revised 1st january 2019 review on microgrids protection

IET Generation, Transmission & Distribution

Special Issue: Intelligent Protection and Control of Microgrids withEnergy Storage Integration

Review on microgrids protection ISSN 1751-8687Received on 28th July 2018Revised 1st January 2019Accepted on 1st February 2019E-First on 22nd March 2019doi: 10.1049/iet-gtd.2018.5212www.ietdl.org

Siavash Beheshtaein1 , Robert Cuzner2, Mehdi Savaghebi3, Josep M. Guerrero1

1Department of Energy Technology, Aalborg University, Pontoppidanstraede 111, Aalborg, Denmark2Electrical Engineering Section, The Mads Clausen Institute, University of Southern Denmark, Odense, Denmark3Department of Electrical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA

E-mail: [email protected]

Abstract: Microgrid, which is one of the main foundations of the future grid, inherits many properties of the smart grid such as,self-healing capability, real-time monitoring, advanced two-way communication systems, low voltage ride through capability ofdistributed generator (DG) units, and high penetration of DGs. These substantial changes in properties and capabilities of thefuture grid result in significant protection challenges such as bidirectional fault current, various levels of fault current underdifferent operating conditions, necessity of standards for automation system, cyber security issues, as well as, designing anappropriate grounding system, fast fault detection/location method, the need for an efficient circuit breaker for DC microgrids.Due to these new challenges in microgrid protection, the conventional protection strategies have to be either modified orsubstituted with new ones. This study aims to provide a comprehensive review of the protection challenges in AC and DCmicrogrids and available solutions to deal with them. Future trends in microgrid protection are also briefly discussed.

1 IntroductionFor over a century, the conventional structure of the power systemhas been defined based on centralised generation resourcesinterconnected to end-users through long transmission lines. Such apower system has some drawbacks involving high power losses,low penetration of distributed energy resources (DERs), poorvisibility, slow response times due to electromechanical devicesand lack of sufficient standardisation for system automation ofpower distribution particularly for, energy storage systems (ESSs)and electric vehicles (EVs), wind turbines (WTs), photovoltaics(PVs).

Regarding probable high penetration of DERs in the future grid,the concept of microgrid had been introduced to address differentissues such as low power quality, high power loss and lowreliability in the conventional structure of power system, wherecentralised generation resources interconnected to end-usersthrough long transmission lines. The US Department of Energydefines microgrid as ‘a group of interconnected loads and DERswithin clearly defined electrical boundaries that act as a singlecontrollable entity with respect to the grid. A microgrid canconnect and disconnect from the grid to enable it to operate in bothgrid-connected and island-modes’ [1].

Implementing of microgrids contributes notable benefitsincluding higher reliability by introducing of self-healing, higherpower quality by controlling loads and DERs, reducing the carbonemission because of high penetration of renewable energyresources, and low implementing cost by reducing distributionlines and using renewable DERs, to both customers and the gridutility [2]. Microgrids can be classified into different groups basedon type (such as military, residential, industrial, and commercial),the size (such as small, medium, large scales), application (such asloss reduction, resilience-oriented, and premium power), andconnectivity (such as islanded and grid-connected modes). Also,according to voltages and currents, microgrid has three typesincluding AC, DC, and hybrid AC/DC [3].

Although considerable progress achieved in the development ofAC microgrids in the past decades, DC microgrids have receivedmany attentions in industry and academia due to the proliferationof PVs, ESSs, EVs, DC home appliances, and so on that are DC bynature [4]. Compared to AC Microgrids, DC microgrids have

advantages, including higher efficiency, no reactive power flow,higher power quality, easier integration of DC DERs, and no needfor synchronisation [3, 5]. Since AC systems have beenimplemented for a long time, having pure DC microgrids is notrealistic. Therefore, implementing hybrid AC/DC microgrids is amore viable scenario. A typical configuration of a hybrid AC/DCmicrogrid is shown in Fig. 1.

Regarding the requirements, features, and architecture of ACand DC microgrids, these microgrids are facing several protectionchallenges. The common challenges to both AC and DC microgridare severe impacts of a microgrid topology change and DERsexistence on protection system, high impedance fault,communication standards for intelligent electronic devices (IEDs),and cyber-attack. Besides these common challenges, the designinga proper grounding system DC circuit breaker (DCCB), andprotection system are other challenges that exist in DC microgrids[6]. Although several review papers have been published [7–10],none of them does cover all mentioned challenges in both AC andDC microgrids. This paper fills this gap by presenting acomprehensive review in AC and DC microgrids.

To address these issues, a comprehensive overview of theconventional protective devices (PDs) with their respectivedrawbacks and benefits is presented in Section 2. In Section 3,challenges in AC microgrid protection are discussed, and severalsolutions and future trends are overviewed. In Section 4, first faultcurrent analysis in DC microgrid is presented, then acomprehensive discussion about grounding systems, DCCBs, faultdetection methods, and standards in DC microgrid protection arediscussed. Finally, Section 5 presents the conclusions of this paper.

Fig. 1 Typical hybrid AC/DC microgrid system

IET Gener. Transm. Distrib., 2019, Vol. 13 Iss. 6, pp. 743-759© The Institution of Engineering and Technology 2019

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2 Conventional protective relaysThe power system is composed of different components, includinggenerators, transmission lines, bus bars, transformers, reactors,capacitors, loads, and so on. All equipment must be protectedagainst various forms of faults. Current-based, voltage-based,frequency-based or impedance-based methods are among the mostwell-known protection systems [1, 11].

2.1 Overcurrent relays (OCRs)

OCRs considered as simple and economical PDs, are employed asthe main protective relays for distribution and backup protectionfor transmission systems. Generally, there are three types of time-current characteristics for OCRs: (i) instantaneous; (ii) definitetime, and (iii) inverse-time. If the current amplitude exceeds a pre-defined value, the relay with instantaneous and definite-timecharacteristics will send a trip signal instantly and after a definitetime, respectively. For the inverse time-current characteristic,operating time is mathematically defined by IEEE C37.112 [12] asfollows:

TP = TDS βIsh/IP

α − 1+ γ (1)

where TDS is the time delay setting of the relay; IP is the pick-upcurrent; Ish is the current detected by relay; α, β, and γ denote theslope of the relay characteristics. According to IEEE C37.112,moderately inverse (MI), very inverse (VI), and extremely inverse(EI) trip characteristics are three classes of inverse time-currentmodes, which are presented in Table 1.

The proper OCR performance is maximised by selecting properpickup currents and TDSs. Pickup currents are usually determinedin proportion to the maximum possible load current; however,choosing TDSs is more complicated [6]. Correct relay coordinationby appropriate settings would ensure isolation of faults in theprotected zone by corresponding primary relays as quickly aspossible, and if they fail, the corresponding backup relays trip aftera coordination time delay. The operating time of the backup relayis equal to the operating time of the primary relay plus thecoordination time interval. This strategy creates a safety marginbetween primary and backup relays to guarantee the selectivitycriteria and prevent maloperation. The coordination problem can beformulated as follows:

Minimisation OF = ∑i = 1

NTi (2)

T j − Ti ≥ CTI ∀(i, j) ∈ Ω

TDSimin ≤ TDSi ≤ TDSi

max

IPmin ≤ IP ≤ IP

max

where Ω is set of the main-backup pairs of the relays.There are three ways of solving the OCR coordination problem

including trial and error, curve fitting, and optimisation methods[13]. Trial and error approach requires a large number of iterationsto reach an optimal result. Therefore, it has a slow convergencerate. In curve fitting methods, characteristics of OCR are modelled.These methods may need a large computing memory to store manypoints of the OCR characteristic curve. Finally, optimisation-basedmethods model OCR coordination with an objective function andseveral constraints, then try to solve it by different methods such asmixed-integer non-linear programming (MINLP) [14], geneticalgorithm [15], particle swarm optimisation [16], and bee colonyalgorithm [17]. Since the evolutionary algorithms are multipointsearch strategies, they can ensure reaching the global optimalsolutions.

OCRs are suitable candidates for protection of radial networkswhere the fault can only flow in one direction. However,introducing DGs into the distribution system converts the nature of

network from radial to meshed distribution system, which leads tobi-directionality of fault currents. Therefore, discrimination of faultcurrent direction is necessary, and this can be realised bydirectional OCRs (DOCRs) [18]. OCRs may erroneously trip dueto some transient phenomena produced as a result of inductionmachine starting, transformer energising, and capacitor switching.Therefore, the distinction of a real fault from the normaloperational transient phenomena is one of the vital elements toensuring secure and reliable protection. Since the conventionalOCR operates only based on the RMS value of the current, furthersignal processing algorithms are required to cope with thisproblem.

2.2 Directional OCR

The DOCR is one of the alternatives for overcoming theshortcomings of the OCRs in DSs including DGs. DOCRs usevoltage reference [19], pre-fault current [20] and post-fault current[21] to detect the direction of the fault. Since the potentialtransformer is typically absent in the distribution system, thevoltage reference is not applicable in the distribution system.Although in the second method pre-fault current is utilised fordetecting the direction of the fault, this method requires the voltagefor the detection of the power-flow direction. The main advantageof the third method is no requirement for voltage and prefaultcurrent. Also, it will not be influenced by the power-flow direction.

Regarding the DOCR functionality, these relays have to becoordinated to disconnect the faulty area from the healthy parts ofthe grid in the shortest possible time. In radial DS, the coordinationprocess starts with the farthest relay from the source in a feeder,and then the relays are set one by one. The concept of break pointset (BPS) is introduced to apply this method for meshed networks[22]. One of the classes of DOCR coordination is topologicalanalysis. This method utilises graph theory to find minimum BPS[23]. There are other approaches for coordination of DOCRs,which are similar to OCR coordination, including trial and errormethods [24], curve fitting [25], linear programming (LP) [26],MINLP [27], non-LP (NLP) [28], and heuristic-based algorithms[29, 30].

In LP, the value of pickup current is selected between acceptedoverload and minimum fault current, and the optimal value of TDSis obtained through LP. In [31], interval analysis is integrated intoLP to consider uncertainty in the network topology. Typically,pickup current is selected near to its minimum value to increaseDOCRs sensitivity; however, it would not lead to the optimumvalues of TDSs and therefore will cause higher relay operatingtimes. MINLP, which is compatible with electromechanical relays,considers pickup currents and TDSs as discrete and continuousvariables, respectively. In NLP, both pickup current and TDS aresupposed to be continuous variables. The continuity of thesevariables is consistent with the nature of modern digital relays.However, long processing time and complexity are the maindisadvantages of NLP [28]. The main advantage of these heuristicalgorithms is that they can search for a wider space of solutions bythe high number of populations and generations to find a globaloptimum solution.

2.3 Distance relays

Distance relays detect a fault based on measuring the apparentimpedance calculated by division of the voltage by the measuredcurrent at the relaying point. Then, the apparent impedance iscompared with the particular value of impedance called reach pointimpedance. If it is less than the reach point impedance, it isassumed that the fault is located between the relay and the reach

Table 1 Parameters for three modes of time-currentcharacteristics of OCR [12]Curve description α β γMI 0.02 0.051 0.114VI 2.0 19.61 0.491EI 2.0 28.2 0.1217

744 IET Gener. Transm. Distrib., 2019, Vol. 13 Iss. 6, pp. 743-759© The Institution of Engineering and Technology 2019

point; therefore, a trip signal is sent to the respective circuit breaker(CB). According to the principle of the distance relay, one of themain advantages of this relay is that the source impedancevariations do not affect the relay performance [11]. Anotheradvantage is the capability to work as the main and backupprotection for transmission lines [32].

Each distance relay has one instantaneous direction zone and upto five/six time-delayed zones. One of these time-delayed zonescould be used to measure in the reverse direction. Distance relayparameters are set to cover 80–85% of the protected line forinstantaneous zone 1 protection. A 15–20% safety margin isnecessary due to measurement errors of the current and voltage,underestimation/overestimation of line parameters, and relayinaccuracy. Zone 2 encompasses 15–20% of the line to 20–50% ofthe adjacent line. If the fault is detected in zone 2, a tripping signalwill be sent after 15–30 cycles. Similarly, zone 3 includes a wholetwo lines plus 20% of the third line. If the detected fault lies along

the third zone, delayed time for the distance relay would be around90 cycles [33]. A typical three-zone distance relay is shown inFig. 2 [11]. In modern digital distance relays, along with forwarddistance zones, a reverse impedance zone could be added to be asbackup protection for the bus bar. It must be noted that the bus baris a rigid and big conductor utilised to carry current, where wiringis impossible. There are two ways of applying the reverse zone:first, when a separate reverse zone reaches setting of 25% of zone 1and second, by setting a small offset reach from the origin for thethird zone. The second approach is also demonstrated in Fig. 2,where TA1, TA2, and TA3 are tripping times of RA in zone 1, zone 2,and zone 3, respectively. Similarly, TD1, TD2, and TD3 are trippingtimes of RD in zone 1, zone 2, and zone 3, respectively. Finally, TB1and TB2 are tripping times of RB in zone 1 and zone 2, respectively.

The characteristics of distance relay defined on R/X diagramform the shape of operation zones. Some relays measure theabsolute fault impedance, while the others compare either therelative amplitude or phase of two input quantities to build theiroperating characteristics. As shown in Fig. 3, impedance, mho,lenticular, blinders, and quadrilateral are among the mostconventional types of distance relays [11].

The distance relay with impedance characteristic is non-directional, and the operation occurs for all impedances within thecircle. Besides non-directional behaviour, being vulnerable topower swings and a heavy load is another disadvantage of thismethod. The directional relay can be combined with this relay toovercome the first disadvantage. The mho characteristic, which is acircle which passes through the origin, is obtained by comparingthe phases of S1 and S2:

S1 = V (3)

S2 = IZn − V

where V, I, and Zn are the fault voltage, current measured at therelaying point, and the impedance setting of the zone, respectively.

The lenticular distance relay is produced by the intersection oftwo mho distance relays to make distance relay immune to loadchanges. This is done at the expense of losing resistance coverage.Both typical mho and lenticular distance relay fail to operate forthe close-up fault condition. For this circumstance, an offset isadded to mho and lenticular characteristics to cover close-up faultas well as protection of busbar. Another option to make mhodistance relay insensitive to heavy load is connecting the mho relayin series with two angle-impedance distance relays called‘blinders’. With the advent of the solid-state microprocessor-basedrelay, more complex and flexible characteristics can be applied.The quadrilateral characteristic can be realised in themicroprocessor-based relay with four measurement units. As theselines can be adjusted independently, this relay has better resistivecoverage than any mho distance relay for short lines [11]. Powerswing and load encroachment occurred when the load impedanceenters the protective zone, high resistance fault, or when theremote-end source (infeed effect) deteriorates distance relayperformance and causes malfunction of the relay [34].

A power swing is generated as a result of a massivedisconnection of loads, generator disconnection, fault clearance,change in the direction of power flow, and loss of synchronisation[34]. As shown in Fig. 4a, when a power swing occurs, themeasured impedance slowly moves toward the relay's zones andleads to unwanted tripping of healthy lines. Some possiblesolutions to cope with this problem utilise the rate of impedancechange [35], the rate of active and reactive powers changes [36],circular locus and centre behaviour of the admittance trajectory[37], symmetrical fault detector [38], wavelet transform [39], andadaptive-network-based fuzzy inference system [40].

In the transmission level, the distance relay sees a heavy load asa low impedance. In this condition, overlap happens between theload and the impedance characteristic of the relay. Fig. 4b showsthis situation, which is known as a ‘load encroachment’. With agiven mho circular diameter, the mho distance relay can be

Fig. 2 Distance protection of electrical power system

Fig. 3 Conventional types of distance relay(a) Impedance, (b) Mho with/without offset, (c) Lenticular, (d) Blinders, and (e)Quadrilateral

Fig. 4 Distance relay challenges(a) Load encroachment and power swing, (b) High resistance and the infeed effect


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adjusted by variation of sensitivity angle. Sensitivity angle can beincreased to be robust against load swing at the expense of loss ofhigh sensitivity angle and resistive coverage. Other conventionalmethods, such as lenticular distance relay and combining twoblinder elements with cutting off mho characteristic with twoblinder elements are designed to reshape the impedancecharacteristic to exclude load from coverage of the relaycharacteristic.

The fault resistance and infeed effect, caused by an intermediatesource, are the main factors that could influence the measuredimpedance. This is shown as follows [41]:

ZR = m ⋅ Z1L + IFIR

⋅ RF = m ⋅ Z1L

+ IR + IBIR

⋅ RF = m ⋅ Z1L + Z f − eq(4)

where Z1L, IR, and IF are the line impedance, measured current bythe relay, and current fed by another side of the line, respectively.As the conventional distance relays measure the apparentimpedance on the assumption of zero fault impedance, for a highresistant fault the apparent impedance is out of the protection zones(see Fig. 4b). Also, based on the direction of injected current fed byanother side of the line, the distance relay may not see the fault orconsider a healthy situation as a fault; these scenarios are calledover-reach and under-reach, respectively. Fault resistancecalculation based on monitoring active power at the relayconnection point is proposed to avoid the under-reach issue forground distance relays [42]. However, this method is not effectivefor high transient resistance taking place during the first half cycleof AC current, because for this particular case symmetricalcomponents are not equal. In [43], fault path resistance is estimatediteratively based on phase coordinates utilising only voltage andcurrent data. This method does not consider multi-infeedtransmission lines. Adaptive protection with composite polarisingvoltage [44], fault distance calculation based on solving a lineardifferential equation [45], and utilisation of fault record data toestimate the fault resistance [46] are recent methods to cope withthe problem of high impedance fault.

Although distance relays are typically designed for transmissionnetworks, advantages such as inherent directional behaviouralcharacteristics, ability to discriminate in-zone and out-of-zonefaults, and compared to DOCRs being less affected by sourceimpedance and change in microgrid conditions make these relaysan attractive option [47]. However, distributions lines are usuallyshort in microgrid; therefore a high-impedance fault may lead tomaloperation of the relays. Besides, the existence of DGs inmicrogrid increases the measured impedance and thus have anadverse effect on the performance of the relay [48].

2.4 Differential relays

A current differential protective relay first measures the currentsflowing in and flowing out the bus, transformer, or transmissionline then compares the shared information which is transferred by apilot wire, fibre-optics communication, power line carrier, or awireless communication network. If the difference between theinput and output currents exceeds a threshold, a fault is detected inthe protected zone [49]. With the rapid increase in communicationtechnology, the implementation of differential protection isbecoming less costly. Furthermore, high sensitivity and selectivity,immunity to power swing and external fault, better performance forhigh impedance fault, and simplicity are among the mainadvantages of this relay.

Differential relays can block the protective action when thepower swing occurs. However, if the fault occurs during the powerswing, the relay must send a tripping signal to CB. Fault detectionduring the power swing will become a challenge when there is asymmetrical fault. In [50], a new method called differential power-based fault detection was proposed to identify symmetrical faultduring slow and fast power swing.

Besides fault detection during power swing, applyingdifferential relays for the long-distance transmission lines

encounters many challenges such as the requirement for acommunication channel, unsynchronised data gathered from twoends of the line, the inaccurately measured current due to thesaturation of the current transformer (CT), and capacitive current.Utilising a Global Position System (GPS) [51], symmetricalcomponent analysis when GPS is disconnected [52], and agent-based differential relays [53] were proposed to cope with theproblem of time delay or traffic conditions. Several methods basedon the distributed line models [54], phasor-based and time-domainbased current compensation [55], wavelet analysis of the spikes inthe faulted phase [56], skewness-based differential protection [57],and impedance-differential protection [58] have been presented todeal with the issue of the distributed capacitive current. Althoughthe differential protection is recommended for transformers of 10 MVA and above [59], some technical challenges such as CTsaturation [60] need to be addressed. Exponential decaying andmagnitude of the DC component of inrush current are the mainreasons for CT saturation. Differential relays are designed tooperate for an internal fault before entering the saturation region.However, there is a concern that transformer saturation occurs forthe external fault and the differential relay detects it as an internalfault [61]. Five main approaches for compensating CT saturationinclude using secondary current during the unsaturated period [62,63], reconstructing current by artificial neural networks (ANNs)[64, 65], reproducing current by CT model [66], correction ofdistorted current by using the wavelet transform [67], and datapreprocessing according to component analysis principle [68]. Onthe other hand, due to transformer energisation, inrush currentmostly presents on one side of the transformer [69]. To address thisissue, the ratio of the second harmonic component to itsfundamental harmonic component is used in commercial relays[69]. However, differential relays operating based on this methodmay experience an extra time delay [70]. Other techniques such asANNs [71], fuzzy systems [72], wavelet analysis [73], andadaptive approaches [74] were proposed to overcome theseshortcomings.

2.5 UV/OV (under voltage/over voltage) and UF/OF (underfrequency/over frequency) protective relays

Voltage sag usually occurs as a result of the fault, overload, andstarting of the large motors [75]. On the other hand, overvoltagetakes place due to many reasons such as lightning, switching,disconnection of bulk loads, ferroresonance, and insulation faults[76]. Recently, it has been recognised that high penetration of PVsystems at the distribution level could lead to overvoltage causedby reverse power flow [77]. Typically, disturbances in the powersystem disrupt the balance between consumption and generation ofactive and reactive powers, and as a result, voltage and frequencystability are jeopardised, simultaneously. Accordingly, UV relayand OV relay, UF relay and OF relay are four other conventionalprotective relays used to enhance stability in the power system.Voltage and frequency relays have many applications such aspassive islanding detection [78], load shedding (LS) [79], andproviding low voltage ride through (LVRT) capability of DGs [80].

According to Table 2, each type of relay has its benefits anddrawbacks. However, the following trends are expected in thefuture grid:

• Adding advanced signal processing methods to the conventionalrelays in order discriminate fault from other transientphenomena.

• Designing and producing DOCR with low production cost (e.g.current-only DOCR that removes potential transformer)

• Designing cost-efficient, high speed, and reliablecommunication systems for the differential relay.

• Designing multifunctional IEDs to quickly perform complexprotection functions.

Adaptively adjust the setting of the relay according to variousgrid conditions.


3 AC microgrid protection system challenges,solutions, and future trendsThe designing protection system is one of the last steps tosuccessfully implement a microgrid. However, according to theconfiguration of the microgrid, conventional protection systems arenot efficient. This section first discusses the protection challenges.Then, some possible solutions are presented to address them.

3.1 Topology changes and DERs impacts

Configuration and the energy resources in the power system affectthe measured current of a relay. This can be roughly presented inthe following equation [81]: (see (5)) , where ki is the impact factorof ith DG, d, GC, and CM denote distance to fault, gridconfiguration, and control mode, respectively. Switching fromgrid-connected mode to islanding mode considerably reduces thefault current, particularly where the microgrid is supplied withInverter Interfaced DGs (IIDGs) such as PVs and WTs because

IIDGs typically limit the fault current to twice of the rated current.The grid-connected and islanded modes affect fault current as it isdemonstrated in the first term of (5). Furthermore, the contributionof fault current is affected by the DG types, the number, andcontrol-mode of them, fault resistance, and finally, placement ofDGs. For example, three possible conditions, including blindingprotection, sympathetic tripping, and weak-infeed loop fault mayhappen in a microgrid as shown in Fig. 5 [18]. In this figure, greyand white blocks denote closed and open CBs, respectively. Itshould be noted that CB is connected with its respective relay,which is not shown in Fig. 5. In blinding mode, measured currentsent to relay 2 decreases because of injected fault current of DG1.In this situation, CB2 and CB4 will open. In sympathetic trippingmode, a DG injects a high level of current resulting in tripping ofrelay 3. Finally, for weak-infeed loop fault due to two differentroutes of fault, relay 4 will receive a higher level of fault currentthan relay 2. This may result in disconnection of CB4 and non-operation of CB2.

In order to mitigate the DGs impact on protection systems,several methods are proposed as follows:

i. Limiting maximum DG capacity: This method attempts to limitthe DG's maximum capacity to prevent adverse impacts on theprotection system [82]. However, this approach limits thepenetration rate of DGs in the future power system.

ii. Utilising a fault current limiters (FCLs): An ideal FCL is theseries device suppressing fault current of DG or branch byincreasing its impedance from zero to a high value in a fastmanner to enable coordination of protection [83, 84]. Up tonow, passive FCL (PFCL), superconducting FCL (SFCL),solid-state FCL (SSFCL), and controlled-based FCL to reachthe promising goal [85]. Although FCL permits higher DG

Table 2 Summary of protective relaysProtectiontype

Applications Advantages Disadvantages

OC protection • transmission anddistribution lines

• generators

• simple• inexpensive

• requires additional analysis to discriminate real-fault from other transient phenomena

directional OCprotection

• transmission anddistribution lines

• generators

• good candidate for the mesheddistribution system

• less expensive than differentialprotection

• more expensive than OCR• more complex coordination process• requires additional analysis to discriminate real-

fault from other transient phenomenadistanceprotection

• transmission anddistribution lines

• busbars• generators

• not influenced by sourceimpedance variations

• high impedance fault, power swing, bidirectionalpower flow, and high resistance fault candeteriorate the protection performance

• more expensive than OCRdifferentialprotection

• transmission lines• busbars• transformers• generators

• relatively simple• high speed and sensitivity• inherent selectivity• immune to power swings and

external fault• high performance for high

impedance fault

• high cost of implementation• error in measured current and communication

delay affect the protection performance

UV, OV, UF,and OF

• islanding detection• LS• satisfying LVRT

requirements

• proper design of underfrequency LS (UFLS) could bean effective tool to preventpower system blackout

• islanding detection based onUF/OF and UV/OV are cost-efficient and have no impact onpower quality

• separate designing of under voltage LS (UVLS)and UFLS protection will not lead to the desiredoutcome for LS purpose

• in conventional UFLS, the amount of shed load isfixed and does not affect by disturbance location,bus voltage, and rate of change of frequency drop

• islanding detection based on UF/OF and UV/OVhave large non-detection zone and thresholds arerequired to be adjusted properly

Irelay = IFaultgrid d, GC, R f × Operation Mode

+ ∑i = 1

nki d, GC × Ifault DGi CM, DG′s type, R f × StatusDGi

(5)

Fig. 5 Impacts of DG and topology change on selectivity(a) Blinding of protection, (b) Sympathetic tripping, (c) Week-infeed loop fault


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installation, the initial cost of FCL, the high recovery time ofSFCL, and switching losses of SSFCL are the main drawbacks.

iii. Utilising storage units: ESSs could provide sufficient amountof fault current into the microgrid in islanded mode [86].However, this method requires a high investment cost.

iv. Modifying DG's control: High penetration of DGs could haveadverse effects on relay coordination and increases thenecessity of dynamic changing of relays settings. One of thepromising solutions is to limit fault current by designing propercontrol in a system with high DG penetration [87]. Thismethod has many advantages such as no additional cost,enabling higher integration of DGs, less need for modifyingrelays settings, and enhanced fault ride through of DGs.However, difficulty in controlling fault current in indirectcurrent control and various control principles and systems indirect current mode are the main drawbacks for this approach.Also, overcurrent transients appear at the beginning of thelimiting process [88].

v. Non-conventional protection: This type of protection approachis classified into pattern-based, harmonic-based, and travelling-based methods. In a pattern-based scheme, signal processingtools such as wavelet transform (WT) and S-transform are usedto extract desired features/signals/components from originalsignal/signals.

In [89], WT extracts features such as change of energy,Shannon entropy, and standard deviation from each phase ofthe current signal. Then, decision tree (DT) classificationmethod is used for detection and classification. Thecombination of WT and DT makes this method improve thedependability and reliability of the relay. In [90], waveletpacket transform (WPT) is employed to extract the first-levelhigh-frequency subband contents of d–q-axis components ofcurrent. The transient disturbances such as grid connection andstarting of induction motor frequency components are locatedin low-frequency half-band (BL) with fixed locations anddecaying magnitude; however, for faulty condition frequencycomponents are located in both BL and high-frequency half-band with changing locations and magnitude. As a result, theWPT that extracts relocated frequency components of signalhelps to determine the type of disturbance. In [91], currentfrom two-sides of feeder are analysed by S-transform, which isa combination of WT and short-time FT, to obtain spectralenergy content, subsequently, and its differential. Bydetermination of different thresholds for these indices, differentsituations such as a fault in islanded mode, a fault in grid-connected mode, and high impedance fault could bedistinguished.

In harmonic-based protection, DGs inject high-frequencyharmonic in particular frequencies to locate the fault [92]. Themain advantages of this method include the fault could belocated regardless of the islanded/grid-connected mode, the

existence of DGs, and topologies of the microgrid. However,the main disadvantage of this method is its complexity.

Once a fault happens, voltage and current travelling waves(TWs) propagate along the transmission lines. According tothis fact, the fault can be located by analysing the TWs’features such as magnitude, polarity and time intervals betweenthe arrival waves. In [93], mathematical morphology techniqueis used to extract both time and polarity of initial current TWsfor identification of a fault in a meshed microgrid. Althoughthis method had fast response, accurate fault location formicrogrid with short power lines require an extremely highsampling rate and fast processing device.

vi. Adaptive protection: Adaptive protective relay anticipatesmicrogrid operation modes, microgrid topologies, and DERsstatus to update the relay settings to match itself with thecurrent conditions of the system [94, 95]. Synchronisedmeasurement of current phasors by the communication systemwith remote relays and GPS synchronisation are the mostcommon methods for adaptive OCR [96]. The communicationsystems used for adaptive protection involve centralised, andagent-based communication [97].

Each of these six solutions has its own advantages anddisadvantages which are presented in Table 3.

3.2 High impedance fault (HIF)

HIF scenarios are becoming one of the major concerns for theprotection system. These faults often occur when a brokenoverhead conductor falls on a high impedance surface or when twoor three phase conductors are connected through a high impedanceelement. In these cases, the levels of fault currents are very low andnot adequate to be detectable by conventional protection relays orfuses [98]. Thus, the energised conductors on the ground surfacepresent a human safety hazard. Furthermore, the arcing, which isthe result of a HIF, pose a fire hazard. One of the distinctcharacteristics of HIFs is ‘random behaviour with unstable andwide fluctuation in the current’, which results in high-frequencycomponents in the current. Proposed methods for HIFs detectionhave evolved from HIF modelling to computation-based methods[99]. Generally, latter methods are classified into time domain[100], frequency domain [101], time–frequency domain [102], andartificial intelligent such as neural networks [103], fuzzy logic[104], DT [105], and support vector machine [106]. However, mostof the methods are not designed for online fault detection.Furthermore, scarcity of HIFs data with high resolution is anotherissue for HIF detection. The methods for online detection of faultoccurrence and location as well as classification must becomputationally efficient as much possible as to be applicable inmodern relays. Recently, power line communication is utilised toinject a test impulse into the network for the detection of HIF andits location [107]. However, challenges such as the scarcity of

Table 3 Different approaches presented to address topology changes and DERs impactsMethods Drawbacks or challenges Benefitslimiting DG capacity • to prevent high penetration of DGs • cost-effective solutionusing FCL • recovery time of superconducting FLC is long

• switching loss of solid-state FLC during normal operation• additional cost for utility or DG owners

• allow higher penetration of DGs

using storage units • high investment cost • provide sufficient fault currentmodifying DG control • control of fault current is difficult in indirect current control

• in direct current, control principle and the system arechanged during the fault and normal condition

• inexpensive solution• no need to modify infrastructure of the utility• need for upgrading PDs will be reduced• allow higher integration of DGs in the power

systemnon-conventionalprotection

• higher computation burden than conventional protection • cost-effective solutions.• higher performance that conventional

relaysadaptive protection • may require communication infrastructures and fast

processing units• adaptable settings of relay bring proper

coordination of relays


available actual data, proper and fast methods for fault detection,location, and classification, and modelling of HIF should beaddressed.

3.3 Low voltage ride through

The high penetration of RESs in power system has led to theelaboration of specific technical requirements in the grid codes.The goal of modification in the existing grid codes is to improvethe stability of the grid [108]. One requirement of connecting awind power plant to an electric network is its LVRT capability, i.e.the ability of a power plant to remain connected to a grid duringvoltage sags and to actively contribute toward stability of thepower system by providing reactive power until fault clearance.Injection of reactive power during the grid fault may jeopardise thecoordination of the protective relays [109]. Also, DG units have totake into account the grid code requirements for protective relayscoordination [80].

3.4 Communication standards and protocols for IEDs

With the advent of power industry deregulation, an increasingnumber of IEDs, such as relays, metering devices, digitaldisturbance recording devices, and power quality devices are beingincorporated into distribution networks [110]. Regarding thisevolution, one of the main challenges of the Substation AutomationSystem is the necessity of a communication standard to supportinteroperability between IEDs, interchangeability of IEDs, self-descriptive devices, and scalability of the power system.

IEC 61850 is a structured and widely-accepted standard forautomation and equipment of power utility and DER units fromdifferent vendors [111]. In IEC 61850, the three main types ofcommunications have been defined: generic object-orientedsubstation event (GOOSE), sampled measured values (SMVs)based on the publisher/describer mechanism, and client–servercommunication between the electrical network monitoring andcontrol system and IEDs operated based on manufacturing messagespecification (MMS). GOOSE is a fast and unsolicited messagethat transmits data from an IED to single or multiple IEDs in apeer-to-peer or one-to-many fashion, respectively. Process bustechnology transfers SMVs of currents and voltages to the IEDs,

then GOOSE messages have been created over Ethernet for manyhigh-speed and high-priority applications including alarm,command, and interlocking signal [112]. MMS is used for non-time-critical applications such as communication betweencontrollers, as well as between stations and controllers. The IEC61850 standards based on the Open Systems Interconnectionmodel, a conceptual model that shows how applications cancommunicate over a network, is presented in Fig. 6. IEC 61850facilitates the implementation of these three ways ofcommunication systems by abstract definition of the data and theservices, which is independent of any protocols.

Then, these data and services map to an actual protocol. Forclient–server communication, the mapping is carried out ontoMMS that supports complex naming and service models of IEC61850; However, for SMV and GOOSE, communications aremapped onto an Ethernet frame. By eliminating the process of anymiddle layers, the data can be transmitted in a fast manner.Recently, more sections are planned to be added to IEC 61850standard to cover new aspects of the smart grid.

These communication services used for power utilityautomation, which are presented in Fig. 7, are summarised asfollows:

i. IEC 61850-7-4xx for modelling of hydro power and DERssuch as diesel generators, solar panels, fuel cells, andcombined heat and power.

ii. IEC 61850-5xx for the user guide.iii. IEC 61850-80-x for mapping to IEC 60870-5-101 and DNP3.iv. IEC 61850-90-x for communication between substations,

communication between substations and control centres,condition monitoring, transmission of synchrophasorinformation, network engineering guide for substations,distribution automation for PV, storage, and EVs schedules.

In addition to IEC 61850, IEC 61970/61968/62325, which arecollectively known as Common Information Model (CIM)standards, is the widely accepted and used to allow interoperabilityin smart grid domain [113]. CIM standards introduce data models,which are based on Unified Modelling Language (UML) bringinginteroperability into the wide range of Energy ManagementSystems (EMSs), distribution management systems (DMSs), andenergy market communications. As there is a significant dataexchange between these domains and substation automation, highcompatibility must exist between CIM and IEC 61850. Althoughthese two standards have different natures and evolutions, severalmethods have been proposed to unify them [114].

IEC 61850 is particularly designed for information exchangebetween IEDs and modelling system's elements. However, onedeficiency in the IEC 61850 standard is a lack of standardisation ofsequential, combinational, rule-based or any other forms of powersystem control and automation logic, such as interlocking logic forcontrol operation [115]. The IEC 61499 standard is proposed tomodel distributed industrial process measurement and controlsystems. The architecture of IEC 61499 standard is based on event-driven function blocks (FBs) encapsulating functionalities,behaviours, and their signal interconnection. These FBs can becombined to constitute a complex and hierarchical systemdescription. The use of the FBs facilitates implementation of thecontrol system. As inferred from previous parts of this paper, aproper protection scheme of AC and DC microgrids may consist ofcommunication links, control system, and intelligent managementcentre. As a result, a promising standard must covercommunications, modelling, and distributed control. Theintegration of IEC 61850, IEC 61499 and CIM standards couldmeet the mentioned requirements. Recently, IEC 61850/61499 wassuccessfully implemented to enhance the flexibility andadaptability of automation systems [116].

3.5 Cyber security

Smart grid implementation will be facilitated widely bycommunication networks transmitting data for data accumulation,control, protection, or energy management. Wide utilisation of

Fig. 6 The IEC 61850 described based on OSI model(a) Blinding of protection, (b) Sympathetic tripping, (c) Week-infeed loop fault


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communication systems makes the power system vulnerable tomalicious cyber-attacks and may deteriorate the performance of theprotection system [117, 118]. Typically, cyber-attacks are classifiedinto several types, including denial-of-service (DOS) attacks,eavesdropping attacks, account cracking etc. [119]. These cyber-attacks can be launched simultaneously, in a coordinated mannerfrom multiple points in either space or in time, or as a combinationof both simultaneous and coordinated attacks. As a result, they canaffect at least one of the security parameters includingconfidentiality, availability, integrity, and non-repudiation/accountability of information [120]. To have a secure and reliablecyber-physical system various domains including risk assessment,attack prevention, detection, mitigation, and resilience, must becarefully accounted for in the design [121] systems, respectively.The comprehensive comparison of IEC 61850, IEC 61499, andCIM is presented in Table 4.

4 DC microgrid protection challenges, solutions,and future trendsSome of the challenges of protection are common in both AC andDC microgrids, e.g. topology changes and DER impacts. However,protection of DC microgrids has specific challenges includinggrounding system design, the need for protection against faultcurrent with high amplitude and without natural zero crossingpoints in a simultaneously fast and coordinated way, prevention ofpropagation of voltage transients during fault isolation and

recovery, designing of advanced DCCBs, and a uniform standard[122].

4.1 DC fault analysis

DC microgrids are categorised into different topologicalconfigurations, such as multi-terminal, zonal, and DC looped. Thedecision to choose a specific topology of DC microgrid depends onthe application, reliability level, and voltage level [123]. Forexample, the U.S. navy focuses on developing zonal DC microgridto achieve a shipboard system with high survivability, low weight,as well as low implementation cost [124]. Regardless of thedifferent topological configurations of the DC microgrids, there aretwo types of DC bus architectures: unipolar DC bus topology usingtwo-level voltage source converters (VSCs) (see Fig. 8a) andbipolar bus topology using three-level neutral-point-clamped VSCs(see Fig. 8b). The bipolar DC-bus topology has differentadvantages over unipolar, including more power capacity,increased reliability, and flexibility in the connections betweenloads and DGs [125]. However, since each bus can operateindependently, imbalanced power flow may occur in the bipolarDC bus, leading to an imbalanced DC bus. As a result, a DCvoltage balancer is required [126]. Regardless of the DC microgridtopology, the DC fault could occur in either in the DC bus or in theDC-cables that interconnect the microgrid components. The DCbus is defined as the in-feed or out-feed point of a DG unit or nodaldistribution unit within the DC distribution system. Since theseunits, of necessity, must be interconnected via cabling, the cable

Fig. 7 Communication networks for power utility automation


interconnections are also a likely entry point for faults. Since thesimplicity of the microgrid is the goal of a DC microgrid, dc busand dc interconnections are intended to act as a single point ofenergy interface between DGs, ESSs and loads. From a protectionstandpoint, the down-side is that a fault on a DC bus or DC cableconnection has the simultaneous effect on DGs, ESSs, and loads.Also, DGs, ESSs, and loads may all contribute to the fault current.Therefore, if the protection system design is inadequate, a singlefault anywhere within the system can have unrecoverable impacts.

4.1.1 Battery and load DC fault analysis: The battery may befar away from DC bus. Therefore it has to be connected to the DCbus with cables. While the fault occurs in either DC bus or line, thefault current can be presented as follows [127]:

τbatt = Lbatt + LLBRbatt + RLB

(6)

ibatt t = VbattRbatt + RLB

1 − e−T /τbatt (7)

where Rbatt, Lbatt, RLB, and LLB are internal resistance andinductance, and line resistance and inductance, respectively.

On the other hand, the loads are categorised into constantimpedance, constant power, and constant current. Based on thetype of load, the fault current contributed by load could becalculated.

4.1.2 VSC DC fault characteristics: The DC microgrid dependsupon the VSC for energy transfer. Each VSC is equipped with aDC microgrid interfacing filter (see Fig. 9a). Because of the VSCstructure, for rapidly applied short-circuit faults, the capacitor firstdischarges through the DC network upon fault inception. Thencontribution from the converter interfaced sources forms the latterpart of the response (see Fig. 9b). Capacitor discharge may result insignificantly high current amplitudes that could damage DCmicrogrid interfacing components. The resulting fault currentcharacteristic is completely defined by the VSC(s) capacitivefilter(s) and the inductance between the fault current contributingVSC(s) and the fault location. This latter behaviour makes it verydifficult for conventional PDs, such as mechanical CBs (MCBs) todiscriminate the location of the fault before an unrecoverable lossof bus voltage occurs. Furthermore, primary unit-protectionfeatures (such as AC-side CBs or fuses) of the VSC come into playduring any fault scenario to protect against the transfer of faultcurrent from the VSC connected AC source to the DC bus [128–130]. As a result, even if the initial capacitor discharge is not anissue (i.e. due to slow fault inception or higher impedance fault), arace condition will always exist between external protectivefeatures and unit protective features, with the worst case resultbeing, again, unrecoverable fault behaviour. To address bothequipment damage scenarios and unrecoverable fault scenarios, afast protection device is required. To understand and analyse theDC fault characteristics, the non-linear system is solved bydefining three different stages including capacitor discharge stage,diode free wheel stage, and grid-side current feeding stage. Fig. 10shows an electrical equivalent circuit for each stage.

4.2 DC microgrid grounding systems

Although grounding systems are well-defined in AC microgrid, aproper design of DC grounding system, which provides personalsafety, ease of fault detection, and low electromagnetic interferencedepend on scale, voltage level, application, and so on [131, 132].

The three main goals of the grounding system are to ease thedetection of a fault, minimise DC stray current, and increasepersonnel/equipment safety by reducing common-mode voltage(CMV) [133]. Stray current and CMV are related to each other bythe grounding resistance. High grounding resistance results in verylow stray current and high CMV. However, the existence of lowresistance ground leads to low CMV and high stray current [123].According to IEC 60364-1, the DC grounding systems arecategorised into five types including TT, TN, and IT [124]. Thefirst letter including T and I refer to a direct connection to the earthand no connection to the earth, respectively. The second letterincluding T and N denotes a direct connection to exposed

Table 4 Main standards utilised in substation automationStandards Potential benefits Potential barrier or drawbacks ApplicationsIEC 61850 • reducing ambiguity thanks to using system

configuration language• lower installation cost by employing GOOSE• lower transducer cost by using merging units• lower commission cost due to the less manual

setting• multivendor interoperability• flexible and expandable structure• reducing network burden because of seven

levels of messages with the different prioritiesEthernet-based communication, whichsupports 10 Mbit/s–10 Gbit/s

• although GOOSE message is fast, it is notreliable because it maps directly onto theEthernet frame and TCP is not available

• probability of overloading in the Ethernetswitch in a case of GOOSE avalanche modeusing multicast communication on edgedevices

• further extension of DERs modelling for theWT, FCL, EV, flywheel, advanced meteringinfrastructure is required

• SMV loss/delay has adverse effects on theperformance of the protection system

• substationautomation

CIM • multivendor interoperability by UML• UML can offer cyber security by definitions for

users, companies, roles, consoles, andfunction authorities

• CIM is not compatible with IEC 61850 • EMS• DMS• energy market

IEC 61499 • encapsulation of behaviour, knowledge,functionalities into FBs

• FBs were designed to achieve portability,configurability, and interoperability

• lack of formal semantics and differentinterpretation of the execution of FBs results indifferent execution behaviours

• distributedautomationand control

Fig. 8 DC microgrid bus architectures(a) Unipolar DC bus, (b) Bipolar DC bus


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conductive parts and to the neutral point, respectively. As shown inFig. 11a, in TT grounding system converter middle point and bodyof the appliance are separately connected to the ground points. TheTT grounding system is straightforward to install, and the faultdoes not transfer to other parts of the grid; however, circulation ofcurrent, as well as the possibility of high voltage stress, are themain drawbacks of this grounding topology [125]. TN is the mostcommonly used DC grounding systems. In this configuration, theconverter middle point is connected to the ground, and the body ofthe appliance is connected to both neural and Protective Earth (PE).The TN topology has three subclasses, including TN-S, TN-C, andTN-C-S. In TN-S, separate PE and N conductors are used (seeFig. 11b); however, TN-C combines these two conductors toProtective Earth and Neutral (PEN) conductor to offer a cost-effective ground system (see Fig. 11c). TN-S system has thehighest Electromagnetic Compatibility (EMC) among differenttypes of TN grounding systems. Also, TN-S has higher safety thanTN-C, because, if the conductor gets disconnected, the protectivefeatures remain. As a result, this grounding system is suitable forinformation technology and communication networks. TN-C-Sgrounding topology is a combination of TN-C and TN-S to havemaximum benefit from two systems (see Fig. 11d). The advantagesof TN grounding systems include having sufficient amount of faultcurrent to be detected, requiring low grounding impedance, and thelimiting fault current by adjusting the ground resistance; however,for high voltage applications, the touch voltage is high [126]. ITgrounding system has no grounding point for the neutral point, andthe appliance body is grounded separately (see Fig. 11e). Thisconfiguration has advantages such as small current under LG faultand ability to continue providing energy to the loads; however, itsdisadvantages include hard-to-locate fault and unpredictable faultcurrent paths through the DGs when a second line to ground faultoccurs [134].

4.3 Fault detection and location

In the DC microgrid, the line impedance is very low, accordinglyfault current deviation is too high and the fault current exceedshundreds of amp in less than tens of milliseconds. As a result, theimplemented sensors must have high sampling rates and speed, andthe communication system must be a very low time delay, andreliable. Regarding installed sensors, communication and controlsystems, protection approaches must identify in a fast, reliable, andhigh precision way. Up to the present, several DC protection

methods including overcurrent, current derivative, directionalovercurrent, distance, and differential protections have beenproposed to detect and identify the faulty section. These protectionmethods are summarised in Table 5.

4.3.1 Overcurrent protection: Similar to the traditional ACovercurrent protection, a threshold is considered to determine theoccurrence of the fault. In [114], a combination of overcurrent andunder-voltage protections are embedded in the converters to locatefaults in multiple terminal DC distribution system within a fewmilliseconds. However, this method does not consider faultimpedances; as a consequence, implementing such a method on aDC microgrid with more complex architecture may result in eitherlonger fault clearance times or the disconnection of larger parts of anetwork than necessary in the event of a fault [115]. One solutionto address high-impedance fault is adding a parallel LC filter toeach pole to have resonance in specific frequency during the faultyconditions. Then, a discrete WT (DWT) is utilised to extract thisfrequency for fault identification [116]. Another solution is tointegrate a differential protection approach, which has highsensitivity, speed, and selectivity, with overcurrent protection tooffer a fast, efficient, and low costs protection solution [115]. Also,the coordination of the OCRs based on time-inverse characteristicsis difficult. To deal with this problem, the OCRs are connectedwith each other by a communication link, which is based on thestandard message of the IEC 61850 protocol, to provide selectivityand disconnect only the faulty parts [117].

4.3.2 Current derivative protection: Once the fault occurs, thecurrent derivative increases from zero to a very high value. Thisvalue can be used to identify a fault in a very short time. Tomeasure the current derivative, the sensors have to operate withhigh sampling rates. Using high sampling rates will amplify noiseand may result in false tripping. To deal with this issue, an efficientfiltering method is required to both have a little time delay andhigh noise cancellation capability [135]. On the other hand, currentderivative value depends on the cable length, line loading, and faultimpedance. Due to this fact, it is very difficult to find a properthreshold, and this threshold has to be adapted for each operatingcondition. To deal with this issue, first and second orders of thederivatives of the current are considered to detect the low and highfault impedance fault [118].

Fig. 9 VSC scheme under a pp fault(a) Equivalent scheme of VSC under a short-circuit fault, (b) Cable current icableduring the short-circuit fault

Fig. 10 VSC scheme under a pp fault(a) Stage 1 – capacitor discharge, (b) Stage 2 – freewheeling diodes, (c) Stage 3 –grid-side current feeding


4.3.3 Directional overcurrent protection: In a complex meshedDC microgrid current flows from both directions. Regarding thisissue, implementing directional overcurrent protection couldenhance the selectivity in such a microgrid. Recently, thedirectional overcurrent protection is proposed for a DC microgridwhere a communication system in place [119, 120]. According tothe presented approach, once the fault occurs, the magnitude and

direction of a fault current in all branches are identified by usingthe communication system to locate the faulty line.

4.3.4 Distance protection: This class of protection method isclassified into active and passive ones. The active impedanceestimation (AIE) is a new family of distance protection that locatesfault based on the injection of a short-duration disturbance into thepower system and measurement of transient response seen from thepoint of measurement; then the system impedance is calculated[121]. This idea had been successfully applied in the DC zonaldistribution system to detect and locate faults. In [136], a short-duration current is injected; then, through measurement of thevoltage and current responses, the system impedance is estimated.The key advantage of this method is its independence fromcommunication networks. Another advantage is that each zone canbe protected by a single unit. This would pave the way toimplementation of reconfigurable, plug-and-play power systems. In[137], AIE is implemented with a portable injection unit thatconsists of DSP/field-programmable-gate-array (FPGA) board, anisolation transformer, a variac, a diode rectifier, DC-link capacitors,an IGBT H-bridge, transducers, connectors, and cables. Also, acontinuous wavelet transform (CWT) is used as a more effectivedata-processing tool than a fast Fourier transform (FFT) whendealing with non-periodic signals. Although the AIE basedmethods different advantages, this method may fail to locate thefault if a load with high resistance exists. Because in this case, theinjected current is divided between the load and the fault circuitpaths, and therefore, the accuracy of fault location is decreased.Also, since a disturbance signal is injected a high-bandwidthmeasurement and high computing capability are required, that willresult in lower time response of this protection. In the passiveimpedance estimation approach, the impedance/inductance isestimated by applying advanced mathematical calculations. Forexample, local signals including voltage, current, and currentderivative are measured at multiple time instants; then an onlinemoving-window least-squares method is used to identify theequivalent inductance between a PD and a fault [138].

4.3.5 Differential protection: Differential relay measures only thecurrent amplitude of each side of a specific element by a currenttransducer and then, based upon the currents differential value,determines whether the fault has occurred or not. In [139], faultresponse of converter-interfaced DC systems is analysed toinvestigate how transient system behaviour, such as poorsynchronisation for the high change rate of a fault condition,influences the operation of differential protection schemes. Then,this analysis quantifies the necessary protection system responsetimes for fast and accurate fault detection. Finally, a centralprocessing device that takes advantage of the natural properties ofDC differential current measurements is designed to achieve high-speed differential protection. In [140], comprehensive protection ispresented for a medium-voltage DC microgrid with variousdistributed energy sources including PV arrays, WTs, a fuel cellstack, an ESS, and mobile generators. The proposed protectionschemes include communication-based differential protection witha solid-state switch for distribution lines, DC overcurrentprotection as a backup for lines protection and communication-based DC directional overcurrent protection devices for bothsource and load protection to support bidirectional power flow.Nevertheless, similar to AC microgrids, differential protection hasdisadvantages such as the need for a communication system, being

Fig. 11 DC microgrid grounding systems(a) TT, (b) TN-S, (c) TN-C, (d) TN-C-S, (e) IT

Table 5 Comparison of five main DC protection methods Overcurrent Current derivative Directional overcurrent Distance Differentialbasic principle I > Ith di/dt > di/dtth I > Ith and current direction change Z < Zline |Iin + Iout| > Ithsampling time high very high high high highcommunication not required not required required not required requiredselectivity moderate low high high highspeed moderate high moderate slow highsensitivity low high low moderate high


753

a high-cost solution, no capability for backup protection, and beingsusceptible to current transducer errors.

4.4 Device technologies

PDs used in the DC system are broadly divided into AC CBs(ACCBs) and DCCBs. ACCB is a simple and economic solutionfor VSC-based DC system. However, the ACCB may not be fastenough to prevent damage to the VSC's freewheeling diodes. Also,employing ACCB leads to disconnection of the whole network.Another solution is a combination of ACCB and fast DC switches[141]. In this method, the DC system has to be completely de-energised until the fault is removed. On the other hand, the increasein penetration of DG and energy demand results in a rise in faultcurrent levels that may exceed the rating of the existing CBs andloss of coordination of the overcurrent protection [142]. Replacingthe network's facilities such as transformers, transmission anddistribution lines, and CBs with higher ratings is not a cost-efficient solution. In the past years, different DCCBs technologiesincluding fuses, MCBs, solid-state CBs (SSCBs), and hybrid CBs(HCBs) have been introduced for DC systems. For LV system,fuses, moulded case CBs and MCBs are three conventional PDs[128]. Table 6 presents the advantages and disadvantages of eachclass of PDs.

4.4.1 Fuse: The fuse, which consists of a link and heat-absorbingmaterial inside a ceramic cartridge, is used as the simplest PD inthe protection of DC systems for voltages up to 4200 V. Fuses areideal to be applied in DC systems with a low inductance or (highdeviation of current) because the time for the fuse to reach meltingpoint would be minimum [143]. Although the fuse is a low-cost PD

with a simple structure, it has disadvantages such replacement aftera successful operation and inability to discriminate betweentransient and permanent fault.

4.4.2 Mechanical CB: MCB is a PD that uses a mechanicaldevice, or so-called interrupter, to stop the current. Interruptionprocess of the current in the MCB is always accompanied by anelectric arc between switching contacts. To suppress the arc,several solutions were offered including generating a voltageopposing the system voltage to drive current to zero as well aspassive and active commutation of DC current [144]. In the passivecommutation method, when interrupter opens, current flowsthrough an L–C circuit, with a capacitor that has not been pre-charged, starts to oscillate and to create current zero points. In thiscase, the mechanical switch completely interrupts current in itspath. During these two stages, the mechanical switch voltage iscontinuously increasing until it reaches a specific value. Once thevoltage passes the definite value, the current flows to energyabsorber circuit, which is typically Metal Oxide Varistors (MOVs)to dissipate the stored energy. On the other hand, in the activecommutation case, the un-precharged capacitor is replaced with theprecharged capacitor. In this type of MCB implementation, whenthe interrupter opens, the charged capacitor injects a negativecurrent equal to fault current to make a zero-crossing current.During the interruption process, magnetic energy is stored in thesystem inductance. Metal-oxide varistors (MOVs) are connected inparallel with interrupter to relieve overvoltage stress and absorb theinductive energy in the path of the fault.

The main advantages of MCBs are low power loss andrelatively low cost; however, slow response time and limitedcurrent interruption capability are their main disadvantages. Themechanical passive and active resonant CBs are shown in Figs. 12aand b, respectively.

4.4.3 Solid-state CB: To cope with the problem of low timeresponse and other limitations, semiconductor-based switches areused in SSCBs. The concept of the SSCB is a new idea. SeveralAC SSCBs based on power semiconductors devices had beenproposed to increase power quality, isolate faulty part within acouple of microseconds [145]. However, implementation of a fast-acting DCCBs is much more important, because the fault currentcould increase up to hundred times of rated value within tens ofmicroseconds.

A typical scheme of SSCB is shown in Fig. 13, where a coolingsystem is employed to ensure high efficiency of the SSCB duringthe conducting condition.

Different semiconductor devices, such as gate-turn-offthyristors (GTOs), silicon insulated-gate bipolar transistor (IGBT),and integrated gate-commutated thyristor (IGCT) have beenutilised in this context, each one of them having their ownadvantages and disadvantages. It is reported that GTOs and IGCTshave much lower on-state losses than IGBTs. Also, the IGCTrectifier has the highest reliability. Based on IGCT and GTOcharacteristics, four SSCB topologies were presented andcompared regarding economical and technical aspects [146].Utilising IGBTs, GTOs, and IGCTs would increase both cost andconduction losses of SSCBs. Therefore, in industrial applications,

Table 6 PDs used in DC microgridsPD Disadvantages Advantagesfuse • not able to distinguish between a transient and a permanent fault

• fuse need to be replaced for successful operation• low cost• simple structure

MCB • long operating times (30–100 ms)• limited interruption current capability

• relatively low cost• very low power loss

solid state CB • expensive• high power loss• big due to heatsink needed

• fastest response time (<100 μs)• very long interruption lifetime

HCB • very expensive • low power loss• no arcing on mechanical contacts• fast response time (few ms)

Fig. 12 MCB with(a) Passive commutation circuit, (b) Active commutation circuit

Fig. 13 Typical SSCB


high voltage and power of SSCBs are seriously hampered. Onesolution is to replace IGBTs, GTOs, and IGCTs with thyristors.The main drawback of thyristors is not able to actively turn-offcurrent. However, this issue is less critical in SSCB applications,because there is no need to switch at high frequency. The on-statelosses of thyristor switch are much less that IGCT switch. Thiswould result in more reduction of overall life-cycle costs of theSSCB and decrease in investment on the cooling system ofthyristor-based SSCBs. Regarding this issue, three differenttopologies for the forced commutated circuit and three differentcharging circuits were proposed and compared with each other interms of the economic and technical aspects [147]. It is noted thatthe topology with a varistor coupling of all phases is an economicalsolution and can meet all requirements of modern SSCBs [147]. Inspite of very fast time response (<100 μs), high or relatively highconduction loss of the silicon power devices is still the majordrawback of this type of CB [146, 148]. Another disadvantage ofSSCB is large size due to the need for a heatsink. With theadvancement in technology and the introduction of newsemiconductor devices such as WBG power switches, DCCB hasbecome a solution that can isolate the fault in a fast and efficientmanner [149–151]. Recently, WBG semiconductors such assilicon-carbide (SiC) and gallium-nitride (GaN) have beenintroduced as ideal materials for switching devices in high-voltage,low on-state losses, high-power, high-temperature, and high-frequency applications (see Fig. 14) [150]. WGBs semiconductorshave the following unique characteristics including higherbreakdown voltage thanks to higher field strength (Si: 0.3, SiC:1.2–2.4, GaN: 3.3), being thinner and having lower on-resistanceand higher thermal conductivity. Although high device cost andreliability concerns are two main obstacles that prevent applyingthese materials in SSCBs, research and development are pursuedintensively in this area [153, 154].

4.4.4 Hybrid CB: According to Table 6, each of the MCB andSSCB has its own drawbacks and benefits. HCB is a new class ofCBs that combines MCB and SSCB to take advantages of both[155]. As a result, HCBs have advantages such as fast response,low power loss, and no arcing at the mechanical contacts.However, the high cost of HCBs is still the main drawback.

Typically, as shown in Fig. 15, HCB consists of four main partsincluding a mechanical switch (MS), a low voltage SS switch as acommutation switch (CS), a high-voltage SS switch as the mainbreaker (MB), and MOVs [156]. In the normal mode, the currentflows through the MCB to reduce power loss and at the instant offault, fault current commutates from the MCB to the SSCB tointerrupt the fault current, and finally the fault current commutatesto MOVs and decreases to zero, which results in decreasing theresponse time. In addition to the classic HCB circuit, severalmodels such as phototransistor-based HCB [157], field effecttransistor (FET)-based HCB [158], multi-stage IGBT-based HCB[159], IGCT-based HCB [160], and thyristor-based HCB [161,162] have been proposed. In low-voltage applications, the CS isnot required, because the generated arc voltage of MS is higherthan the voltage drop of MB and therefore the current naturallycommutated from the MS to the MB. Whereas in the high voltagesystems, the MS arc voltage could not reach to the MB voltagedrop. As a result, CSs are required to ensure successfulcommutation [163, 164].

4.5 Standards and regulation

Although DC microgrids have been studied for a couple of years,there is no comprehensive standard. Recently, differentorganisations such as EMerge Alliance, EuropeanTelecommunications Standard Institute (ETSI), IEC, IEEE, and soon are actively working to develop the required standards for DCsystems.

ETSI EN 300 132-3-1 defines operating voltage range innormal condition for DC system [165]. For example, the nominalvoltage range at the interface of telecommunications and datacomequipment must be between 260 and 400 V DC. On the other hand,voltage variations, voltage dips, short interruptions, and voltagesurges are also defined in abnormal conditions. Also, forprotection, maximum steady state and different limitations oninrush current are specified.

A revised version of IEEE standard 946 presents a guideline fordesigning lead-acid batteries, selection number of the batteries, andso on. Also, this standard briefly explains the impact of groundingon the operation of the DC system [166]. IEC SG4 focuses onenergy efficiency, EMC reduction, protection, and grounding issues[167].

Although many efforts have been made to develop standards tofacilitate implementation of DC microgrid, there is still a lack ofpractical standardisation on grounding systems for different voltagelevels, cyber-security, and protection system.

5 Conclusion and future trendsProper protection of AC and DC microgrids is one of the lastbarriers for implementing microgrids. As a result, designing aproper protection system that copes with all of the challenges in theAC/DC microgrid is an essential task. To this end, this paper hasinvestigated protection issues and viable solutions in microgrids.

Overcurrent, directional overcurrent, distance, differential, over/under voltage, and over/under frequency relays are classicalprotection systems that could present an acceptable performance inthe conventional power system. However, with the introduction ofthe microgrid, a higher number of DERs are allowed to beintegrated into the grid. One of the main challenges arises whenmicrogrid contains only/mostly power electronics interfaced DGs.In such a case, since these DGs limit output current to a specificvalue, fault current level will be low. This level will be much lowerin the islanded mode of the microgrid. The above of all existenceof DGs and the different configuration of microgrid significantlyaffect of fault current level. In such a gird, the conventional relaysshow poor performance. To deal with this challenge, severalsolutions including limiting maximum DG capacity, Modifying DG

Fig. 14 Summary of Si, SiC, and GaN relevant material properties [152]

Fig. 15 Typical HCB


755

control, using FCLs, ESSs, modification of conventional relays,and adoptive relays have been presented and compared with eachother. High impedance fault is another challenge that limits thelevel of fault current and results in misoperation of relays.Furthermore, with the advent of massive amounts of IEDs andwidespread use of communication systems in the grid, two otherchallenges arise including cyber security issues and the need for acomprehensive standard to cover data modelling, distributedcontrol, and substation automation. Towards a comprehensivestandard, three main standards including CIM, IEC 61850, and IEC61499 were discussed in details. Since each of these standards hasits own applications and goals, these standards do not havecompatibility, or it has a lack of specific features. Accordingly,researchers put tremendous efforts to unify/integrate/harmonise thestandards.

To demonstrate the destructive effects of fault current on powerelectronics devices, this paper first analyses the DC fault current inthree stages including the capacitor discharge stage, freewheelingdiodes stage, and grid-side current feeding stage. In the first stage,depending on the fault impedance and the impedance between thefault point and the VSC, the fault current increases up to 100 timesof nominal values in less than a couple of milliseconds/microseconds. This current will be discharged through the cableuntil its voltage reaches zero, then this current passes through thefreewheeling diodes. As this current has high value, it has to beinterrupted to prevent the VSC from being damaged. The type ofgrounding system could also influence on safety, the ability toidentify the fault, and survivability of DC microgrid under faultyconditions. Regarding this, three classes of grounding systemsincluding IT, TT, and TS, with their own advantages anddisadvantages, have been discussed and compared. Also, this paperhas reviewed five main DC fault detection/location methods:overcurrent protection, current derivative protection, directionalovercurrent protection, differential protection, and distanceprotection. Each method has its own advantages and disadvantages.For example, differential protection has high accuracy and fastresponse to identifying the fault, however, its performance ishighly deteriorated by communication delay/disconnection. Mostof the proposed methods are applied for low fault impedance,radial DC system, grounded system. However, for example it isvery hard to detect and locate a high impedance fault in a multi-looped DC microgrid with the unground system. As a result, tolocate a fault in such a DC microgrid, new fault location algorithmsare required that are independent of sensor error, communicationsystem, and communication delay. Since the nature of DC faultcurrent is different from the AC fault current, the PDs have to bedesigned to meet the requirements. Four types of PDs includingfuses, MCBs, SSCBs, and HCBs have been presented andcompared regarding cost, time response, power loss, and size.MCB and SSCB are two dominant PDs in industry and academia.MCBs have very low power loss and longtime response. On theother hand, SSCB has high fast response and power loss and veryfast time response.

Regarding all challenges in AC and DC microgrids, the majorfuture trends could be as follows:

Fault detection/location in AC and DC microgrids: Sincemicrogrid topology changes and the existence of DERs severelyimpact on the performance of microgrid, the promising protectionsystem must figure out the microgrid status with/withoutcommunication. According to this requirement, adaptive protectionis one of the main candidates. Recently, a method based on a high-frequency signal injection has been proposed. This approachdetects and locates a fault in the higher frequency domain, andtherefore is independent of topology changes and the existence ofDERs. Thanks to this prominent feature, this method assumes to bea major trend for fault detection/location shortly. On the otherhand, fault detection/location in DC microgrid must be much fasterdue to the DC fault current characteristics. Although it seems thatdifferential protection is the most viable approach in such amicrogrid, other approaches such as current derivative, directionalovercurrent, distance protections could be appropriate candidates if

advanced signal processing or other methods will address theirchallenges.Cyber-attack: Communication networks will equip futuremicrogrids for data accumulation, control, monitoring, protection,or energy management. Wide utilisation of communication systemsmakes microgrids susceptible to malicious cyber-attacks and mayworsen the performance of the protection system. Typically, cyber-attacks have several types including confidentiality attacks, DOS,eavesdropping, account cracking, false data injection, replay, man-in-middle attack, and so on. These cyber-attacks could be launchedsimultaneously, in a coordinated manner from multiple points ineither space or in time, or as a combination of both simultaneousand coordinated attacks. To have a secure and reliable cyber-physical system, different areas including risk attack prevention,detection, mitigation, and resilience, have to be designed carefully.Therefore, it is critical to redesign the protection schemes with aspecific focus on cyber security.Standardisation: The ‘IEC Smart Grid Standardization Roadmap’suggests different key standards including IEC 61850, CIM, IEC62351, and IEC 62357 for the implementation of smart grids. Also,German DKE smart roadmap adds IEC 61499 to other keystandards. Each of these standards covers specific domain and mayhave limitations. For example, IEC 61850 and IEC 61499 lack thespecification of functions and communication service, respectively.As a result, the integration of these two standards is a promisingsolution for the implementation of flexible and adaptable gridautomation. Furthermore, because of the lack of compatibilitybetween IEC 61850 and CIM, these two class of standards have tobe harmonised/unified to facilitate seamless data exchange betweensubstation and management levels. Although standards have beendeveloped for long times, the DC stardisation is in the early stages.Several organisations involved in developing standards, but there isstill a need for a comprehensive and practical standard for DCmicrogrids.Grounding systems for DC microgrid: Generally groundingsystems are classified into TT, TS, and IT. However, regardingsize, application, scale, voltage level, and other characteristics ofDC microgrid, an optimum grounding system could be designed byconsidering a mixed configuration or more active components.DCCBs: Design of a proper DCCB depends on applications. Inlow-power application, the future trends will be designing DCCBsbased on WBG based DCCBs to offer a DCCB with lower powerloss, higher voltage, temperature, and frequency capability. On theother hand, for high-power application, it is recommended todesign a DCCB based on the integration of a SSCB and fast-actingMCB to present a reliable DCCB. Besides this general trend,recently DCCBs are integrated to present a multi-port DCCB. Thistype of DCCB has salient features such as reducing cost, size andcontrol signals. Another promising and cost-effective topology so-called breaker-less topology eliminates the need of DCCBs byusing fully controllable power converters that coordinate with noload contractors or segmentising contractors to isolate faults.

6 AcknowledgmentThis work was supported in part by the National ScienceFoundation, grant no. 1439700, USA.

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