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International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 12, Number 3 (2019), pp. 405-418 International Research Publication House. http://www.irphouse.comA Review on Loss of Mains and Islanding Detection in Microgrids withMultiple Renewable Energy SourcesMogaka L. O.1, Nyakoe G.N.2 and Saulo M. J.31Department of Electrical Engineering, Pan African University Institute of Basic Science, Technology and Innovation (PAUSTI),Kenya.2Department of Electrical and Electronic Engineering, Jomo Kenyatta University of Agriculture and Technology, Kenya.3Department of Electrical and Electronic Engineering, Technical University of Mombasa, Kenya.and voltage regulation especially when there is multiple RESsof different technologies in the same MG [1] [2] [3].AbstractThe conventional distribution systems are radially configuredand power flows unidirectionally from the transmissionsubstation to the downstream loads. However, unlike thetraditional system, power flows bi-directionally in the moderndistribution systems which incorporate Renewable EnergySources (RESs) as the grids evolve to smart grids. This modernsystem comes with inherent challenges in terms of systemprotection as the system loads receive power supply frommultiple Distributed Generators (DGs) in the same microgrid(MG). The MG operation is a new concept for future energydistribution system that enables the RES to be integrated atpoints close to the load centres. This consists of a number ofDGs that are usually interfaced to the grid either directly orthrough power inverters. For the islanding operation of multipleDGs in a MG, the main consideration is on how to maintain thevoltage and frequency within the required standards andimprove power supply reliability. This paper reviews andcategorizes various approaches of loss of mains and islandingdetections in MGs with multiple RES, compare them in termsof their respective advantages and disadvantages and alsohighlight how the various islanding detection methods haveevolved over time. In addition to this, the future trends of thesemethods will be analyzed and discussed in detail as found inthe research literature.Now that most of the RESs produces power intermittently,there is a consequent variation of system voltage and frequencywithin the MG. To overcome this challenge of intermittency inpower production, various RESs like wind and photovoltaic(PV) are usually integrated in the same MG to form a hybridsystem. This improves the reliability of power supply to someextent. This hybrid of RESs may be connected to the grid toshare the excess or get the deficit power as the situation mayarise [4]. In addition to this, most of the RES employ powerelectronic interfaces to convert the generated power into usableelectrical energy before connection to the utility grid. However,these electronic converters introduces harmonics into thesystem.This high penetration of RES in distribution systems has led toislanded operation mode of DG units and MGs as a suitableapproach to maintain continuity and reliability of supplyinstead of disconnecting the DG when a fault occurs [5]. Thisis because the use of RES is an economical and preferred choicein remote areas where it is not economically feasible toconstruct transmission lines to reach those customers. It alsoassists in transmission loss reduction by having generationbeing close to load centres. Among the distributed generationsources, RESs are developing very fast because of favourablegovernment policies that are geared towards reducinggreenhouse gas emissions as per the international standards andalso to reduce the consumption of fossil fuels which are limitedin supply [6].Keywords: Loss of mains detection, DGs, RES MGs, Islanding1.INTRODUCTIONThe use of RESs DGs is on the rise of recent due to a numberof factors. These include; an effort to supply the increasingpower demand in proportion to increasing population, thegeneral response to the global warming and environmentalconcerns, as a result of privatization and deregulated electricitymarket, among others. Unlike the conventional distributionsystems where power flows in one direction from the mainsubstation into the downstream loads, the power flows in bothways in grids with high penetration of RESs connected at thedistribution level and smart grids in general. This brings achallenge when it comes to the system protection coordination,islanding, safety, system stability, reliability, supply securityThe operation of DG units in MGs can broadly be classifiedinto two categories based on their configurations. That is, theycan be configured to operate on the voltage-controlled mode orcurrent controlled mode [7]. In grid-connected mode, the unitsare operated in the current controlled mode while in islandedcondition the DGs operate in voltage and frequency controlmode. The most adopted control strategies for current controlmode and inverters are discussed in references [8] and [9]. Inislanding mode, the electronic converter interfaces between theloads and the micro-source acting as voltage sources. These areresponsible for the power sharing according to their ratings and405

International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 12, Number 3 (2019), pp. 405-418 International Research Publication House. http://www.irphouse.comavailability of power from their corresponding energy sourcesor prime movers [10].technology only while others a combination of DG sources.Figure 2 below shows a generic system for islanding analysisas per IEEE1547 [11].In order to maintain continuity of the power that is supplied tothe loads, most utilities recommend islanded mode of operationfor DG units in case of a contingency leading to loss of mains.To attain the required efficiency and reliability, the LOMdetection methods should be fast, precise, and cost-effective.Hence, various methods of LOM detection and theircorresponding control strategies have been developed anddeployed on the RES units and MGs. In accordance to IEEEStandard 1547-2003, islanding should be detected as quickly aspossible, within 2 seconds after it occurs, to be precise.Islanding operation can be defined as a condition whereby apart of the entire power system grid with loads and generatorsare isolated from the rest of the grid [11]. The necessaryconditions for MG LOM and islanding are published andelaborated in standards such as IEEE-1547 [12] and IEC-62116[13]. An islanding concept of typical topologies of MGs assuggested in IEEE 1547.4 [14] [15], is illustrateddiagrammatically in figure 1 below.Figure 2: IEEE general system for islanding analysisThe RLC and frequency values can be determined by using theequations (1)-(4) below;๐‘… ๐‘‰2๐‘ƒ๐ฟ๐‘‚๐ด๐ท๐ฟ ๐‘‰2(2๐œ‹ ๐‘“ ๐‘„๐‘“ ๐‘ƒ๐ฟ๐‘‚๐ด๐ท )๐‘ƒ๐ฟ๐‘‚๐ด๐ท๐ถ ๐‘„๐‘“ (2๐œ‹ ๐‘“ ๐‘‰ 2 )1๐‘“ (2๐œ‹ ๐ฟ ๐ถ )Figure 1: Illustration of Islanding ConceptThe rest of this paper is organized as follows: In Section 2, thecurrent standards and methods used for islanding detection aredescribed. The technical considerations of islanding arepresented in Section 3. Section 4 illustrates the performancecriteria for islanding detection. Finally, the islanding detectionconsidering multiple DGs in MGs and its future trends arediscussed in Sections 5.2.(1)(2)(3)(4)More often than not, Over/Under Voltage (OUV), Over/UnderFrequency (OUF) and the Rate of Change of Frequency(ROCOF) at the PCC are often used as the most conventionalislanding detection parameters. This is illustrated by theequations (5)-(9) below;CURRENT STANDARDS AND METHODS USED FORISLANDING DETECTIONAs of now, there is no specifically designed and standardmethod for testing the islanding detection and operation forMGs. Some of the existing methods make use of one DG๐‘ƒ 2๐‘ˆ๐‘ƒ๐ถ๐ถ๐‘…(5)๐‘„ 2๐‘ˆ๐‘ƒ๐ถ๐ถ1()๐‘…๐œ”๐ฟ ๐œ”๐ถ(6)So2๐‘ˆ๐‘ƒ๐ถ๐ถ ๐‘ƒ๐‘…406(7)

International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 12, Number 3 (2019), pp. 405-418 International Research Publication House. http://www.irphouse.com๐œ” ๐œ”0 ( ๐‘„ 21 ๐‘„() 1 . )22ฮป ๐‘ƒ4ฮป ๐‘ƒ1anti-islanding algorithms cannot detect the islanding situation[28] [17] [29]. As a result of this, these methods are not suitablefor LOM and islanding detection in MGs with multiple RESs[30] [31]. Figure 4 below shows the flow chart for the LOMand islanding detection using passive method.(8)And๐‘‘๐‘“๐‘ƒ๐ฟ ๐‘ƒ๐บ ( ๐‘“)๐‘‘๐‘ก2๐ป ๐‘†๐บ๐‘ ๐‘Ÿ(9)StartParameters Measured at PCCAs a result of the continued interconnection of RESs, islandedmode operation of these RESs will be an unavoidable issue inthe future [16]. LOM detection is crucial for the safe operationand stability of the power system grid with RES integration.Islanding detection methods can be broadly classified into theremote methods and local methods of islanding detection ashighlighted in figure 3 below [17] [18].YesParameters in the rangeof reference setting?NoIslanding condition detectedClassification of islanding detection methodsAlert alarm and disconnect DG from local loadsRemote MethodsLocal MethodsStopPassive MethodsActive MethodsFigure 4: Flow chart for passive methodFigure 3: Islanding Detection Methods Techniques2.1.22.1 Local Islanding Detection MethodsThese techniques of loss of mains were devised to overcomethe weaknesses of passive methods of islanding detection. Theyidentify an LOM condition by applying small perturbations oruse a positive feedback [32] and then observe the changes insystem parameters when the DG is islanded [33] [34].Moreover, these techniques are characterized by simplicitywith a high efficiency, good accuracy and very reliable [35].Examples of active methods include impedance measurement[36] [37], active frequency drift [38], frequency jump method,Sandia frequency shift [39], sliding mode frequency shift,phase distortion method [40], harmonic injection method [41],Sandia voltage shift, power variation monitoring, and reactivepower export error detection.Local islanding detection methods can be grouped into twocategories: that is, active and passive techniques of LOMs andislanding detection. These are elaborated in the subsequentsubsections below.2.1.1Active LOM and islanding detection TechniquesPassive LOM and islanding detection methodsThese methods use the local measurements at the DG side andmonitor for sudden electrical parameters variations. The gridvoltage, frequency, phase angle and total harmonic distortionare monitored to detect the LOM and islanding condition [19].If these monitored parameters goes outside the set limits, thenthe connected relay opens and disconnects the DG from thesystem. Using changes in OUV or OUF [20] [21] [22], ROCOFor ROCOV [23] [24], fast changes in the voltage phase, voltageharmonic monitoring, etc., these methods discern LOM and theoccurrence of islanding condition of the monitored MG.Most of these active IDMs have not discussed the islandingdetection in multiple generators. In reference [42], Reigosa andothers discussed briefly a case of multiple DGs islandingdetection using the injection of signals at high frequency levels.However, they proposed the injection of these signals from themaster DG as other DGs act as slaves in the system. This lowersthe islanding detection accuracy in the absence of the masterDG. Despite their merits, these active methods introduceundesirable disturbance in the grid, which should be kept assmall as possible especially in MGs with multiple RESs as thiscan cause interference between DGs operating in parallel [17].These methods also have poor performance in the presence ofThese methods are generally simple and cheap to implement.Moreover, they donโ€™t have any effect on the power quality inislanding detection [25] [26]. They however show a large nondetection zone (NDZ) [27], i.e., when the generated power bythe DG is equal to the power absorbed by the local loads andno power is therefore imported from the utility grid. In thiscase, the variation of the grid variables is negligible, and the407

International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 12, Number 3 (2019), pp. 405-418 International Research Publication House. http://www.irphouse.commultiple DGs in a MG [43]. A detailed survey on LOM andactive islanding detection methods is discussed in references[44], [45] and [46]. Figure 5 below shows a flow chart for activeloss of mains and islanding operation detection techniques.2.3 Hybrid islanding detection methodsThese techniques were devised in order to overcome thedrawbacks of both the active and passive techniques in LOMsand islanding detection and to optimize on their merits. Toachieve this, the two methods are combined so as to improvethe accuracy of the system [51] [52]. The two combinedmethods are not used concurrently. The active method is onlyemployed after the island is first detected by the passivemethod. This acts to verify the fact that an island has occurredor not, hence increasing the accuracy. This does not have agreat impact on the quality of power and significantly reducesthe NDZ.StartInject disturbance signal to PCCParameters Measured at PCC(Voltage, current, harmonics)YesParameters in the range ofreference setting?2.4 Modern islanding methodsThese are the most used techniques currently due to theirefficiency and high accuracy. They make use of signalprocessing methods in the extraction of the signal and itsclassification as islanded or not islanded. This greatly improvesthe accuracy in LOMs and islanding detection and theperformance of the entire system. Examples of these techniquesof islanding detection include and not limited to: mathematicalmorphology, HH transform, TT transform, Wavelet transform,S-transform [11], among others.NoIslanding condition detectedAlert alarm and disconnect DGStopThe current research trend has shifted to the combination of thepassive techniques and artificial intelligence (AI) algorithms inislanding detection. These methods have proven to be havinghigh efficiency, accuracy, reliability and can be applied foronline monitoring of the power grids [35]. There are a numberof examples of these modern hybrid techniques. For instance,Gaing combined DWT with PNN in power disturbanceclassification [53], Othman combined PNN with WT for faultidentification and location [54], Yin also used a combination ofFast Fourier Transform and Artificial Neural Network forislanding detection [55] and also Elnozahy used a combinationof WT and ANN in islanding detection [56] among other hybridmethods.Figure 5: Flow chart of Active Method2.2 Remote Islanding Detection MethodsThese methods were developed to overcome the challenges anddemerits of local LOMs and islanding detection methods. Theyare also known as communication based islanding detectionmethods. These methods can be broadly categorized into threegroups: Supervisory Control and Data Acquisition basedmethods (SCADA) [47], Power Line Carrier Communication(PLCC) methods [48], and methods that monitor the deviceused to connect or disconnect from the grid [49] [50].Basically, the SCADA technique monitors the grid systemparameters like currents, voltages and frequencies of the circuitbreakers and transfer them to the main control centres. On theother hand, PLCC method, has transmitters which are used toproduce the status signals and transmit them on the power linesto the central point for making decision on islanding condition.These methods do not have NDZs and are grid friendly. Theyare applicable to the systems with multiple DG units. Thisinclude both inverter-based sources and synchronousgenerators. However, they are not very unpopular due to theneed of an ample and reliable communication system betweenthe DG and the utility grid. This is a costly undertakingespecially for small MGs [43].2.5 Analysis of Classical LOM Detection MethodsThere are many and different LOMs detection methodsdeveloped that have been used for different types of DGs andtheir operating conditions. Some islanding detection methodsare suitable for synchronous-based generators while others aresuitable for inverter-based DGs [57]. Generally, there is nouniversal islanding technique that can work for all the differentDG systems. Each technique has its merits and demerits and thechoice of methods to implement requires a compromisebetween merits, demerits, cost and desired reliability amongothers. Table 1 below presents a comparative analysis of someof the conventional characteristics of the different classicalislanding detection techniques.408

International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 12, Number 3 (2019), pp. 405-418 International Research Publication House. http://www.irphouse.comTable 1: Comparative analysis of islanding detection DetectionReliabilityEffect on multipleDGsPassive methodsLargeShortNo effectLowLowNoneActive onissuesHybrid issuesMachine learningSmallVariableNo effectHighHighNoneRemote methodsNoneVery shortNo effectHighVery highIncrease in cost TECHNICAL ASPECTS ANALYSIS OF LOMsAND ISLANDING DETECTIONIslanding condition can be broadly classified intounintentional and intentional islanding operations. Bydefinition, intentional islanding can be defined as the processof knowingly and willingly splitting the grid into small andseparate controllable sections so as to mitigate cascadingeffects and its eventual blackouts [58]. This is done in orderto mitigate or minimize the effects of unintentional LOMsand islanding. This immensely improves the systemefficiency, voltage profile and also reduces both transmissionand distribution congestion. The system frequency andvoltage are expected to come back to their normal limits uponaccurate islanding detection so as to avoid further orimpending blackouts in the system [59]. In case of intentionalislanding scenario, each islanded MG may have different unitprice which should also be controlled to avoid exploitation ofcustomers. The producers should give the customers thechoice of either buying electricity or not under this condition[13].The IEEE 1547-2003 standard requires an immediatedisconnection of any unintentional island within two secondsof its occurrence. This is illustrated in the table 2 below [60]and other international standards of operation limits forensuring a safe, reliable, and standard quality supply to allend user loads existing within the island as shown in table 3[61].Table 2: IEEE Abnormal Voltage Range and theirrecommended Clearing Times.On the other hand, an unintentional islanding occurs when themain grid is lost without the knowledge of both consumersand utility supplier. The DGs in the isolated MG continuessupplying the local loads. The LOMs in unintentionalislanding can take place under any of the following conditions[11]: Through a natural occurrence and catastrophes, forinstance an earthquake among others.The presence of a fault upstream of a MG that isdetected by the protection mechanism of the mainutility grid but goes undetected by the DGโ€™sprotection systemFailure of the utility equipment that eventually leadsto unintentional LOMs and islanding and409Percentagevoltage rangeClearing time insecondsV 50%0.1650% V 88%288% V 110%Normal operation110 V 120%1V 120%0.16

International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 12, Number 3 (2019), pp. 405-418 International Research Publication House. http://www.irphouse.comTable 3: Standard Power Systems Voltage and FrequencyLimitationsReference standardFrequencyRangeVoltageRangeIEEE P15470.99 โ€“ 1.010.9 - 1.1VDE 0126, OVE E27500.94 โ€“ 1.020.8 โ€“ 1.15CEI 0-21, CEI 0-160.994 โ€“ 1.006This is one of the main methods of determining theperformance of IDM. The small the NDZ in a given method,the higher the performance. The power mismatch space andthe load parameter space are mainly used in the detection ofan island [63]. The NDZ methods based on monitoringvoltage, frequency or phase deviation is often

remote methods and local methods of islanding detection as highlighted in figure 3 below [17] [18]. Figure 3: Islanding Detection Methods Techniques 2.1 Local Islanding Detection Methods Local islanding detection methods can be grouped into two categories: that is, active and passive tec