energies

Article Detection and Location of Earth Fault in MV Feeders Using Screen Earthing Current Measurements

Krzysztof Lowczowski 1,* , Jozef Lorenc 1, Jozef Zawodniak 2 and Grzegorz Dombek 1

1 Institute of Electric Power Engineering, Poznan University of Technology, Piotrowo 3A, 60965 Poznan, Poland; [email protected] (J.L.); [email protected] (G.D.) 2 Department of Network Asset Management, Enea Operator, Strzeszynska 58, 60479 Poznan, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-61-665-2270



Received: 4 February 2020; Accepted: 8 March 2020; Published: 10 March 2020


Abstract: The paper analyzes the utilization of cable screen currents for earth fault identification and location. Attention is paid on cable and mixed feeders—cable and overhead lines. The principle of operation is based on utilization of 3 criterion values: Ratio of cable screen earthing current and zero sequence cable core current—RF110/15, phase shift between cable screen earthing current and zero sequence cable core current—α and cable screen admittance defined as a ratio of cable screen earthing current and zero sequence voltage—Y0cs. Earth fault location is possible thanks to discovered relation between RF110/15 and α, whereas Y0cs allows for reliable detection of earth faults. Detection and identification are very important because it allows to increase the reliability of supply—reduce downtime and number of consumers affected by the fault. The article presents a phase to ground fault current flow for different power system configurations. At the end solution, which improves location capabilities is proposed. The solution is analyzed in PSCAD software and verified by network experiment.

Keywords: cable screen; earthing system distribution feeder; mixed cable-overhead line; protection relay; fault location; fault detection

1. Introduction Nowadays one can notice a tendency to increase the number of measuring equipment installed in power system networks. The topic is analyzed by different organizations i.e., CIGRE WG B3.44—Substation servicing and supervision, present and expected future use of mobile devices and sensors. It is however noted that similar trends are observed in different industries. In industrial applications, one can observe the tendency to install 4.0 generation sensors, whereas in the consumer electronics industry one can observe the tendency to install more and more IoT devices [1]. It is, however, necessary to underline that, increasing the number of sensors leads to an increase in the amount of data. If data is not utilized properly will have a negative impact on the cost of IT system connected with data transmission, processing, and storage [2]. In order to maximize the effectiveness of installed sensors, it is necessary to process data locally, particularly in the case of wireless sensors, which have a limited amount of energy [3]. One has considered a fact that data transmission is more energy-consuming process than local data analysis. The paper focuses on the utilization of cable screen currents of MV power cables. In previous papers, a few functionalities were proposed—utilization of cable screen currents for identification of cable screen erroneous connection and for identification of earth faults in cable lines [4,5]. Moreover, proof of concept regarding the identification of line affected by phase to ground fault was presented. The literature presents additional functions like the identification of cable sheath faults. The paper presents earth fault for different configurations

Energies 2020, 13, 1293; doi:10.3390/en13051293 www.mdpi.com/journal/energies Energies 2020, 13, 1293 2 of 24 of power lines, the criterion of phase to ground fault detection is elaborated and the methodology of identification line affected by the fault is presented. Identification of line type affected by the fault is important in order to increase the reliability of the supply of MV distribution systems by reducing the time of fault removal and blocking of operation of auto-reclose. Reduced number of unsuccessful auto-reclose operations, helps to reduce thermal and mechanical tensions, which lead to oxidation of connection surface and PD discharges [6]. Moreover, a reduced number of unsuccessful auto-reclose operations helps to increase the quality of supply since voltage transients and sags connected with fault occurrence are reduced [7]. One has to hover notice that not all faults at the MV side are seen on the LV side. Thermal and mechanical tensions resulting from unsuccessful AR operation can be reduced by PulseCloser technology [8]. One has to underline that overvoltages still exist and could damage insulation and therefore utilization of prosed solution or combination of both is recommended. Analysis of phase to ground faults is important since approximately 70–80% of all faults are phase to ground faults i.e., 72% in Portuguese [9]. In the case of cable lines, approximately a few earth faults are observed per 100 km, whereas in case of overhead lines a number of faults are in the range of 20/100 km [10]. The proposed method is developed for 3 phase 3 wire system with 1 neutral point grounding or isolated systems. Four wires network, multi-point grounded network and others require different methods of fault location and blocking of AR [11]. The presented criteria are not valid in multi-point grounded system since additional earthing points would create parallel paths for earth fault current flow. As a result of the parallel current flow paths, cable screen earthing current could be bigger than zero sequence cable core current. The effect could be compared to reverse capacitive current presented in paragraph simulation. An exception would be a few point earthed system i.e., 2, but only if the neutral earthing points would be outside the protected feeder. The schematic diagram of the proposed solutions is presented in Figure1. The second paragraph presents a method of fault location in MV networks. The next paragraph presents a simulation model and describes relations important for fault location. Simulation results are confronted with the measurement taken in MV network during network experiments. One previous last chapter presents the algorithm of identification of line type affected by fault and branch as well as the system for ground fault identification. The summary presents the most important features of the system and presents the scope of future works. Energies 2020, 13, 1293 3 of 24 Energies 2020, 13, x FOR PEER REVIEW 3 of 24

FigureFigure 1. SchematicSchematic diagram diagram of of the the developed developed system system (the (the elements elements described described in this in study this are study marked are markedin blue, in and blue, the and elements the elements in blue [in5]. blue [5].

2.2. Methods Methods of of Fault Fault Loca Localizationlization TheThe literature literature presents presents a a big big number number of of solutions solutions developed developed for for the the identification identification of of the the line line affectedaffected by by the the fault. The The topic topic is is important important for for power power system system distribution distribution networks, networks, industrial industrial and and railwayrailway networks networks [ [1212].]. Despite Despite a a big big number number of of loca localizationlization algorithms algorithms presented presented in in the the literature, literature, onlyonly 2 2 methods methods are are utilized utilized on ona bigger a bigger scale scale—fault—fault current current indicators indicators and distance and distance protection protection relay [13,14].relay [13 Effectiveness,14]. Effectiveness of distance of distance protection protection relays under relays phase under to ground phase to fault ground is strongly fault is limited strongly— [15]limited—[ claims15 that] claims identification that identification of 1/3 of the of 1feeder,/3 of the which feeder, could which be affected could be by aff theected fault by is the a faultsuccess. is a Distancesuccess. Distanceprotection protection is commonly is commonly used in HV used and in UHV HV and lines UHV and linescan identify and can the identify type of the line type affected of line byaff ectedfault as by long fault as as the long fault as is the not fault close is to not the close point to of the line point transition of line— transition—cablecable to overhead to [16]. overhead Principle [16]. ofPrinciple operation of of operation distance of relay distance is presented relay is in presented Figure 2— inprotection Figure2—protection zone determined zone determined via impedance via settingimpedance is marked setting with is marked blue lines with blueand linesfeeder and impedance, feeder impedance, including including branches, branches, is marked is marked with black with color.black Un color.fortunately Unfortunately measured measured fault impedance fault impedance can be cangreatly be greatly shifted shifted from the from line the impedance line impedance what iswhat caused is caused mostly mostly by measuring by measuring and parameter and parameter errorserrors as well as as well fault as impedance fault impedance—on—on MV level MV only level resistanceonly resistance is considered. is considered. As a result, As a result, it is almost it is almost impossible impossible to identi to identifyfy faulted faulted branch. branch.

Energies 2020, 13, 1293 4 of 24 Energies 2020, 13, x FOR PEER REVIEW 4 of 24

FigureFigure 2. 2.Characteristics Characteristics ofof distancedistance protection.protection.

UtilizationUtilization ofof distancedistance relaysrelays inin MVMV networksnetworks isis connectedconnected withwith manymany problemsproblems i.e.,i.e., didifficultiesfficulties withwith a a determinationdetermination of ofk kparameter parameter EquationEquation (1)(1) requiredrequired forfor properproper calculationcalculation ofof impedanceimpedance underunder phasephase to to ground ground fault fault [17 ,18 [17,18].]. Effectiveness Effectiveness of distance of distance relays is relays affected is by affected load before by load fault before occurrence, fault faultoccurrence, resistance, fault presence resistance, of additional presence of power additional sources power and otherssources [ 19 and]. Due others to [19]. many Due parameters to many aparametersffecting impedance affecting measurement, impedance measurement, there is uncertainty there is range uncertainty of impedance range of measurement impedance measurement and distance to fault is only approximation. and distance to fault is only approximation. ! 1 Z k = 0 1 (1) 1 푍0 푘 =3 (ZL −− 1) (1) 3 푍퐿 where Z0 is zero sequence impedance, ZL is positive sequence impedance. whereThe Z0 next is zero alternative sequence for impedance, the phase Z toL is ground positive fault sequence location impedance. is the utilization of the phase shift betweenThe current next alternative transients for recorded the phase in di tofferent ground parts fault of networkslocation is [20 the]. Theutilization method of is the developed phase shift for cablebetween lines, current however, transients it is necessary recorded to in develop different a method parts of for networks cable—overhead [20]. The method lines. The is developed disadvantage for ofcable the lines, method however, is connected it is necessary with the to necessity develop a of method human for interpretation cable—overhead of results lines. andThe thedisadvantage necessity ofof high-qualitythe method is devices—recording connected with the signal necessity with of high human sampling interpretation rate—in rangeof results of MHz, and the which necessity in turn of increaseshigh-quality the costdevices of the—recording hardware. signal Automation with high of devices. sampling Similar rate— method—travelingin range of MHz, waves which method in turn inincreases 2 places the uses cost additional of the hardware. processing Automation methods—Teager of devices. Energy Similar operator method to—traveling identify the waves moment method of travelingin 2 places wave uses occurrence additional and processing wavelet methods Hilbert Huang—Teager transform Energy operator for identification to identify of transientsthe moment [21 of]. Ditravelingfferent methods wave occurrence of identification and wavelet line type Hilbert affected Huang by faulttransform are the for installation identification of additional of transients sensors [21]. alongDifferent the protectedmethods of feeder identification i.e., PD sensorsline type on affected cable joints by fault [22 ].are The the concept, installation however, of additional requires sensors high investmentalong the protected costs and feeder operational i.e., PD problems. sensors on Thecable next joints identification [22]. The concept, concept however, is connected requires with hi angh optimizationinvestment costs algorithm and operational for earth fault problems. location. The The next method identification can be used concept in isolated is connected and compensated with an mvoptimization networks. Unfortunately,algorithm for earth the e fffaultectiveness location. of methodThe method operation can be depends used in on isolated fault resistance and compensated and local energymv networks. sources. Unfortunately, The effectiveness the of effectiveness identification of is method in the range operation of 80–95% depends [23]. on Despite fault resistance the presented and method,local energy harmonic sources. based The methods effectiveness like Zon, of identification Volna, ZVN, YC3,is in etc.the range or symmetrical of 80–95% component-based [23]. Despite the i.e.,presented negative method, current are harmonic used [24 based,25]. methods like Zon, Volna, ZVN, YC3, etc. or symmetrical componentProbably-based fault i.e., current negative indicator current gained are used the [24,25]. biggest popularity in MV networks. The fault indicatorsProbably indicate fault the current area between indicator indicators gained as the an biggest area under popularity fault conditions. in MV networks. The disadvantage The fault ofindicators the indicators indicate is manythe area complications between indicators and technical as an area constraints under fault i.e., installationconditions. andThe maintenancedisadvantage ofof thethe faultindicators indicators is many [26 complicat,27]. Effectivenessions and technical of fault constraints location can i.e., be installation increased ifand electrical maintenance signals of arethe processedfault indicators via advanced [26,27]. Effectiveness tools i.e., wavelet of fault and location correlation can with be increased other systems if electrical like geographic signals are informationprocessed via system—GIS advanced i.e., tools it i.e.,is possible wavelet to and claim correlation that fault waswith created other systems because likeof contact geographic with tree,information when trees system are marked—GIS i.e., in it GIS is systempossible one to claim can reduce that fault the areawas ofcreated search because to place of with contact trees with i.e., foresttree, when [28]. Ittrees is also are possible marked to in analyze GIS system voltage one sag can type reduce in order the area to identify of search fault to reasonsplace with and trees localize i.e., theforest fault [28]. [29 It,30 is]. also possible to analyze voltage sag type in order to identify fault reasons and localize the faultPresented [29,30]. methods are based on phase currents measurements. The presented method is based on thePresented screen current methods measurement. are based on phase The literature currents measurements. presents information The presented about the method utilization is based of screenon the currentscreen current measurement measurement. for identification The literature of line,presents however, information 3 phase about cable the screens utilization currents of screen and complexcurrent measurement algorithms are for necessary, identification whereas of inline, the however, case of developed 3 phase cable method screens only currents cable screen and earthingcomplex current—algorithmsI0cs areis needed necessary, [31]. whereas Only 1 side in the current case measurement of developed is method needed only in the cable method screen and earthing simple signalcurrent processing—I0cs is needed method [31]. what Only significantly 1 side current reduces measurement the cost of measuringis needed in equipment the method comparing and simple to signal processing method what significantly reduces the cost of measuring equipment comparing to the other presented solutions. Due the specific operation—identification of the segment affected by

Energies 2020, 13, 1293 5 of 24 the other presented solutions. Due the specific operation—identification of the segment affected by the fault is compared with similar solutions—fault indicators and distance protection is compared in Table1. Di fferent fault location methods are compared in [32].

Table 1. Comparison of earth fault location methods in MV grids.

Methods Fault Current Indicator Distance Protection Proposed Solution Detection of 1fg faults Yes Yes Yes Detection of 2fg and Yes Yes Yes 1 phase to phase faults Estimated location to faults in Fault location The section between 2 The estimated location of cable lines and identification identification indicators 2 phase to phase faults 3 of line type affected by faults 4 Yes, if additional current Yes, if there are significant Identification of branches indicators are installed and In exceptional cases 6 differences in electric affected by faults integrated with the fault parameters between lines 4 management system 5 Average – utilization of The complexity of Very big – many measuring Big – complex algorithm additional current sensors for measuring system systems and hardware 7 I0cs measurement Good operational Possibility to install indicators Gives the possibility to use 2 Advantages performance in case of in convenient places different time settings phase to phase faults Difficult installation and Difficult – expensive maintenance actions in Operational issues protection relay tester is Average difficulty 10 different places. required 9 Communication is required 8 Proper operation in case No No Yes of neutral point failure Auto reclose blocking Possible 11 In exceptional cases 12 Possible 1 According to preliminary investigation it is possible to detect 2 phase to ground faults using cable screen earthing current. Due to complexity, the topic is excluded from the paper content. 2. The effectiveness of fault current indicators strongly depends on number of indicators used. Theoretically fault current indicators could be installed at every support structure of the overhead line. At the same time one has to notice that cost would increase greatly. 3 The estimated location in case of an earth fault in distribution network is 1/3 of the protected feeder. 4 Detailed description in the following sections of the paper. 5 From technical point of view earth fault current indicators can be adopted to any complex feeder. One has to underline that performance (unwanted trips or missing trips) of simple fault current indicators has often been questioned. 6 In exceptional cases it is possible—if points on RF110/15 = f(α) differ significantly, it is possible to indicate branch affected by fault (the principle is presented in paragraph 4). 7 In case of conventional distance relays one can observe complex, sophisticated algorithms, which require fast hardware for real time control of 6 fault impedance loops. 8 Communication failure can compromise the system. 9 Verification of distance protection performance require a tester with 4 current and 4 voltage outputs. Additional training for utility employees responsible for testing is required. 10 Detailed description and analysis is difficult what is presented in the paper. At the same time however one can notice that interpretation of method, development of model is relatively simple. It is believed that a few hours training would be sufficient to educate a new utility employee. 11 If earth fault indicator is installed at the end of the cable lines. 12 In exceptional cases—long overhead line and short cable line.

In the literature, much attention is paid on the analysis of local energy sources on protection relay [33]. Proposed solution work properly in active system networks with many additional power sources and new devices like fault current limiters (FCLs) etc. since zero-sequence current flow is not affected by Dy transformers and YYn transformers, ungrounded at MV side [34,35]. Local energy sources could affect zero-sequence current flow if YNyn transformer is used [36,37]. In such case local energy sources i.e., prosumers could have an impact on zero-sequence current flow. In most cases however prosumers have small power and as a result, the impact is also small. At the same time one has to underline that in the polish powers system, apart from exceptional cases, networks are grounded in only one point, what allows to neglect the impact of local energy sources on zero sequence current completely. Despite the possibility to distinguish line type affected by the fault, the proposed algorithm is capable of identifying branch affected by the earth fault. One has to underline that the possibility Energies 2020, 13, x FOR PEER REVIEW 6 of 24

sources could affect zero-sequence current flow if YNyn transformer is used [36,37]. In such case local energy sources i.e., prosumers could have an impact on zero-sequence current flow. In most cases however prosumers have small power and as a result, the impact is also small. At the same time one has to underline that in the polish powers system, apart from exceptional cases, networks are grounded in only one point, what allows to neglect the impact of local energy sources on zero sequence current completely. Energies 2020, 13, 1293 6 of 24 Despite the possibility to distinguish line type affected by the fault, the proposed algorithm is capable of identifying branch affected by the earth fault. One has to underline that the possibility of ofbranch branch identifi identificationcation is limitedis limited and and depends depends on on electrical electrical parameters parameters of powerof power lines lines—mostly—mostly on oncables cables length length and and earthing earthing resistances, resistances, what what is presented is presented in the in theparagraph paragraph—simulation—simulation results. results. An Anadditional additional factor factor affecting affecting the the proposed proposed algorithm algorithm is is connected connected with with power power sys systemtem reconfiguration reconfiguration i.e.,i.e., tietie openopen pointpoint modificationmodification [[38].38]. To compensatecompensate forfor networknetwork reconfiguration,reconfiguration, oneone cancan sendsend faultfault datadata andand networknetwork information information to to the the central central system. system. Based Based on on the the data data simulation simulation model model is created is created i.e., ini.e., PowerFactory in PowerFactory (version (version 2019 SP4, 2019 DIgSILENT SP4, DIgSILENT GmbH, GmbH, Gomaringen, Gomaringen, Germany) Germany) or PSCAD or software PSCAD (v6.1,software Manitoba (v6.1, HydroManitoba International Hydro International Ltd., Winnipeg, Ltd., Manitoba, Winnipeg, Canada). Manitoba, Simulation Canada software). Simulation is used tosoftware obtain datais used for todi ff obtainerent fault data locations, for different which fault are locations, further compared which are with further measured compared fault data. with Simulationmeasured fault software data. can Simulation be run in software background can be mode runthanks in background to API interfaces mode thanks [39]. Itto is API also interfaces possible to[39]. increase It is also the possible accuracy to ofincrease results the if weatheraccuracy data of results or other if weather will be useddata or to other assess will earthing be used resistance. to assess Theearthing proposed resistance. method The can be proposed used in cable method lines canwith be di ff usederent bonding in cable configurations lines with different i.e., cross bonding bonded linesconfigurations or one point i.e., bonded cross bonded lines [40 lines]. or one point bonded lines [40].

3.3. Measuring System TheThe proposedproposed solutionsolution utilizes utilizes Rogowski Rogowski coils coils for for current current measurement. measurement. The The measuring measuring system system is presentedis presented in Figurein Figure3[ 5 3]. [5]. In many In many places places around aroun thed globe,the globe, conventional conventional current current transformers transformers are used, are however,used, however, utilization utilization of Rogowski of Rogowski coils fits coils in world fits trends.in world One trends. of the One main of reason the main for CT reason transformer for CT installationtransformer are installation historical are issues—current historical issues transformers—current nottransformers only provide not measurand, only provide but measurand, also energy but to runalso analogenergy relays to run [41 analog]. Nowadays relays [41]. relays Nowadays are connected relays to are an connected external power to an supply external and power power supply from CTand is power not necessary. from CT is It not is, however,necessary. di Itffi is,cult however, to replace difficult conventional to replace CT conventional transformers CT because transformers some protectionbecause some relays protection are not able relays to readare not Rogowski able to coilread signal Rogowski so an oldcoil CTssignal are so replaced an old withCTs are a new replaced a one andwith when a new protection a one and relay when needs protection to be replacedrelay needs it is to adopted be replaced to an it existing is adopted current to an sensors. existing Authors current believesensors. that Authors protection believe relays that manufacturer protection relays should manufacturer equip protection should relays equip with protection bigger relays number with of inputs—Rogowskibigger number of inputs and conventional—Rogowski and or provide conventional accessories or provide for signal accessories conversion. for signal In manyconversion. cases, newIn many protection cases, relays new already protection use Rogowski relays already coil to use measure Rogowski secondary coil to side measure transformers secondary andsome side manufacturerstransformers and already some o manufacturersffer relays with already both current offer relays input typeswith both [42]. current input types [42].

FigureFigure 3. 3.Measuring Measuring system; system;NP —neutralNP—neutral point point of ZNyn of transformer,ZNyn transformer,BB—MV BB busbars,—MV R busbars,e1—resistance Re1— ofresistance 110/15 earthing of 110/15 system; earthingRe 2system;—resistance Re2— ofresistance MV earthing of MV system. earthing system.

4. Simulation Tests 4. Simulation Tests Earth fault current flow in the MV cable line network is presented in Figure4. The model is Earth fault current flow in the MV cable line network is presented in Figure 4. The model is built built from the following components: 110/15 transformer, grounding transformer, earthing system, from the following components: 110/15 transformer, grounding transformer, earthing system, cable, cable, and overhead line [43]. The cable line is modeled as a Frequency-dependent line and considers distributed parameters.

Energies 2020, 13, x FOR PEER REVIEW 7 of 24 and overhead line [43]. The cable line is modeled as a Frequency-dependent line and considers distributed parameters. Figure 5 presents earthing current sources. The current sources marked with yellow, grey, red and green are presented in the previous author’s publications [4,5]. Sources marked with red-blue and light blue are presented in the next paragraphs. Simulations were made in PSCAD software. Attention is paid on earthing resistances and the number of cable earthing points because those parameters have the biggest impact on simulation results. Moreover, earthing system resistance is a function of time i.e., resistance made of pipe ¾ inch with a length 3 feet in rocky, clay ground is varying in the range of 42 do 77 Ω. Depth of earthing system installation has a big impact on resistance variation i.e., 10 feet long ¾ pipe is varying in the range of 37 to 58 Ω [44]. Variability of earthing Energiesresistance 2020 ,depends 13, x FOR PEERon the REVIEW earthing system's spatial layout and material of the earthing system7 ofand 24 Energiessoil type2020 [45]., 13, It 1293 is possible to assess earthing system resistance using weather station data or humidity7 of 24 andsensors overhead installed line in [43].the distribution The cable linesystem is modelednetwork. as a Frequency-dependent line and considers distributed parameters. Figure 5 presents earthing current sources. The current sources marked with yellow, grey, red and green are presented in the previous author’s publications [4,5]. Sources marked with red-blue and light blue are presented in the next paragraphs. Simulations were made in PSCAD software. Attention is paid on earthing resistances and the number of cable earthing points because those parameters have the biggest impact on simulation results. Moreover, earthing system resistance is a function of time i.e., resistance made of pipe ¾ inch with a length 3 feet in rocky, clay ground is varying in the range of 42 do 77 Ω. Depth of earthing system installation has a big impact on resistance variation i.e., 10 feet long ¾ pipe is varying in the range of 37 to 58 Ω [44]. Variability of earthing resistance depends on the earthing system's spatial layout and material of the earthing system and soil type [45]. It is possible to assess earthing system resistance using weather station data or humidity sensors installed in the distribution system network. Figure 4. Cable screen current flow. flow.

Figure5 presents earthing current sources. The current sources marked with yellow, grey, red and green are presented in the previous author’s publications [4,5]. Sources marked with red-blue and light blue are presented in the next paragraphs. Simulations were made in PSCAD software. Attention is paid on earthing resistances and the number of cable earthing points because those parameters have the biggest impact on simulation results. Moreover, earthing system resistance is a function of time i.e., 3 resistance made of pipe 4 inch with a length 3 feet in rocky, clay ground is varying in the range of 42 do 77 Ω. Depth of earthing system installation has a big impact on resistance variation i.e., 10 feet 3 long 4 pipe is varying in the range of 37 to 58 Ω [44]. Variability of earthing resistance depends on the earthing system’s spatial layout and material of the earthing system and soil type [45]. It is possible to assess earthing system resistance using weather station data or humidity sensors installed in the distribution system network. Figure 4. Cable screen current flow.

Figure 5. Earthing current sources.

Figure 5. EarthingEarthing current current sources. sources.

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In the previous previous author’s paper paper [5], [5], a a concept concept of of faulted faulted line line identification identification is presented. In In case of fault between cable screen andand cablecable core,core, thethe phasephase shiftshift betweenbetweenI I00cscs and II0cc currentcurrent is is 0, 0, whereas whereas in case of of a a fault fault in in overhead overhead line line installed installed after after the the cable cable the the phase phase shift shift is is90. 90. The The presented presented concept concept is ais valid a valid hover hover is issimplified. simplified. In Inre realal cable cable lines, lines, the the phase phase shift shift can can vary vary in in a a relatively relatively broadband, broadband, which is presented in the paragraph. Simulations were were aimed aimed at at sensitivity sensitivity analysis analysis on on a reduction a reduction factor factor of 110/15 of 110 kV/15 substation kV substation and phaseand phase shift shift between between the the zero zero-sequence-sequence current current measured measured in in cable cable cores cores and and earthing earthing current. current. Attention is paid on cable parameters, mostly on cable screen crosscross-section-section and cable length and earthing resistances. Mathematical description description of cableof cable lines lines reduction reduction factor is factor given by is the given formula by the [46 ]. formula Mathematical [46]. Mathematicaldescription of reductiondescription factor of reduction of cables with factor ECC of cableswire can with be found ECC wire in [47 can], the be reduction found in factor [47], of the 3 reductioncore cables factor in [48 of] and 3 core in [ 49cables] reduction in [48] and factor in of[49] parallel reduction lines. factor Most of publications parallel lines. focus Most on electrocutionpublications focusprotection on electrocution and consider protection a case of phase and consider to screen a fault, casewhere of phase touch to voltagescreen fault, has the where biggest touch amplitudes. voltage hasA reduction the biggest factor amplitudes. of mixed A power reduction lines—cable, factor of mixed overhead power lines lines is based—cable, on overhead the scheme lines presented is based onin Figurethe scheme6. presented in Figure 6.

Figure 6 6.. CouplingCoupling impedances impedances in in case of earth earth fault in cable, based on [50]. [50].

Relations presented in Figure 6 6 allowsallows toto developdevelop followingfollowing setset ofof equationsequations describingdescribing thethe cablecable screen earthing current: a11Icsa + a12Icsb + a13Icsc = b13I0cc (2) 푎11퐼푐푠푎 + 푎12퐼푐푠푏 + 푎13퐼푐푠푐 = 푏13퐼0푐푐 (2) a22Icsb + a21Icsa + a23Icsc = b23I0cc (3) 푎22퐼푐푠푏 + 푎21퐼푐푠푎 + 푎23퐼푐푠푐 = 푏23퐼0푐푐 (3) a33Icsc + a31Icsa + a32Icsb = b33I0cc (4)

푎33Igr퐼푐푠푐= +3I푎cc31퐼푐푠푎(Icsa+ +푎32I퐼푐푠푏+=Icsc푏3)3퐼0푐푐 (4)(5) 0 − csb a = (R + Lczc + Re ); i [1, 3] (6) ii 퐼푔푟e1= 3퐼0푐푐 − (퐼푐푠푎2 + 퐼푐푠푏 + 퐼∈푐푠푐) (5)   aij = Re1 + Lczmi,j + Re2 ; i , j (7) 푎 = (푅 + 퐿 푧 + 푅 ); 푖 ∈ [1,3] (6) 푖푖 푒1 푐 푐 푒2  b = R + Lcz + Re ; i [1, 3] (8) i e1 mp,i 2 ∈ 푎푖푗 = (푅푒1 + 퐿푐푧푚푖,푗 + 푅푒2); 푖 ≠ 푗 (7) After solving the presented set of equations using method of substitution following formulas are obtained: 푏푖 = (푅푒1 + 퐿푐푧푚푝,푖 + 푅푒2);   푖 ∈[1,3] (8)  3R + Lc zc z   e1 − mp,1  After solving the presented3I0cs = set3I 0ofcc equations1 using method of substitution following formulas(9)  − Lc(zm1,2 + zz) + 3Re1 + 3Re2  are obtained: Earth fault in overhead section is also connected with electrocution hazard, it is, however, smaller 3푅푒1 + 퐿푐(푧푐 − 푧푚푝,1) than in case of fault along the3퐼 cable= 3 line.퐼 ( Additional1 − attention is paid on HV) and EHV line because(9) of 0푐푠 0푐푐 퐿 (푧 + 푧 ) + 3푅 + 3푅 high zero sequence currents amplitude resulting푐 from푚1,2 direct푧 earthing푒1 of neutral푒2 points. Electric shock protectionEarth requires fault in additional overhead sectionanalysis is and also is presented connected in with [51]. electrocution hazard, it is, however, smaller than in case of fault along the cable line. Additional attention is paid on HV and EHV line

Energies 2020, 13, x FOR PEER REVIEW 9 of 24 because of high zero sequence currents amplitude resulting from direct earthing of neutral points. Energies 2020, 13, 1293 9 of 24 Electric shock protection requires additional analysis and is presented in [51]. It is also possible to identify line affected by earth fault in microgrids, what actually is simpler sinceIt lengths is also of possible lines in to such identify grids line are atypicallyffected by smaller. earth faultAt the in same microgrids, time, one what has actually to remember is simpler that microgridsince lengths requires of lines additional in such grids protection are typically relay— smaller.interface At protection the same [52]. time, Identification one has to remember and location that ofmicrogrid 2fg faults requires requires additional separate protectionattention and relay—interface is part of current protection research [52 ].[53]. Identification and location of 2fg faults requires separate attention and is part of current research [53]. 4.1. Cable Lines 4.1. Cable Lines Among factors, which could influence angle α is the self-capacitive current of the cable line. In case Among of uniform factors, cable which line could (without influence additional angle α earthingis the self-capacitive resistances) current practically of the whole cable line. capacitive In case currentof uniform will cable flow line through (without the additional earthing earthing system of resistances) 110/15 substation. practically The whole capacitive capacitive current current from will healthyflow through phases the is earthingthen flowing system to ofthe 110 grounded/15 substation. phase through The capacitive MV windings current fromof transformers. healthy phases For caseis then of flowingsimplicity to theonly grounded D winding phase of 110/15 through kV MV transformer windings is of considered transformers. and For isolated case of network simplicity is considered.only D winding Blue of color 110/15 represents kV transformer the capacitive is considered current and isolated of a cable network line, is whereas considered. orange Blue color color represents thecapacitive capacitive current current of overhead of a cable line. line, Idealized whereas orangeearth fault color (fault represents resistance capacitive is zero) current occur in of placeoverhead EF1 and line. all Idealized capacitive earth current fault (faultis flowing resistance to the is place zero) of occur earth in fault. place EFIf earth1 and fault all capacitive would occur current in cableis flowing line tomarked the place with of red earth color fault. (location If earth EF fault2), would the capacitive occur in earth cable linefault marked current with of cable red color line measured(location EF at2 ),location the capacitive marked earth with faultI0cs would current be of compensated cable line measured because atblue location arrows marked have opposite with I0cs woulddirection. be compensatedAt the same time because self blue-capacitive arrows current have opposite measured direction. in cable At cores the same would time be self-capacitivecompensated forcurrent the same measured reason. in The cable current cores flow would is presented be compensated in the Figure for the 7 same. Unfortunately, reason. The with current the increase flow is ofpresented cable length in the and Figure increase7. Unfortunately, of earthing system with the number, increase factors of cable presented length andas negligible increase ofwill earthing play a biggersystem role number, and factors consequently, presented angle as negligibleα will be will varying. play a The bigger biggest role and α and consequently, smallest RF angle110/15 canα will be observedbe varying. for The the biggestcase of αearthand fault smallest at theRF end110/15 ofcan the becable observed line. Simulation for the case results of earth consider fault at only the endthis— of worstthe cable case. line. Simulation results consider only this—worst case.

Figure 7.7. Capacitive componentcomponent ofof earth earth fault fault current current flow flow in in mixed mixed feeder; feeder; cable cable line line marked marked with with red; red;overhead overhead line marked line marked with yellow; with yellow; another anotherpower lines power marked lines with marked green; withTe—grounding green; Te— transformer;grounding Tstransformer;—secondary Ts winding—secondary of 110 winding/15. of 110/15.

Another factor, which have an impact onon RFRF110110/15/15 andand angle angle αα isis reverse reverse self self-capacitive-capacitive current, current, which f flowslows to place of fault from load side and reverses induced current resulting from the flow flow of capacitive current as is presented in Figure 88.. L1L1 isis aa lengthlength ofof cablecable betweenbetween 110110/15/15 substation and fault location, location, whereas whereas L2 L2 is is the the length length between between fault fault locat locationion and and cable cable end. end. As can As be can observed be observed self- capacitiveself-capacitive current current flows flows through through whole whole cable cable length length (L1 (L1 + + L2).L2). On On the the length, length, L1 L1 current current is compensated since since current current from from 2 2 ‘healthy’ ‘healthy’ phases phases is isflowing flowing to tothe the grounded grounded phase. phase. On Ondistance distance L2 howeverL2 however one one can canobserv observee only only current current flowing flowing to supply to supply side, side, as a as result a result zero zero-sequence-sequence current current on distanceon distance L2 L2is greater is greater than than 0 and 0 and according according to to induction induction law, law, the the current current induces induces additional additional zero sequence current in cable screens. The The induced induced zero zero-sequence-sequence cable cable screen screen current current has has a a different different phase shift than zero-sequence current resulting from the galvanic connection. As a result, angle α

Energies 2020, 13, x FOR PEER REVIEW 10 of 24

phase shift than zero-sequence current resulting from the galvanic connection. As a result, angle α of the resulting zero sequence current is different. Despite described currents, in Figure 7, one can notice additional current marked with light blue, which is a part of self-capacitive current flowing from opposite direction. The current is usually small and negligible, however, in case of high capacitance behind the fault location i.e., long cable or MV substation the current can also influence angle α and RF110/15. The same effect is observed in overhead line and the current is marked with yellow. Red color represents earth fault current component flowing through neutral point. The bigger the component Energiesis, the2020 smaller, 13, 1293 is the effect of reverse capacitive current. 10 of 24 Energies 2020, 13, x FOR PEER REVIEW 10 of 24 Analysis of the Figures 9 and 10 allows to observe that with the increase of cable length, angle α ofphase theis rising. resulting shift When than zero zero cable sequence-sequence length increases,current current isresulting an di ffamplitudeerent. from Despite the of galvanicearth described fault connection. current currents, returning As ina result, Figure to theangle7, one110/15 α canof station is reduced at the same time a difference between I0cs and I0cc is rising—what results in a rise of noticethe resulting additional zero current sequence marked current with is different. light blue, Despite which isdescribed a part of currents, self-capacitive in Figure current 7, one flowing can notice from a magnetic field, which induces a current in cable screens. Current flowing through cable screens is oppositeadditional direction. current Themarked current with is light usually blue, small which and isnegligible, a part of self however,-capacitive in case current of high flowing capacitance from a combination of current resulting from galvanic connection and induction. With the rise of cable behindopposite the direction. fault location The current i.e., long is cableusually or small MV substationand negligible, the current however, can in also case influence of high capacitance angle α and length, an amplitude of MV earthing system current rises and at the same time voltage resulting from RFbehind. Thethe fault same location effect is i.e., observed long cable in overhead or MV substation line and the the current current is can marked also influ withence yellow. angle Red α and color 110current/15 flow through earthing resistance increases. As a consequence, voltage difference between RF110/15. The same effect is observed in overhead line and the current is marked with yellow. Red color representsearthing earth system fault at load current side component and supply flowingside increases, through what neutral in turn point. can Theinduce bigger a flow the of component circulating is, represents earth fault current component flowing through neutral point. The bigger the component thecurrent. smaller is the effect of reverse capacitive current. is, the smaller is the effect of reverse capacitive current. Analysis of the Figures 9 and 10 allows to observe that with the increase of cable length, angle α is rising. When cable length increases, an amplitude of earth fault current returning to the 110/15 station is reduced at the same time a difference between I0cs and I0cc is rising—what results in a rise of a magnetic field, which induces a current in cable screens. Current flowing through cable screens is a combination of current resulting from galvanic connection and induction. With the rise of cable length, an amplitude of MV earthing system current rises and at the same time voltage resulting from current flow through earthing resistance increases. As a consequence, voltage difference between earthing system at load side and supply side increases, what in turn can induce a flow of circulating current.

FigureFigure 8. 8The. The idea idea of of reverse reverse self-current self-current(capacitive) (capacitive) and reverse self self-induced-induced (capacitive) (capacitive) current. current.

Analysis100 of the Figures9 and 10 allows to observe that with the increase of cableR=10; length, 150/25 angle α is rising. When cable length increases, an amplitude of earth fault current returningR=7; to the150/25 110/15 station is reduced at the same time a difference between I0cs and I0cc is rising—what results in a rise of 95 R=5; 150/25 a magnetic field, which induces a current in cable screens. Current flowing through cable screens is a R=3; 150/25

combination) of current resulting from galvanic connection and induction. With the rise of cable length, %

an amplitude( 90 of MV earthing system current rises and at the same time voltage resultingR=2; from 150/25 current

15 / flow through earthing resistance increases. As a consequence, voltage difference betweenR=1; 150/25 earthing 110

systemRF at load side and supply side increases, what in turn can induce a flow of circulating current. Figure85 8. The idea of reverse self-current (capacitive) and reverse self-induced (capacitive) current.R=1; 150/50 R=2; 150/50 100 R=10; 150/25 80 R=3; 150/50 R=7; 150/25 R=5; 150/50 95 R=5; 150/25 R=7; 150/50 75 R=3; 150/25

) 1 2 3 4 5 R=10; 150/50 %

( 90 length (km) R=2; 150/25

15 / R=1; 150/25 110 Figure 9. RF110/15 for faults between cable core and screen at the end of the cable line in a trefoil RF 85configuration. R=1; 150/50

R=2; 150/50

80 R=3; 150/50 R=5; 150/50 R=7; 150/50 75 1 2 3 4 5 R=10; 150/50 length (km)

FigureFigure 9. 9. RF110/15110/15 forfor faults faults between between cable cable core core and andscreen screen at the at end the ofend the cableof the line cable in a line trefoil in a trefoilconfiguration. configuration.

Energies 2020, 13, x FOR PEER REVIEW 11 of 24

Energies 2020, 13, 1293 11 of 24 20 Energies 2020, 13, x FOR PEER REVIEW R=10;11 of 150/25 24 18 20 R=7; 150/25 16 R=10; 150/25 18 R=5; 150/25 14 R=7; 150/25 16 R=3; 150/25 R=5; 150/25

α (⁰) α 12 14 R=2; 150/25 R=3; 150/25 10 R=1; 150/25 α (⁰) α 12 R=2; 150/25 8 R=1; 150/50 10 R=1; 150/25 6 R=2; 150/50 8 R=1; 150/50 4 R=3; 150/50 6 R=2; 150/50 2 R=5; 150/50 4 R=3; 150/50 R=7; 150/50 02 R=5; 150/50 1 2 3 4 5 R=10; 150/50 R=7; 150/50 0 length (km) 1 2 3 4 5 R=10; 150/50 Figure 10. α for faults between cable corelength and (km) screen at the end of cable line in a trefoil configuration. FigureFigure 10. 10.α αfor for faults faults betweenbetween cablecable core and screen screen at at the the end end of of cable cable line line in in a atrefoil trefoil configuration. configuration. 4.2.4.2. Cable—OverheadCable—Overhead LinesLines 4.2.In CableIn case case—Overhead of of earth earth L faultfaultines in in overhead overhead section section of of mixed mixed feeder feeder in in the the ideal ideal case—short case—short cable cable line line grounded with high resistance, angle α is 90°. With an increase in the length of cable, more and more groundedIn case with of high earth resistance, fault in overhead angle α is section 90◦. With of mixed an increase feeder in in the the length ideal of case cable,—short more cable and line more currentcurrentgrounded is is flowingflowing with high through through resistance, the the earthing earthing angle α system issystem 90°. With instead instead an increase of of screens screens in ofthe of ‘healthy’ ‘healthylength of’ phases. phases.cable, more EarthingEarthing and more systemsystem ofofcurrent MVMV networknetwork is flowing hashas through resistiveresistive the charactercharacter earthing and systemand thereforetherefore instead angle angle of screensα αis is changed. changed.of ‘healthy ’ phases. Earthing system of MVTheThe network numbernumber has of resistiveearthing character systems systems and and thereforeresistance resistance angle has has aα a bigis big changed. impact impact on on earth earth fault fault current current flow. flow. In order to assess the impact of the parameters on α and RF110/15, a model presented in Figure 11 is In orderThe to number assess theof earthing impact ofsystems the parameters and resistance on α hasand a RFbig110 impact/15, a modelon earth presented fault current in Figure flow. In11 is developed.developed.order to assess ItIt isis assumed theassumed impact that that of the thethe same parameterssame cable cable type ontype α is andis used used RF 3110/15 3x150/25150, a /25 model or or 3 3x150/50, presented150/50, which in Figure is clearlyclearly 11 is aa × × simplificationsimplificationdeveloped. It sincesinceis assumed normallynormally that cross-section crossthe same-section cable ofof type cablecable is corecore used andand 3x150/25 screenscreen or isis reduced3x150/50,reduced sincesincewhich faultfault is clearly currentcurrent a isis reducedreducedsimplification by by increasingincreasi sinceng normally lengthlength of ofcross lineslines-section (lines(lines impedance),ofimpedance), cable core whatandwhat screen naturallynaturally is reduced limitslimits short shortsince circuitcircuitfault current powerpower is andand thermalthermalreduced stress.stress. by increasi ng length of lines (lines impedance), what naturally limits short circuit power and thermal stress.

Figure 11. A model for testing the impact of the number and earthing systems of a cable line on the FigureFigure 1111.. AA modelmodel for testing thethe impactimpact of of the the number number and and earthing earthing systems systems of ofa cable a cable line line on onthe the earth fault current distribution. earthearth fault fault currentcurrent distribution. Simulation results for faults outside the cable line are presented in Figures 12 and 13. It is assumed SimulationSimulation results results for faults outside outside the the cable cable line line are are presented presented in in Figures Figures 12 1and2 and 13 . 1 It3. is It is that resistance of the earthing system is below 10 Ω—1, 2, 3, 5, 7 or 10 Ω and earth resistivity is assumedassumed that that resistanceresistance of the earthingearthing system system is is below below 10 10 Ω Ω——1,1, 2, 2,3, 3,5, 5,7 or7 or 10 10 Ω andΩ and earth earth resistivity resistivity

Energies 2020, 13, 1293 12 of 24 EnergiesEnergies 20 202020, ,1 133, ,x x FOR FOR PEER PEER REVIEW REVIEW 1212 of of 24 24

200isis 200200Ωm ΩmΩm [54 [54].].[54]. Surface SurfaceSurface earth earthearth resistivity resistivityresistivity has hashas an impact anan impactimpact on current onon currentcurrent induced inducedinduced in the iinn earth thethe earth aroundearth aroundaround the cable thethe linecablecable [55 lineline]. [55].[55].

100100 R=1;R=1; 150/25 150/25 9090 R=2;R=2; 150/25 150/25 8080 R=3;R=3; 150/25 150/25 7070 R=5;R=5; 150/25 150/25 6060 R=7R=7 150/25 150/25

5050 R=10;R=10; 150/25 150/25

α (⁰) α α (⁰) α 4040 R=1;R=1; 150/50 150/50 3030 R=2;R=2; 150/50 150/50 2020 R=3;R=3; 150/50 150/50 1010 R=5;R=5; 150/50 150/50

00 R=7;R=7; 150/50 150/50 11 22 33 44 55 length (km) R=10;R=10; 150/50 150/50 length (km) FigureFigure 12.1122. .α α for for faults faults in in overhead overhead section section of of mixed mixed feederfeeder (cable(cable inin aa trefoiltrefoil configuration).configuration).configuration).

100100 R=1;R=1; 150/25 150/25 9090 R=2;R=2; 150/25 150/25 8080 R=3;R=3; 150/25 150/25 7070

R=5;R=5; 150/25 150/25 (%) (%) (%) (%) 6060 R=7;R=7; 150/25 150/25

5050 110/15

110/15 R=10;R=10; 150/25 150/25 RF RF 4040 R=1;R=1; 150/50 150/50 3030 R=2;R=2; 150/50 150/50 2020 R=3;R=3; 150/50 150/50 1010 R=5;R=5; 150/50 150/50 00 R=7;R=7; 150/50 150/50 11 22 33 44 55 lengthlength (km) (km) R=10;R=10; 150/50 150/50

FigureFigureFigure 13.1 133. .RFRF RF110110/15110/15/15 for for faults faults in in overhead overhead section section of of mixed mixed feeder feeder (cable (cable in in a a trefoil trefoil configuration). configuration).configuration).

AsAs isis presentedpresentpresenteded inin FiguresFigures 121122 andand 131133,, cablecable screen screen cross cross section section has hashas limited limitedlimited impact impactimpact on onon results. results.results. IncreaseIncrease ofofof cablecablecable length lengthlength leads leadsleads to toto an anan increase increaseincrease of ofof the thetheRF RFRF110110/15110/15/15 andand decrease decrease ofof thethe αα.. Increase Increase of of RR earthearthearth leadsleadsleads tototo decrease decreasedecrease of ofof the thethe RF RFRF110110/15110/15/15 andand increase increase of ofof the the αα... Presented PresentedPresented relations relationsrelations make make identification identificationidentification ofof linelineline typetype moremoremore di difficultdifficultfficult—the——thethe precise preciseprecise measurement measurementmeasurement of ofofI 0II0cc0cccc and andand I I0I00cscs currentscurrentscurrents is is required.required. At At somesome point,point, itit isis practicallypractically impossible.impossible. TheThe maximummaximum cablecable length,length, whichwhich allowsallows forfor properproper identificationidentificationidentification inin casecase ofof cablecacableble lineslines withwith justjust oneone earthingearthingearthing systemsystem atat thethethe endendend isisis approximatelyapproximatelyapproximately 101010 km.km.km. One One hashas toto howeverhowever underlineunderline that that that when when when the the the length length length of the of of thecable the cable cable increases increases increases the uncertainty the the uncertainty uncertainty of simulation of of simulation simulation results also results results increase also also thereforeincreaseincrease thereforetherefore results are resultsresults limited areare to limitedlimited 5 km long toto 55 cable kmkm longlong to avoid cablecable hasty toto avoiavoi conclusions.dd hastyhasty conclusions.conclusions. AA bigbig numbernumber ofof factorsfactors aaffectingaffectingffecting RF RFRF110110/15110/15/15 and andand angleangle angle αα makemake thethe analysisanalysis moremore difficult,difficult, difficult, however, however, thethe presented presented presented result result result confirms confirms confirms that that that the presented the the presented presented concept concept concept can be usedcan can be inbe real-life used used in in scenarios real real--lifelife to scenarios scenarios distinguish to to betweendistinguishdistinguish faults betweenbetween in cable faultsfaults and inin overhead cablecable andand sections. ooverheadverhead sections.sections.

EnergiesEnergies2020 2020, 13, 1,3 1293, x FOR PEER REVIEW 13 of 24 24 Energies 2020, 13, x FOR PEER REVIEW 13 of 24 There is boundary earth fault condition—fault in a transition point—change of line type. In this There is boundary earth fault condition—fault in a transition point—change of line type. In this case,There RF110/15 is boundary and α can earth be strongly fault condition—fault affected if earthing in a transitionresistance point—change is comparable ofwith line surface type. Inearth this case, RF110/15 and α can be strongly affected if earthing resistance is comparable with surface earth case,resistivityRF110/15 andand resultingα can be resistance strongly be afftweenected cable if earthing screens resistance and fault islocation. comparable with surface earth resistivity and resulting resistance between cable screens and fault location. resistivityFigures and 1 resulting4 and 15 resistancepresent impact between of earthing cable screens system and number fault location.on RF110/15 and α. Increasing cable Figures 14 and 15 present impact of earthing system number on RF110/15 and α. Increasing cable lengthFigures and number14 and 15 of presentearthing impact systems of makes earthing the system identification number of on lineRF type110/15 affectedand α. by Increasing the fault more cable lengthlengthand moreand and number numberdifficult of ofand earthing earthing at some systems systems point, makesit makes becomes the the identificationimpossible. identification 3 coreof of line line cable, type type which affected affected is almost by by the the completelyfault fault more more andandreplaced more more difficult diwithfficult 1 coreand and atcable at some some in point,most point, countries it it becomes becomes could impossible. impossible. have a different 3 3 core core cable, cable, type whichof which cable is is sheath,almost almost completelybut completely in most replacedreplacedcases, thewith with sheath 1 1 core core was cable cable made in in most mostof uninsulated countries countries could couldPb alloy. have have The a a different di sheathfferent actstype type as of of acable cablemulti sheath, sheath,-point but(indefinitely but in in most most cases,cases,large) the the grounding sheath sheath was was system made made and of of uninsulatedtherefore uninsulated presented Pb Pb alloy. alloy. criteria The The sheath are sheath not acts actsoperational as as a a multi multi-point in -thesepoint types(indefinitely (indefinitely of lines large)large)i.e., AKnFtgrounding grounding [56]. system It is possible andand therefore thereforeto use the presented presentedpresented criteria criteriacriteria are inare 3 not corenot operational operationalcables with in XLPEin these these insulation, types types of linesof which lines i.e., i.e.,AKnFtare AKnFt installed [56 ].[56]. It isin It possible Siscotch possible MV to usetonetworks use the the presented [57].presented criteria criteria in 3in core 3 core cables cables with with XLPE XLPE insulation, insulation, which which are areinstalled installed in Scotchin Scotch MV MV networks networks [57 ].[57]. 70 70 OV_R1 60 OV_R1 60 OV_R2 OV_R2 50 OV_R3 50 OV_R3 40 OV_R4 40 OV_R4 α (⁰) α 30 OV_R5

α (⁰) α 30 OV_R5 20 CAB_R1 20 CAB_R1CAB_R2 10 CAB_R2 10 CAB_R3 0 CAB_R3CAB_R4 0 1 2 3 4 5 CAB_R4 1 number2 of sections in3 5 km long 150/25 cable4 5 CAB_R5 CAB_R5 number of sections in 5 km long 150/25 cable

FigureFigure 14. 14.Impact Impact of of earthing earthing system system numbernumber onon angleangle αα.. Figure 14. Impact of earthing system number on angle α. 100 100 90 OV_R1 90 80 OV_R1OV_R2 80 70 OV_R2OV_R3 60 70(%) OV_R3OV_R4 60 50 (%) OV_R4OV_R5 110/15 40 50RF OV_R5CAB_R1 110/15 40 30 RF CAB_R1CAB_R2 30 20 CAB_R2CAB_R3 20 10 CAB_R3CAB_R4 10 0 1 2 3 4 5 CAB_R5 0 CAB_R4 number of sections in 5 km long 150/25 cable 1 2 3 4 5 CAB_R5 number of sections in 5 km long 150/25 cable FigureFigure 15. 15Impact. Impact of of earthing earthing system system number number on onRF RF110110/15/15..

Figure 15. Impact of earthing system number on RF110/15. FurtherFurther analysisanalysis allows allows to toconclude conclude that thatthe presented the presented method method could be couldadjusted be to adjusted complex— to complex—multi-branchmulti-branch feeders (Fi feedersgure 16 (Figure), which 16 are), which quite arecommon quite commonin distribution in distribution system network system networkand are Further analysis allows to conclude that the presented method could be adjusted to complex— andincluded are included in benchmark in benchmark models i.e., models CIGRE i.e., [58]. CIGRE The [idea58]. is The based idea on is previously based on previouslypresented relations presented— multi-branch feeders (Figure 16), which are quite common in distribution system network and are relations—changechange of induced of current induced in currentthe function in the of functionline length. of lineIf branches length. are If created branches by are cables, created which by included in benchmark models i.e., CIGRE [58]. The idea is based on previously presented relations— cables,lengths which differ lengthssignificantly differ or significantly resistances of or earthing resistances systems ofearthing in different systems branches in didifferfferent significantly branches change of induced current in the function of line length. If branches are created by cables, which dioneffer can significantly observe the one significant can observe change the of significant induced currents. change of As induced a result, currents. one can reduce As a result, time to one locate can lengths differ significantly or resistances of earthing systems in different branches differ significantly reducefailure time since to a locate significant failure part since of a the significant feeder can part be of excluded the feeder from can the be search excluded area. from The the proposed search one can observe the significant change of induced currents. As a result, one can reduce time to locate failure since a significant part of the feeder can be excluded from the search area. The proposed

Energies 2020, 13, 1293 14 of 24 Energies 2020,, 13,, xx FORFOR PEERPEER REVIEWREVIEW 14 of 24 area.methodology The proposed is very methodology interesting, is but very unfortunately interesting, but not unfortunately universal. In not some universal. feeders, In the some proposed feeders, themethod proposed could method be a very could effective be a very tool eforffective earth tool fault for location earth fault and locationidentification, and identification, whereas in different whereas infeeders different it is feeders only it possible is only possible to identify to identify the type the of type line of affectedline affected by bythe the fault. fault. It It is is believed believed that distributiondistribution feeders should should be be analyzed analyzed in in the the context context of of the the effectiveness effectiveness of of the the proposed proposed method. method. If Ifthe the method method is is effective effective one one can can resign resign from from the the installation installation of of fault fault current indicators, which are eeffective,ffective, butbut cumbersomecumbersome solution.solution.

Figure 16.16.. Analysis of branches affected affected by ground faults.faults.

5. Research in the MV Network 5.. ResearchResearch inin thethe MVMV Network Measurements in MV networks are taken at a new mixed feeder. Cable line connected with 110/15 Measurements in MV networks are taken at a new mixed feeder. Cable line connected with station is made of 3xNA2XS(F)2.Y 150/25 12/20 kV cable, which is 370 m long. The measurement 110/15 station is made of 3xNA2XS(F)2.Y 150/25 12/20 kV cable, which is 370 m long. The system is presented in Figure 17. Rogowski coils are installed on cable cores and screens and additional measurement system is presented in Figure 17.. RogowskiRogowski coilscoils areare installedinstalled onon cabcablele corescores andand screensscreens fault recorder is installed at the secondary side of instrument transformers. The role of the additional and additional fault recorder is installed at the secondary side of instrument transformers. The role recorder is to verify measuring errors at the secondary side. In this particular case, brand new current of the additional recorder is to verify measuring errors at the secondary side. In this particular case, transformers with low ratio 75:5 allowed to get the acceptable measuring error—amplitude error below brand new current transformers with low ratio 75:5 allowed to get the acceptable measuring error— 5% and phase error in range 10–15 . amplitude error below 5% and phase◦ error in range 10–15°..

FigureFigure 17.17.. Diagram of the measuring system.

The tests were based on applying an artificial connection between the working conductor and the cable screens with the help of a controlled circuit breaker. During the tests, the number of cable

Energies 2020, 13, 1293 15 of 24

The tests were based on applying an artificial connection between the working conductor and the cable screens with the help of a controlled circuit breaker. During the tests, the number of cable screens connected to the earth earthing system was changed and no effect on the natural current flowing in healthy phases was noticed. Overvoltages in grounded return screens were also measured during the tests to determine the risk to cable screens, but the subject is analyzed in separate project. Table2 presents the results of the network tests. After test number 2b one of the lines was switched and as a result earth fault current is changed significantly since tests were carried out in compensated network with tap regulated coil in neutral point. As can be observed simulation results and measurement results were similar with an error below 5%. Figure 18 presents cable screen and core waveforms for case number 1, after operation of the active current forcing system, presented in Table2, as can be observed currents flow evenly in all phases, but one can observe additional distortions in Energies 2020, 13, x FOR PEER REVIEW 15 of 24 cable, through which an earthing current is flowing. screens connected to the earth earthing system was changed and no effect on the natural current flowing in healthy phases was noticed.Table Overvolta 2. Resultsges of in network grounded tests. return screens were also measured

during the testsMeasurements to determine in MV the risk to cable Connection screens, with but Ground the subject Simulation is analyzed Results in separate Simulation project. Errors Lp. Table 2 presents the results of the network tests. After test number 2b one of the lines was I0˙zr I0˙zp αr RF110/15 ABC I0˙zp αs RF110/15 RF110/15 ∆α switched and as a result earth fault current is changed significantly since tests were carried out in 1 44.025 42.706 1.75 97.0 1 1 1 43.384 0.03 98.5 1.59 1.91 − compensated2a 44.066 network 41.812 with 1.62 tap regulated 94.9 coil 1 in neutral 0 point. 1 As43.125 can be0.14 observed 97.9 simulation 3.12 results1.64 − and2b measurement 42.67 40.208 results 2.7 were similar 94.2 with 1 an error 0 below 1 5%. 41.759Figure 0.1418 presents 97.9 cable screen 3.89 and2.84 − 3a 34.865 33.347 1.98 95.6 0 0 1 33.401 0.29 95.8 0.21 1.88 core waveforms for case number 1, after operation of the active current forcing system, presented in− 3b 33.391 31.278 1.44 93.7 0 0 1 31.992 0.29 95.8 2.25 1.28 − Table4a 2, 33.402 as can 31.018 be observed 3.35 currents 92.9 flow 0 evenly 1 in all 1 phases,31.168 but 0.04 one can 93.3 observe additional 0.44 3.68 − 4b 31.814 29.631 2.79 93.1 0 1 1 31.168 0.04 98.0 5.23 3.06 distortions in cable, through which an earthing current is flowing. − 5 31.601 0.677 86.9 2.14 1 1 1 0.891 81.84 2.8 31.75 5.62 High harmonics content could have a negative influence on the operation of protection relays− and therefore the signal is typicallyαr —anglefiltered measured to remove on theunwanted primary side.components of the signal—in case of the proposed solution only fundamental component is left [59].

FigureFigure 18.18. Fault between between cable cable screen screen and and core core (nr (nr 1 in 1 inTable Table 2) 2of) of2 points 2 points bonded bonded cable. cable.

High harmonics content couldTable have 2. a Results negative of network influence tests. on the operation of protection relays and therefore the signal is typically filteredConnection to remove with unwanted components of the signal—inSimulation case of the Measurements in MV Simulation Results proposedLp. solution only fundamental componentGroundis left [59]. Errors I0żr I0żp αr RF110/15 A B C I0żp αs RF110/15 RF110/15 ∆α 6. Developed Algorithms 1 44.025 42.706 1.75 97.0 1 1 1 43.384 0.03 98.5 1.59 −1.91 2aPresented 44.066 41. results812 1.62 can 94. be9 expressed1 as0 the RF1 110/1543.125in a0 function.14 97.9 of α,3. which12 −1. allows64 for simpler2b 42 identification.67 40.208 2. of7 line94. type2 aff1 ected0 by the1 fault. 41. Figure759 0 .1419 presents97.9 results3.89 presented−2.84 in Figures3a 34.9, 86510, 1233. and347 131.98expressed 95.6 in RF0 110/15/α0 plane for1 di33.fferent401 grounding0.29 95.8 resistances 0.21 1, 2,−1. 3,88 5, 7 and 3b 33.391 31.278 1.44 93.7 0 0 1 31.992 0.29 95.8 2.25 −1.28 4a 33.402 31.018 3.35 92.9 0 1 1 31.168 0.04 93.3 0.44 −3.68 4b 31.814 29.631 2.79 93.1 0 1 1 31.168 0.04 98.0 5.23 −3.06 5 31.601 0.677 86.9 2.14 1 1 1 0.891 81.84 2.8 31.75 −5.62

αr—angle measured on the primary side.

6. Developed Algorithms

Presented results can be expressed as the RF110/15 in a function of α, which allows for simpler identification of line type affected by the fault. Figure 19 presents results presented in Figures 9, 10, 12 and 13 expressed in RF110/15/α plane for different grounding resistances 1, 2, 3, 5, 7 and 10 Ω and length of cable lines in range of 1 to 5 km. Shades of gray represent faults at the end of the cable line and shades of blue represent faults in overhead lines. Dashed lines determine 2 zones—fault in cable

Energies 2020, 13, 1293 16 of 24

10 Ω and length of cable lines in range of 1 to 5 km. Shades of gray represent faults at the end of the Energies 2020, 13, x FOR PEER REVIEW 16 of 24 cable line and shades of blue represent faults in overhead lines. Dashed lines determine 2 zones—fault inline cable is marked line is marked in section in section defined defined by dotted by dotted black blacklines linesand earth and earth faults faults in overhead in overhead section section of the of theprotected protected feeder feeder are are marked marked with with blue, blue, dotted dotted lines. lines. As As can can be be seen seen zones zones are are slightly shiftedshifted fromfrom simulationsimulation results,results, what what isis neededneeded toto copecope withwith simulationsimulation errorserrors and and modelingmodeling inaccuracies. inaccuracies. PrecisePrecise valuevalue of of correction correction factors factors is is not not known—typically known—typically in in case case of protectionof protection relays relays correction correction factors factors are setare basedset based on operational on operational experience experience [60]. [60].

100 OV_cs_25_R=1

90 CAB_cs_25_R=1

80 OV_cs_25_R=2

70 CAB_cs_25_R=2

60 OV_cs_25_R=3 (%)

50 CAB_cs_25_R=3 110/15

RF 40 OV_cs_25_R=5

30 CAB_cs_25_R=5

20 OV_cs_25_R=7

10 CAB_cs_25_R=7

0 OV_cs_25_R=10 0 20 40 60 80 CAB_cs_25_R=10 α (⁰)

FigureFigure 19. 19RF. RF110110/15/15/α characteristic.

PresentedPresented considerationsconsiderations allowedallowed forfor thethe developmentdevelopment ofof anan algorithmalgorithm forfor earthearth faultfault locationlocation andand identificationidentification of the the faulted faulted line. line. The The big big advantage advantage od line od linerecognition recognition is the is possibility the possibility to block to blockauto-reclose auto-reclose devices devices in case in case of failures of failures along along the the cable, cable, which which are are permanent. permanent. Moreover, Moreover, 2 time time settings—fastsettings—fast forfor earthearth faultsfaults alongalong thethe cablecable lineline andand slowslow forfor faultfault inin overheadoverhead lineline sectionssections cancan bebe usedused toto ensureensure compromisecompromise betweenbetween thethereliability reliabilityof of supply supply and and insulation insulation stresses. stresses. TheThe algorithmalgorithm presentedpresented in in Figure Figure 20 20 analyzes analyzes angle angleα andα andRF RF110110/15/15 and and use use conventional conventional comparators comparators to to determine determine if measurandsif measurands are are inside inside the the protection protection zones. zones. If If measured measured values values are are within within area area 1 1 a a fault fault in in the thecable cable isis recognized,recognized, inin thethe rangerange 2—fault2—fault in overhead overhead section. section. Analysis Analysis of of Figures Figures 8,8 ,9,9, 1313 andand 1414 allowsallows to tonotice notice that that in in case case of of short short cable cable lines lines——<< 33 km, km, it is possible possible to to analyze analyze only only RF RF110110/15/15 inin order order to to determinedetermine if if fault fault occurred occurred in in a a cablecable lineline oror outsideoutside ––in in overheadoverheadsection. section. OneOne cancan concludeconclude practicalpractical conclusion—inconclusion—incase caseof of shortshort cablecable lineslines itit isis notnot necessarynecessary toto replacereplace conventionalconventional CTCT withwith RogowskiRogowski coilcoil inin orderorder toto useuse thethe presentedpresented criterion.criterion. AsAs isis presentedpresented inin previousprevious author’sauthor’s paper,paper, inin casecase ofof lowlow currentcurrent flowing flowing through through primary primary side side conventional conventional CT CT can can introduce introduce big measuringbig measuring error error – even – 20%.even In20%. order In order to avoid to avoid negative negative impact, impact, i.e., unwanted i.e., unwanted tripping, tripping, of the of error the error on protection on protection algorithm algorithm CT loadingCT loading block block is introduced is introduced (I_load (I_load> Imin_load). > Imin_load). If If loading loading of of primary primary side side CT CT is is belowbelow threshold,threshold, algorithmalgorithm makes makes the the decision decision about about tripping tripping based based solely solely on onY Y0cs0cs,, which which is is modified modified admittance admittance protectionprotection criterion, criterion, obtained obtained according according to to formula: formula:

퐼 I0cs 푌Y0cs==0푐푠 (10)(10) 0푐푠 U 0 푈0 where 푈0 is zero sequence voltage. where U0 is zero sequence voltage. Preliminary research confirmed that similar characteristics are obtained for cable lines in depths of the network—not connected with 110/15 kV station. Topic is currently analyzed and is a part of future research.

Energies 2020, 13, 1293 17 of 24 Energies 2020, 13, x FOR PEER REVIEW 17 of 24

Figure 20. An algorithm for fault detection and identification of line type affected by earth fault. Figure 20. An algorithm for fault detection and identification of line type affected by earth fault. Preliminary research confirmed that similar characteristics are obtained for cable lines in depths of theAfter network—not protected feeder connected is de energized, with 110/15 the kV fault station. location Topic general is currently algorithm analyzed presented and isin a Figure part of 2future1 is started. research. The general procedure is based on utilization of simulation software or mathematical model.After Detailed protected description feeder isof de cable energized, line model the fault can locationbe found general in PhD algorithm thesis [61]. presented Due to inthe Figure model 21 complis started.exity, the The algorithm general procedure requires relatively is based big on amount utilization of time of simulation to identify software branch affected or mathematical by earth faultmodel. or determine Detaileddescription distance to ofearth cable fault. line As model a result, can it be is found not possible, in PhD thesisassuming [61]. reasonable Due to the cost model of protectioncomplexity, relay, the algorithmto use the algorithm requires relatively in real time. big amount of time to identify branch affected by earth faultThe or determinefault location distance procedure to earth is started fault. Asafter a result,measured it is signals: not possible, U0, I0cc assuming, I0cs and information reasonable about cost of present system configuration are send to the system. After obtaining information about the system protectionEnergies 2020 relay,, 13, x FOR to use PEER the REVIEW algorithm in real time. 18 of 24 configuration the simulation model is updated. Moreover, it is possible to adjust earthing resistances if weather, particularly rain information is provided. Afterwards faults in key locations are simulated—in case of an earth fault in one of overhead sections fault at the beginning of overhead lines are simulated, whereas if an earth fault is identified in cable sections—fault is simulated in the places of cable line branching. Simulation results are compared with measurement results. Differences between simulation results and measured values are calculated. The fault location characterized via the smallest difference is chosen as faulted branch. After faulted cable branch is identified, simulations are resumed, however this time, simulated fault locations are changed and the fault is simulated along faulted cable branch. Simulation results are again compared with measurement results and place characterized by the smallest difference is indicated as fault location. In some cases, similar electric parameters of branches, it is not possible to identify just 1 fault location since simulation results could be very similar i.e., just 5% difference between results. In case of small differences between simulation results, a list of results is send to grid operator. Finally, operator obtain simulation results—if one branch is affected information to repair team is send immediately, however in case of more potential fault locations are indicated, the grid operator need to coordinate repair action. It is also possible to use statistical data or operator experience to choose branch, which is more likely to be short-circuited.FigureFigure Both, 21.21 simulation. GeneralGeneral faultfault and locationlocation measurement algorithm.algorithm. results are saved in the system database in order to observe effectiveness of the proposed solution. In order to understand performance of the developed algorithm, a performance comparison is given in Table 3.

Table 3. Performance of the RF/α, fault current indicators and distance relay for earth faults in MV

networks.

Identification of Line Location of Case 1 Type Affected by Fault Location of Earth Fault in Earth Fault in Identification of (Figure 1, 5, and Auto-Reclose Cable Line Overhead Branch 10 km) Blocking for Earth Line Faults in Cable Line Case 1 (Figure 6) cable 1 km overhead section 1 km Distance No No no na Yes, but because of Fault No1 operation Yes2 na indicator issues is not installed Developed Yes No Yes na criteria Case 1 (Figure 6) cable 5 km overhead section 5 km Distance No No limited na Fault No1 yes Yes2 na indicator Developed Yes No Yes na criteria Case 2 (Figure 7) cable 1 km overhead section 1 km and 10 km cable line Distance No No No na Yes, but because of Fault No operation yes na indicator issues is not installed Developed Require special attention in No Yes na criteria compensated networks Case 2 (Figure 7) cable 5 km overhead section 5 km and 10 km cable line Distance No No No na Fault Yes, but No yes na indicator because of

Energies 2020, 13, 1293 18 of 24

The fault location procedure is started after measured signals: U0, I0cc, I0cs and information about present system configuration are send to the system. After obtaining information about the system configuration the simulation model is updated. Moreover, it is possible to adjust earthing resistances if weather, particularly rain information is provided. Afterwards faults in key locations are simulated—in case of an earth fault in one of overhead sections fault at the beginning of overhead lines are simulated, whereas if an earth fault is identified in cable sections—fault is simulated in the places of cable line branching. Simulation results are compared with measurement results. Differences between simulation results and measured values are calculated. The fault location characterized via the smallest difference is chosen as faulted branch. After faulted cable branch is identified, simulations are resumed, however this time, simulated fault locations are changed and the fault is simulated along faulted cable branch. Simulation results are again compared with measurement results and place characterized by the smallest difference is indicated as fault location. In some cases, similar electric parameters of branches, it is not possible to identify just 1 fault location since simulation results could be very similar i.e., just 5% difference between results. In case of small differences between simulation results, a list of results is send to grid operator. Finally, operator obtain simulation results—if one branch is affected information to repair team is send immediately, however in case of more potential fault locations are indicated, the grid operator need to coordinate repair action. It is also possible to use statistical data or operator experience to choose branch, which is more likely to be short-circuited. Both, simulation and measurement results are saved in the system database in order to observe effectiveness of the proposed solution. In order to understand performance of the developed algorithm, a performance comparison is given in Table3. Comparison of different earth fault location solutions clearly shows that in case of single point grounded cable lines, developed solution is preferred solution. Furthermore, developed solution has big performance in case of short lines since installation of additional fault current indicators is not justified from economical point of view. Furthermore, the proposed solution is the preferred choice in case of lines with branches. At the same time one has to notice that the proposed solution operates properly if length of cable lines creating different branches differ significantly—at least 1 km for the same grounding resistance or grounding resistance differ significantly i.e., 2 and 5 ohms. Therefore, if the proposed solution is not working one can use fault current indicators. As is presented in table distance relay performance for earth fault location is small, but one has to notice that distance protection can locate phase to phase faults with relatively high precision. Fault current indicators are preferred solutions for overhead lines. Energies 2020, 13, 1293 19 of 24

Table 3. Performance of the RF/α, fault current indicators and distance relay for earth faults in MV networks.

Identification of Line Type Affected Case 1 (Figure1, 5, 10 km) Location of Earth Fault in Cable Line Location of Earth Fault in Overhead Line by Fault and Auto-Reclose Blocking Identification of Branch for Earth Faults in Cable Line Case 1 (Figure6) cable 1 km overhead section 1 km Distance No No no na Fault indicator No 1 Yes, but because of operation issues is not installed Yes 2 na Developed criteria Yes No Yes na Case 1 (Figure6) cable 5 km overhead section 5 km Distance No No limited na Fault indicator No 1 yes Yes 2 na Developed criteria Yes No Yes na Case 2 (Figure7) cable 1 km overhead section 1 km and 10 km cable line Distance No No No na Fault indicator No Yes, but because of operation issues is not installed yes na Developed criteria Require special attention in compensated networks No Yes na Case 2 (Figure7) cable 5 km overhead section 5 km and 10 km cable line Distance No No No na Fault indicator No Yes, but because of operation issues is not installed yes na Developed criteria Require special attention in compensated networks No Yes na Case 2 (Figure 10) cable 1 km overhead section 1 km Distance No No No na

Fault indicator Segment between Re1 and Re2 or segment between Re2 and Re3 d Yes, but because of operation issues is not installed yes na Developed criteria yes No Yes na Case 2 (Figure 10) cable 1 km overhead section 10 km Distance No No No na

Fault indicator Segment between Re1 and Re2 or segment between Re2 and Re3 Yes, but because of operation issues is not installed yes na Performance depend on number of earthing points and earthing Developed criteria No Yes na resistance. Case 2 (Figure 15) cable 1 km and 3 km, overhead section 10 km Distance No No No No

Fault indicator Segment between Re1 and Re2 or segment between Re2 and Re3 Yes yes Yes, if earth fault indicator is installed Performance depend on number of earthing points and earthing Developed criteria resistance.—If points on RF110/15 = f(α) differ significantly No Yes Yes, for faults in overhead lines (Figure 18). 1. 2. Can be installed only in places where cable screen is removed. Earth fault current indicator installed at transition point (Re2). Energies 2020, 13, 1293 20 of 24

7. Summary At the beginning of the paper, review of materials about earth fault location in distribution system network is presented. Furthermore, comparison of chosen earth fault location methods—fault current indicators, distance protection and proposed solution is made. New criterion values are presented—phase shift between zero sequence current and cable screen earthing current and ratio of cable screen earthing current and zero sequence current—return factor (RF110/15). Different earthing screen current sources are presented and analyzed in order to ensure proper, sensitive operation of earth fault protection and at the same time eliminate a risk of unwanted tripping of earth fault protection. Simplified mathematical model describing earth fault current flow in cable lines is presented, what allows to understand the principle of operation. Further, different feeder configurations and factors affecting performance of the developed solutions are presented. Graphs showing criterion quantities are described. After simulation result, the developed algorithms for earth fault identification and location are presented. Additionally, SWOT analysis results and performance analysis of the proposed solution and conventional fault location methods is presented below. Strengths:

1. no communication required; 2. cost effective single point measuring; 3. the developed algorithm can detect earth faults in case of zero sequence voltage or current transformer failure; 4. operate properly in case of neutral point impedance/transformer failure.

Weakness:

1. simplified analytical expression is not sufficient for different cases and therefore it is recommended to use simulation software for analysis of real cable line. Proposed approach may be problematic in some countries; 2. proposed solution is not universal and, in some cases, (presented in Table3), the e ffectiveness could be reduced.

Opportunities:

1. further features can be integrated i.e., monitoring of cable screen connections or monitoring of stray currents; 2. improvement of reliability of supply in place where conventional technologies are not justified from economical point of view i.e., suburban areas.

Threats:

1. training for utility employees is needed; 2. it is not always clear what is the lowest value of earthing resistance of complex earthing system and therefore further research activities may be needed.

8. Conclusions The contribution of the paper is connected with identification of new value, which is used as a criterion for earth fault detection and location. The proposed value—phase shift between zero sequence current and cable screen earthing current is combined with previously presented ratio of cable screen earthing current and zero sequence current. It is proposed to present values on x-y plane, where the angle is on x axis and the ratio, referred as return factor (RF110/15) is on y axis. Proposed representation of criterion values greatly simplifies analysis. The proposed criterion is based on single point measurement and allows for clear identification of line type affected by earth fault. Proposed solution has unique feature, which allow to identify branch of line affected by an earth fault. Principle of operation of the proposed solutions is presented and the different screen earthing Energies 2020, 13, 1293 21 of 24 current sources are analyzed in order to ensure sensitive earth fault protection and eliminate a risk of unwanted tripping of earth fault protection. Presented solutions can be applied in distribution system networks and reduce downtime after earth faults. Downtime reduction is really important for power system reliability, which among power distribution loss reduction are 2 main driving forces of power system research [62]. In some cases, just RF110/15 is enough to identify the type of line affected by an earth fault, but in case of longer cable lines ( 2.5 km), one has to measure angle α between I and I . Results of ≥ 0cc 0cs conducted network tests and experience indicate that residual connection current transformer is often characterized by big measuring errors (angle) and therefore it is recommended to use Rogowski coils for measurements. The proposed solution is characterized by very good operational performance and practically 100% effectiveness in case of an earth faults of many MV lines. It is however not universal. First and foremost a distribution feeder should be analyzed in the context of proper operation of the proposed method. Conducted tests confirm that models included in simulation software work properly, however, it has to be underlined that tests were carried out in short cable line 370 m and further tests are planned. It is also planned to develop a criterion for 2fg fault detection and an expert system, which utilize cable screen earthing current measurement to maximize effectiveness of measuring system. The proposed algorithm can be combined with distance protection and location algorithms for phase to phase fault location, what allows to combine advantages of both solutions.

Author Contributions: Conceptualization, K.L.; methodology, K.L, J.L.; software, K.L.; validation, K.L., J.L., J.Z.; formal analysis, G.D.; investigation, K.L.; resources, J.Z., J.L., G.D., K.L.; data curation, K.L.; writing—original draft preparation, K.L.; writing—review and editing, K.L., J.L., J.Z.; visualization, K.L.; supervision, J.L., J.Z.; project administration, K.L.; All authors have read and agreed to the published version of the manuscript. Funding: The research was financed from resources of the Ministry of Science and Higher Education for Statutory Activities No. 0711/SBAD/4410, name of the task: Improvement of Reliability of Supply in Distribution Network. Conflicts of Interest: The authors declare no conflict of interest.

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