeIslanded modeDG units

During the last decade, besides the rapid increase in the penetration level of Distributed Generation (DG)units of micro-grids, the connection of micro-grids as a developed technology to the existing distributionnetworks has also attracted much attention. One of the major challenges associated with the protectionof micro-grids is to devise a proper protection strategy that is effective in the grid-connected as well asthe islanded mode of operation. In order to deal with the challenge, many researchers have recently pro-posed various techniques. The purpose of the current study is to provide a comprehensive review of theavailable protection techniques that are applied to address micro-grid protection issues in both grid-con-nected and islanded mode. The most up to date relevant options are described and categorized into spe-cific clusters. A comparative analysis is carried out in which the advantages and disadvantages to eachtechnique are assessed. Lastly, after the appraisement of the existing protection techniques, some conclu-sions and suggestions are put forward for the protection of micro-grids in the future.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Increasing concerns regarding global warming caused by green-house gases, which are substantially generated by conventionalenergy resources, e.g., fossil fuels have created significant interestin the research and development in the field of renewable energies[1,2]. Such interests are also intensified by the finitude availabilityof conventional energy resources. To take full benefit of renewable-energy resources, e.g., wind and solar energy, interfacing powerelectronics devices are essential, which together with the energyresources form DG units [3–6]. The increasing proliferation of DGunits, such as wind turbines, micro-gas turbines, photovoltaic gen-erators and fuel cells is anticipated, and this inevitably challengesthe traditional operating principles of the power networks [7–11].An emerging philosophy of operation to alleviate the technical is-sues with regard to high penetration of DG units, and to offer addi-tional values is to designate relatively small areas of a distributionnetwork that embed DG units and loads, and to operate them in adeliberate and controlled way. Such sub-networks, referred to asmicro-grids [12]. The structure of a typical micro-grid is depictedin Fig. 1.

The most important benefit of micro-grids is to provide high-reliability and high-quality power for the consumers who require

uninterruptible power supplies [13,14]. Furthermore, micro-gridsbring significantly economic benefits with the utilization ofcombined heat and power technology. In fact, micro-grids havethe potential to generate the electrical and useful thermal energysimultaneously (hot, cold, or both) to optimize the consumedenergy efficiency by applying cogeneration or tri-generation sys-tems [15].

Micro-grids have the ability to operate independently or in con-junction with the rest of the distribution network. The philosophyof the micro-grid’s operation is that under normal conditions themicro-grid operates in the grid-connected mode but when the util-ity damages or has a power failure; it expeditiously disconnectsfrom the utility by the static switch at the Point of CommonCoupling (PCC) and then operates in isolation from the rest of thenetwork [16–22].

In spite of numerous advantages provided by micro-grids, thereare some technical challenges, which require to be met for theengineers and one of them such as micro-grid protection and itsentities. Since the protection devices of the existing distributionsystems are designed according to the large fault currents, theydo not have the ability to protect micro-grids. This is because whena fault takes place in the micro-grid with the widespread prolifer-ation of electronically-coupled DG units, operating in autonomousmode, the DGs are not able to contribute adequate currentstowards the total fault current. It is due to the inverters have alow thermal overload capability, limiting their maximum output

Fig. 1. The structure of a typical micro-grid.

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current to about 2–3 times the rated current [23]. On the otherhand, despite the traditional distribution networks, power flowwithin micro-grids can be bidirectional owing to DG connectionsat the different locations. Accordingly, in order to protect micro-grids in both grid-connected and islanded mode of operation, novelprotection schemes should be employed [24,25].

This paper aims to present a brief analysis of various protectionschemes based on the published papers in attempting to providean appropriate protection strategy which is capable of protectingmicro-grids in both grid-connected and autonomous mode of oper-ation. The organization of this paper is as follows: Section 2 dis-cusses the available techniques for the protection of micro-grids,and Section 3 analyzes the proposed techniques as well as puttingforward some suggestions for the protection of micro-grids in thefuture and finally; Section 4 concludes the paper.

2. Available techniques for the protection of micro-grids

An appropriate technique for the protection of micro-gridshould have the ability to respond to both utility grid and micro-grid fault incidents. In other words, if a fault occurs on the utilitygrid, the desired response is to isolate the micro-grid from the restof the network. This leads to an autonomous operation of micro-grid, and if a fault takes place within the micro-grid, the protectionsystem should remove the smallest possible faulted area of micro-grid to clear the fault. In recent years, various techniques have beenproposed to present an adequate protection strategy for micro-grids. These techniques are precisely illustrated in the followingsubsections.

2.1. Adaptive protection: available techniques and their challenges

Adaptive protection schemes have the ability to solve the prob-lems associated with the protection of micro-grids in both grid-connected and islanded mode of operation. In such protectionschemes, there is an automatic readjustment of relay settingswhen the micro-grid alters from grid-connected mode to islandedmode and vice versa. In fact, adaptive protection is an online sys-tem that modifies the preferred protective response to change insystem circumstances or requirements in a timely manner through

external generated signals or control actions. Numerical directionalover-current relays, which have the potential of using several set-tings groups, are employed in the practical implementation ofadaptive protection systems. In order to provide more effectiveprotection, a communication system can be applied such that indi-vidual relays can communicate and exchange information with acentral computer or between different individual relays.

The work by Tumilty et al. [26], suggested an adaptive protec-tion strategy without the need of the communication system. Theauthors simulated the voltage response for both short-circuit andoverload events. The results of simulations indicated that the volt-age magnitude has a reduction in both events. Nevertheless, themagnitude of this reduction was such that these two events couldbe differentiated. In fact, the voltage drop resulting from short-cir-cuits were significantly greater than that of overloads. Accordingly,they employed a voltage based fault detection method to discrimi-nate the voltage drop in short-circuit and over-load incidents.

Based on centralized architecture, Oudalov and Fidigatti [27]presented a novel adaptive protection scheme using digital relay-ing and advanced communication technique. In the scheme, theprotection settings were updated periodically by the micro-gridcentral controller with regards to the micro-grid operating states.The scheme was realized using numerical directional relay withthe directional interlock capability to act selectively to protectthe micro-grid.

In the following year, Han et al. [28] analyzed the fault behaviorof an inverter-based micro-grid, and proposed an adaptive faultcurrent protection algorithm. They deployed the voltage andcurrent fault components at the installation of protection todetermine the system impedance. Afterwards, the current instan-taneous protection adjusted the settings automatically by compar-ing with the utility grid and micro-grid impedances.

In another study conducted by Dang et al. [29], they proposedan adaptive strategy using Energy Storage (ES) and isolation trans-formers to protect low-voltage micro-grids in the islanded mode aswell as the grid-connected mode of operation. Firstly, in order torecognize the micro-grid’s mode of operation, the over-currentprotection and dq0 voltage detection were utilized for the grid-connected mode and islanded mode, respectively. Then, the differ-ent protection zones could be discriminated by comparing the zerosequence current and a threshold value.

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However, Ustun et al. [1] suggested an additional adaptive pro-tection technique that made use of extensive communication formonitoring and updating settings of relays in accordance withdifferent micro-grid’s operation mode. In the proposed scheme,micro-grid was equipped with a Central Protection Unit (CPU)which communicated with the relays to update their operatingcurrents and with DGs to store their status as ON/OFF.

The work of Khederzadeh [30] used the numerical relays in themicro-grid to adapt the settings of the relays to the status of themicro-grid. Different settings for the over-current relays were cal-culated off-line and saved in the relays. Whenever the micro-gridwas disconnected from the grid, the relay settings were automati-cally changed to the associated group of settings.

The main challenges associated with the implementation of theabove-mentioned adaptive protection techniques are as follows:

– The need for updating or upgrading the protection deviceswhich are currently utilized in the power networks.

– The necessity to know all possible configurations of micro-grid before the implementation of these schemes.

– Establishment of a communication infrastructure can becostly.

– Short-circuit calculations will be complicated for a micro-grid with different operating modes.

2.2. Differential protection: available techniques and their challenges

Differential protection is applied to protect many elementswithin a power system. The structure of the differential protectionis shown in Fig. 2. The protected element might be a length of thecircuit conductor, a bus section, etc. From Fig. 2 it can be seen thatdifferential relaying is a basic application of Kirchhoff’s CurrentLaw (KCL). The relay operates on the sum of the currents flowingin the CT secondaries, I1 + I2. For through current conditions, suchas load or an external fault, the currents in the two CT’s will beequal in magnitude and opposite in phase (assuming the CT’s havethe same ratio and are properly connected), and there will be nocurrent flow in the relay operate coil [31]. In the event of ashort-circuit occurrence within the protected section betweenthe two CT’s, current will flow through the operate circuit causingthe relay to issue a trip output.

Nikkhajoei and Lasseter [32] presented a combined techniqueto protect micro-grids by differential protection and symmetricalcomponent calculations against Single Line-to-Ground (SLG) andLine-to-Line (LL) faults. In this method, a micro-grid was dividedinto several protection zones with relays. The differential currentcomponents were deployed to detect faults that occur in theup-stream zone of protection, whereas the symmetrical currentcomponents (zero and negative sequence current) were used to de-tect SLG fault in the downstream zone of protection and LL faults in

Fig. 2. The structure of the differential protection.

all zones of protection. Simulations were performed at differentlocation of faults for inverter-based DG micro-grid and the resultsindicated that the scheme has the ability to protect the micro-gridsin islanded mode of operation.

Zeineldin and his co-workers [33] discussed the future of micro-grids and regarded two major challenges ahead, including voltage/frequency control and protection. In the developed strategy, theymade use of differential relays at both ends of each line. Theserelays designed to operate in 50 ms could protect the micro-gridin both grid-connected and autonomous operation modes.

The work reported by Conti et al. [34], utilized a differentialprotection scheme in a test micro-grid containing synchronous-based and inverter-based DGs. They described three protectionstrategies to detect phase-to-ground faults in isolated neutralmicro-grids. Additionally, three more protection schemes werepresented for three-phase faults in micro-grids with synchro-nous-based and inverter-based DGs.

The progress of differential protection was further establishedby Sortomme et al. [35]. They offered a novel protection schemebased on some of the principles of synchronized phasor measure-ments and microprocessor relays to recognize all kinds of faults,including High Impedance Faults (HIFs). In this scheme, the pri-mary protection for each feeder relies on the instantaneous differ-ential protection. If absolute values of the two samples were foundto be above the trip pre-determined threshold, a tripping signalwas sent to the switching device.

Prasai et al. [36], suggested a multi-level approach wouldprovide the most effective form of network protection of a meshedmicro-grid, while ensuring a high level of reliability and powerquality by expeditiously and automatically identifying faultedpoints in the system, and actively isolating them. One of the advan-tages of this method was that the proposed protection schememade used of the power line itself (power line carrier technology).Therefore, the communication architecture was highly reliable.

Last but not least, a novel methodology based on differentialprotection was put forward by Dewadasa et al. [37] in 2011. Themethodology, which includes all the protection challenges suchas bidirectional power flow and reduction of the fault current lev-els in islanded operation mode were taken into account. It couldprotect micro-grids in both grid-connected and islanded modesof operation. In the method, the authors not only concentratedon feeder protection, but they also proposed some solutions to pro-tect buses and DG sources within the micro-grid.

One of the most significant benefits of the implementation ofdifferential protection approaches is that they are not sensitive tobidirectional power flow and reduction of fault current level ofislanded micro-grids. However, some drawbacks associated withthem can be summarized as follows:

– As the communication system may fail, providing a second-ary protection scheme is necessary.

– Establishing a communication infrastructure is relativelyexpensive.

– Unbalanced systems or loads may result in some difficul-ties in the above-mentioned protection schemes.

– Transients during connection and disconnection of DGsmay bring about some problems.

2.3. Distance protection: available techniques and their challenges

Since the impedance of a line is proportional to its length, fordistance measurement, it is appropriate to use a relay capable ofmeasuring the impedance of a line up to a predetermined point(the reach point). Such a relay is described as a distance relayand is designed to operate only for faults occurring between therelay location and the selected reach point.

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The main strategy for this group was developed by Dewadasa[38,39]. It was based on an admittance relay with inverse time trip-ping characteristics. The relay could distinguish and isolate thefaults in both grid-connected and autonomous micro-grids.Distance relays having Mho characteristic with two zones of pro-tection were employed in the study. Zone settings were chosensuch that Zone-1 covers 80% of the protected line and Zone-2 cov-ers the whole protected line, plus 50% of the adjacent line. In thestrategy, the fault currents in the faulted phases were restrictedby reducing the converter output voltage. Afterwards, by analyzingfault characteristics, the sequence currents and voltages at therelay locations were calculated. Simulations were done for grid-connected and autonomous modes of operation for different typesof faults at different locations with changes in fault resistance andload conditions. Nevertheless, the effectiveness of this scheme isstill not validated.

Some challenges associated with the application of these typesof relays are as follows:

– Harmonics and transient behavior of current may result insome problems with the accuracy of extracting fundamentals.

– Fault resistance may make some errors in the measuredadmittance.

– The measurement of the admittance for short lines in dis-tribution networks is challenging.

2.4. Voltage-based protection: available techniques and theirchallenges

Voltage-based protection techniques substantially make use ofvoltage measurements to protect micro-grids against differentkinds of faults.

The main approach in this field was confidently proposed byAl-Nasseri and Redfern [40] in 2006. The scheme, in which outputvoltages of DG sources were monitored and then transformed intodc quantities using the d-q reference frame, had the ability to pro-tect micro-grids against in-zone and out-of-zone faults. Moreover,a communication link was deployed to discriminate in-zone andout-of-zone faults.

Subsequently, Hou and Hu [41] proposed a new fault judgmentmethod based on detecting the positive sequence component ofthe fundamental voltage such that it could provide reliable and fastdetection for different types of faults within the micro-grid. In thismethod, the waveforms of the three-phase voltages and the volt-age magnitudes under symmetrical and unsymmetrical fault con-ditions were transformed into the d-q reference frame andcompared to the amplitude of the fundamental positive sequencevoltages in the d-q coordinate system.

Within the same year 2009, Loix et al. [24] proposed a novelprotection technique. This technique which was based on the effectof different fault types on Park components of the voltage wascapable of protecting micro-grids against three phase, two phaseand one phase-to-earth faults. The protection methodology didnot rely on the communication system during its operation, butit could be optimized through communication links between dif-ferent detection modules. The salient feature of this scheme com-pared to the one in [40] was that the proposed strategy was notonly designed for a certain micro-grid but could also be used toprotect different micro-grids with various configurations.

The recent report by Wang et al. [42] introduced an additionalprotection strategy based on busbar fault direction to protectmicro-grids in both grid-connected and autonomous operationmodes. Furthermore, the authors designed the relay protectionhardware and software using Industrial Personal Computers (IPCs).

The main challenges in regard to possible implementation ofvoltage-based protection strategies are:

– Any voltage drop within the micro-grid may lead to mis-operation of protection devices.

– HIFs cannot be identified using above-mentionedmethodologies.

– Most of these techniques are designed and tested for spe-cific micro-grids. In fact, they are strongly dependent onthe micro-grid configuration and on the definition of theprotection zone. Therefore, they may not be convenientfor micro-grids with different structures.

– Less sensitivity in the grid-connected mode of operation.

2.5. Protection techniques with the deployment of external devices andtheir challenges

As mentioned earlier in the introduction, the fault current levelsare significantly different between grid-connected and the autono-mous operation modes. Therefore, the design of an adequate pro-tection system, which performs in both modes of operation, canbe a real challenge. In this regard, there is a possibility of employ-ing a different approach which effectively modifies the fault cur-rent level when the micro-grid alters from grid-connected to theautonomous operation mode and vice versa, through specificexternally installed devices. These devices can either increase ordecrease the fault level. The main options are as follows:

– To decrease the aggregated contribution of many DG sources,which can change the fault current level enough to exceed thedesign limit of various equipment components, as well as toguarantee an adequate coordination in spite of the feedingeffect of DG to fault current, Fault Current Limiters (FCL) canbe used [43]. This effect is particularly evident with synchro-nous machine based DG.

– To level the fault current level in both grid-connected andislanded modes of operation, owing to the limited fault contri-bution by inverter-interfaced DG sources. This can be achievedin two different ways:(a) By applying the energy storage devices (flywheels, batteries,

etc.) in the micro-grid will increase the fault current to adesired level, and allowing over-current protection to oper-ate in a traditional way [44,45].

(b) By installing specific devices between the main grid and themicro-grid, to reduce the contribution of fault current fromthe utility grid [46,47].

The main problems associated with the deployment of thesetypes of devices embedded in the micro-grid are as follows:

– Storage devices require large investment.– The deployment of schemes based on a FCL technology is

only possible up to a certain amount of DGs connected.For very high levels of DGs, it can be difficult to determinethe impedance value of the FCL, due to the mutual influ-ence of the DGs.

– Sources with high short-circuit capability (flywheels, etc.)require significant investments, and their safe operation isdependent on the correct maintenance of the unit.

– The methods based on an additional current source arehighly dependent on the technology of islanding detectionand the proper operation of the current source.

2.6. Protection techniques based on over-current and symmetricalcomponents and their challenges

These protection techniques which are mainly based on theanalysis of current symmetrical components, attempt to improve

Table 1Comparison of the available micro-grid protection techniques.

Protection technique Type of relay used Micro-gridoperation mode

Type of DG connected Availability of thecommunication link

Cost

Adaptive protection Voltage restrained over-current relay/numericaldirectional over-current relay

Grid-connectedand islanded

Rotating-based andinverter-based

Depending on thetechnique used

Reasonable

Differential protection Digital relay Grid-connectedand islanded

Rotating-based andinverter-based

Yes Expensive

Distance protection Distance relay Grid-connectedand islanded

Inverter-based No Reasonable

Voltage-based protection – Islanded Inverter-based Yes ReasonableDeployment of external

devicesOver-current relay Islanded Inverter-based No Very

expensiveOver-current and

symmetricalcomponents

Digital relay Grid-connectedand islanded

Rotating-based andinverter-based

Depending on thetechnique used

Reasonable

304 S. Mirsaeidi et al. / Electrical Power and Energy Systems 58 (2014) 300–306

the performance of traditional over-current protection and providea robust protection system for micro-grids.

The main proposal in this field was developed by Nikkhajoei andLasseter [32] in 2006. They presented a possible solution to recognizethe fault in autonomous micro-grids based on the measurements ofcurrent symmetrical components. To be precise, the authors pro-posed to use zero-sequence current detection in the event of an up-stream single line-to-ground fault (coordinated with unbalancedloads) and negative sequence current for line-to-line faults.

Two years later, Best et al. [48] proposed a three-stage commu-nication assisted selectivity scheme. In the scheme, stage-one rec-ognized the fault event in accordance with the localmeasurements. Stage-two deployed inter-breaker communica-tions, and stage-three adapted the settings of the relays via asupervisory controller.

Subsequently, Zamani et al. [12] designed a microprocessor-based relay with directional element, in conjunction with the faultdetection module. The relays were discriminated based on the def-inite-time scheme, starting from the load side of secondary mainand ending at the micro-grid interface point. This caused a longerfault clearance time from the generation side of the grading pathbut not damaging the micro-grid equipment. The upper limits forthe definite time delays at the generating side were determinedbased on some limitations such as the sensitivity of the criticalloads to voltage disturbances, the duration over which electroni-cally-coupled DGs can contribute to the fault current, and the sta-bility of the rotating-machine based DGs. The salient feature of theproposed scheme was that it had no need of communication linksbetween the relays.

The main disadvantage of the majority of the above-mentionedprotection techniques is related to the necessity of communicationsystems. In such techniques, the protection coordination may beendangered in case of communication system failure.

3. Comparative analysis and suggestions for the protection ofmicro-grids in the future

According to the recent publication [49], the attributes of a pro-tection technique are reliability, selectivity, speed, cost as well assimplicity. Nevertheless, it is impossible to have all the attributesin a single protection technique owing to many contributing factors,like topology change, bi-directionality as well as relay characteris-tics. Consequently, each protection technique is designed only fora specific test system and DG Technology. Table 1 shows the compar-ison of the available protection techniques based on the type of relayused, micro-grid operation mode, type of DG connected to the mi-cro-grid, availability of the communication link and cost.

Realization of future micro-grids requires that all technicalissues are solved. Micro-grid protection and its entity is one of

them. Based on the analysis of the wide range of technical publica-tions proposed in the previous section, some conclusions and sug-gestions for the micro-grid protection in the future are as follows:

– In spite of many efforts which have been performed in thefield of micro-grid protection, there are still limited num-bers of publications. Furthermore, the information givento the micro-grid structures and the proposed techniquesare too brief or incomplete. The present observation real-izes that most of the published works are generally a formto an idea than a practical proposal.

– The majority of the presented techniques to the area ofmicro-grid protection are strongly dependent on the net-work architecture. In fact, the techniques do not have theability to protect different micro-grids with variousconfigurations.

– A reliable protection technique must be able to distinguishhigh impedance faults, which may have current magni-tudes similar to those of normal loads, such that it willnot be causing any noticeable voltage drop. However, onlya small number of publications from the references haveconsidered such a kind of fault.

– Most of the proposed techniques are designed only for themicro-grids with radial feeders and are not capable of pro-tecting micro-grids containing looped feeders.

– Regardless of the protection technique, it is highly likelythat some kind of communication system is going to benecessary, either centrally operated or distributed.

– In order to possess an optimal protection system for micro-grids, a combined action of different protection techniqueswill be necessary.

4. Conclusion

Micro-grids have been designed to meet the reliability andpower quality needs of customers. Nevertheless, the emergenceof micro-grids has been accompanied with significant challenges.Micro-grid protection and its entities is one of them. In recentdecades, numerous approaches have been put forward to pres-ent an adequate protection technique for micro-grids. A robustprotection technique should be able to protect the micro-gridagainst different types of faults and ensure its safety and secureoperation in both grid-connected and autonomous mode. Thegoal of the current study was to provide a comprehensivereview of the existing proposals to deal with the micro-grid pro-tection issues. Additionally, an attempt was made to classifythese proposals into specific groups and finally; some conclu-sions and practical suggestions were derived from the analyzedreferences.

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