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Published in IET Generation, Transmission & DistributionReceived on 30th November 2009Revised on 18th February 2011doi: 10.1049/iet-gtd.2009.0681

ISSN 1751-8687

Impact of distributed generators on arcing faultsin distribution networksR. Ciric1 H. Nouri2 V. Terzija3

1Secretariat for Science and Technological Development of the Government of Autonomous Province of Vojvodina,Bul. Mihajla Pupina 16, Novi Sad 21108, Serbia2School of Engineering, Frenchay Campus, University of the West of England, Coldharbour Lane, Bristol BS16 1QY, UK3School of Electrical and Electronic Engineering, The University of Manchester, Ferranti Building B6, Sackville Street,PO Box 88, Manchester M60 1QD, UKE-mail: rade.ciric@vojvodina.gov.rs

Abstract: Increased fault level is one of the main concerns connected to the integration of distributed generation (DG) intodistribution networks. To accurately calculate fault currents in distribution systems with a high penetration of DG, a realisticfault model must be developed that includes the electrical arc existing at the fault point. The results of an assessment of theimpact of DG on such arcing faults are presented. The significance of the study is that the fault model includes the electricalarc element, which brings additional non-linear resistance into consideration. Since the arc resistance is a non-linear functionof the fault current, the problem of simultaneous fault currents and arc resistance calculation has been tackled using a noveliterative algorithm. In this work, a typical medium voltage distribution network is considered. Results of the simultaneousfault analysis and arc resistance calculation in the IEEE-34 distribution network with a distributed generator are presented anddiscussed.

1 Introduction

Various system studies have demonstrated that the integrationof distributed generation (DG) in distribution networks maycreate different technical and safety challenges [1–6]. Themain concerns regarding the integration of DGs intodistribution networks are the increasing fault level and aneed for changes in the concept of the system protection.The traditional methods for system analysis applied indistribution network planning and operation are based onunidirectional flow of power and the radial networktopology. Fault currents are assumed to flow downwards,which enables relatively simple protection schemes. Theintroduction of DG changes this situation significantly.

The protection challenges connected to the DG integrationinto distribution systems can be classified into severalcategories:

† sensitivity,† selectivity,† reclosing, and† islanding problems.

Calculation of short-circuit currents in distributionnetworks presents standard types of network planningstudies. The results of such studies are used for the designof switchgear and protection co-ordination. The introductionof DG has made these studies more important than ever.Since the short-circuit capacity margin in urban distribution

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networks is relatively limited, the introduction of newgeneration units might be limited, especially during theworst demand-generation scenarios.

In general, the maximum and minimum fault currents areboth calculated for a given distribution system with DG. Themaximum fault current is calculated based on the assumptionsthat all generators are connected, that the load is maximal andthat the fault is a pure bolted (metallic) one, with the faultimpedance equal to zero. On the other hand, the minimumfault current is calculated based on the assumptions that thenumber of generators connected to the network is at aminimum, that the load is at a minimum and that the fault isnot bolted. Under these circumstances the fault impedance,consisting of the tower footing resistance and arc resistance,must be considered. In practical studies, empirical values areoften used for the fault resistance. If they are far away fromthe actual value, a large error in the short-circuit currentscalculation will appear and erroneous conclusions can be drawn.

Statistically, arcing faults occur in over 80% of all faults[7], thus consideration of the arc resistance in short-circuitstudies is essential [8, 9]. Since the fault current depends onthe arc resistance, which is itself a non-linear function offault current, the question here is how to calculate theunknown arc resistance and fault current accurately. It isobviously not possible just to represent the arc as a lumpedresistance and to include it into the short-circuit study as apurely resistive line. This would lead to erroneous results,since an arc cannot be represented in this simplifiedmanner. By assuming a constant average value for arc

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resistance, selected empirically (e.g. 0.5 V), or simply byneglecting the arc existing at the fault location, the problemis directly solvable using a number of classical short-circuitmethods. The arc resistance is determined by its length andthe current flowing through it: it is directly proportional tothe arc length and inversely proportional to its current.

In Fig. 1 a three-phase arcing fault on an overhead mediumvoltage transmission line is presented. The fact is that arcingfaults have not featured greatly in distribution systemanalysis, especially in the presence of DG. One of the mainreasons why this phenomenon has not been investigatedsufficiently is the difficulty in defining the arc resistance ina suitable way for short-circuit studies. In Section 2, thisimportant issue will be discussed and some new solutionsfor the modelling of the fault arc will be proposed.

In this paper an assessment of the impact of DG on arcingfaults is presented. The objective is to draw conclusions onhow DG affects the fault level and how important it is toinclude the arc model in the protection co-ordination studies.The results of this assessment can help to investigate thearcing faults which create challenges for DG and the network.A solution for the short-circuit calculation in the network withthe DG, which takes into account the arc resistance existing atthe fault location, is presented. It is based on an efficientiterative method for short-circuit current calculation indistribution networks and a new arc resistance formula.Compared to the existing methods, in which the phenomenonof arc resistance has been totally neglected or solved byassuming an arbitrary constant arc resistance value, thismethod offers a more accurate short-circuit current calculationin medium voltage distribution networks with/without DG.This will be beneficial for different system studies in whichthe exact fault currents are to be calculated for such protectionco-ordination studies.

The paper is organised as follows: Section 2 describes thealgorithm applied for simultaneous calculation of faultcurrents and arc resistance. In Section 3, a description of thetest network used for the algorithm validation is given and inSection 4, the results of short-circuit currents and the arcresistance calculation on the test example of the IEEE-34distribution network with/without DG are presented anddiscussed. Conclusions are given in Section 5.

2 Fault analysis and arc resistancecalculation

Methods for short-circuit studies in distribution networks usesymmetrical components or phase network representation [9].

Fig. 1 Three-phase arcing fault on an overhead line

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The main advantage of conventional symmetrical componentmethods is that the three sequence matrices are consideredseparately, simplifying the network representation andmaking the method more practical. However, this group ofmethods does not enable the exact modelling of four-wiredistribution networks and cannot be applied for unbalancedand/or unsymmetrical networks. On the other hand, severalexplicit and efficient methods for fault analysis in actualmedium- and low-voltage distribution networks in the phasedomain are proposed [10–15]. The contribution todistribution network fault levels from the connection of DGis well documented and has been reported in many papers[1, 4, 5, 16, 17]. In this study, the distribution short-circuitanalysis approach based on the hybrid compensationmethod [14] is applied. This efficient and robust methoduses the solution of the three-phase distribution power flowas a pre-fault condition [18] and performs one backward–forward sweep to generate the post-fault state after updatingthe hybrid current injections. The method requires thecreation of a fault Thevenin equivalent impedance matrix,combining three basic compensations, that is, the loopsbreak-point compensation, the DG compensation and thefault compensation using phase co-ordinates.

Based on experimental testing in a high power testlaboratory and exhaustive analysis and processing ofdigitised arc voltage and current data records, the followingpractical formula for arc resistance is derived [19]

Ra =2��2

√

p

EaL

If

(1)

where Ra is the arc resistance in V, L is the arc length in m, If

is the arc current in A and Ea is the arc voltage gradient in V/m.In the open literature [7, 20], the following expression for

Ea calculation is proposed

Ea = 950 + 5000/If (V/m) (2)

Equation (2) is applicable to medium and high voltage levels.The physical explanation of (2) is rather complex. In brief, thelarger the fault current is, the higher the temperature and thesmaller the resistance of the arc will be. Smaller resistanceproduces a smaller voltage drop, and under the assumptionof a constant arc length, the arc voltage gradient Ea is smaller.

By introducing Ea from (2) into (1), the following equationfor arc resistance is obtained [19]

Ra =855.3

If

+ 4501.6

I2f

( )L (3)

From the above expression, it is obvious that the arc resistanceis a non-linear function of its current and that it is proportionalto its length. The challenge here is how to include the non-linear arc resistance in the fault model and thereby improvethe accuracy of the existing short-circuit method. A directsolution is not possible. In this paper, an advanced solution,based on an iterative hybrid compensation short-circuitmethod, which models the arc at the fault location, isapplied [21]. Through an iterative procedure, the methodcalculates short-circuit currents and arc resistancesimultaneously.

The applied iterative procedure for fault currentsand arc resistance calculation can be presented

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through the following algorithm steps (here k is an iterationindex)

1. Set the start value of arc resistance to zero, Ra(k ¼ 1) ¼ 0.2. Calculate the fault current, If(k ¼ 1), by using thecompensation short-circuit method and arc resistance at thefault location assumed in the above kth step.3. Increment the iteration index: k ¼ k + 1.4. Calculate the arc resistance Ra(k) using (3) and the faultcurrent at the fault location calculated in the previous step.5. Calculate the fault current, If(k), using the updatedThevenin equivalent impedance matrix, Zt, which includesthe arc resistance Ra(k) at the fault location.6. Test the accuracy criterion, ‘if |Ra(k) 2 Ra(k 2 1)| .1(¼0.0001), then go to step 3, or STOP the procedure’.

The above presented methodology can be applied for anykind of expressions for arc resistance in which the arcresistance depends on the fault current.

Depending on the contract and control status of the DG, itmay be operated in one of the following modes:

1. in parallel operation with the feeder where the DG isdesignated to supply a large load with fixed real and reactivepower output;2. to export power at a specified power factor; or3. to export power at a specified terminal voltage.

Considering power flow, the DG node in the first two casescan be represented as a PQ node. This requires a modificationof the power flow algorithm as the current is injected into thebus. In the third case where the source controls the voltagemagnitude at the corresponding node, the node is referredto as a PV node. If the computed reactive power generationis out of the reactive generation limits, the reactive powergeneration is set to that limit and the unit acts as a PQnode. However, the model of the DG node does not affectthe results of the fault currents and arc resistancecalculation in the proposed iterative algorithm.

The proposed methodology for fault current and arcresistance calculation is of general usage since it can beapplied to most of the existing medium voltage distributionnetworks: three-wire, four-wire, with (solidly) groundedneutral or isolated neutral wire [15]. Different transformerconfigurations (earthing options) and load models alsoaffect the values of the short-circuit currents. Hence, propernetwork modelling and correctly selected networkparameters can be critical in short-circuit analysis and arcresistance calculation [22].

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3 Test network

The methodology for fault currents and arc resistancecalculation described above is applied on the modified IEEE34-node radial overhead distribution network (IEEE-34 DN)[23] (see Fig. 2). The total demand is 1770 kW, and 72% ofthe loads are concentrated 56 km away from the connectionpoint to the supplying network. The most distant node is59 km away from the supplying network. The base voltageof the network is Vb ¼ 24.9 kV. Simplifying, the in-lineauto-transformer 24.9/4.16 kV/kV between nodes 832 and888 of the original IEEE-34 DN test feeder is replaced withline 19–20, and the network is modelled with a singlevoltage level. The automatic voltage regulators in nodes 814and 852 of the original IEEE-34 DN test feeder are also notrepresented and are replaced with lines 7 and 19. The loadand line data of the test network are given in [24]. Theapplied equivalent system impedance at the substation69 kV/24.9 kV (D/grounded Y ), 2500 kVA isZsys ¼ (5 + j5)V/phase.

In order to analyse the fault currents and arc resistances in theIEEE-34 DN with DG, DG is connected at node 23 andmodelled as a PV node. The generator data are as follows: realpower Pg ¼ 300 kW, voltage Vg ¼ 25.100 kV, the internalimpedance of the generator Zgen ¼ (0.6 + j1.5)V/phase. In allof the simulations the arc length in (3) is set to be L ¼ 1 m,which is an arbitrary assumption. One could assume someother realistic arc length (e.g. 0.5 m), but the methodologywould be the same. The longer the arc, the larger the arcresistance and the larger is its influence on the fault level.

In the next section, results of the short-circuit study withand without DG are presented.

4 Application examples

For the purposes of assessing how the DG affects the fault leveland how important it is to take the arc model into consideration,two main sets of simulation studies are carried out. In everysingle set of simulations, four traditional fault types atdifferent points in the network are considered: (i) single-line-to-ground, (ii) line-to-line, (iii) double-line-to-ground and(iv) three-line-to-ground faults. In the first set, the network istreated as passive, without DG. In the second set, thedistribution network is considered as an active network, withDG connected at node 23. In parallel to the importance ofthe modelling of the arc at the fault point, the influence ofthe DG on the fault level is analysed.

The constant admittance load modelling approach isapplied. In all simulations the ground fault resistance is

Fig. 2 IEEE-34 node 24.9 kV test distribution network

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ignored and set to zero (metallic short circuit). In the case ofthe line-to-line fault and double-line-to-ground fault, thealgorithm delivers two values of fault current and twovalues for arc resistance. In case of the three-line-to-groundfault, three phase currents and three values for arc resistanceare obtained [21].

Figs. 3 and 4 represent the arc resistance at different nodesin the test network with/without a DG in the case of single-line-to-ground faults and three-line-to-ground faults,respectively. Figs. 5 and 6 display the fault currentsin the test network with/without a DG in the case ofsingle-line-to-ground faults and three-line-to-ground faults,respectively. Analysis of these figures reveals that the DGincreases the fault level in the network and the values of arcresistance become smaller. Very similar diagrams for faultcurrents and arc resistance are obtained in cases of double-line and double-line-to-ground faults.

In the IEEE-34 test feeder with DG, the maximum faultlevel is obtained when considering a three-phase fault at thebus bar at node 0. This short-circuit power results fromboth the upstream grid and the total DG capacity.

The DG in node 23, which represents a suitable choiceregarding the multi-objective index (active and reactivepower losses, maximum voltage drop, reserve capacity ofconductors and single-phase and three-phase short-circuit)[4], significantly increases the fault currents level. Theincrement of the fault currents level is highlighted at theDG and the neighbouring nodes. Consequently, the arc

Fig. 3 Arc resistance for single-line-to-ground faults in differentnodes

Fig. 4 Arc resistance for three-line-to-ground faults in differentnodes

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resistance is decreased. By changing the DG location, thefault current will be changed and consequently the valuesfor arc resistance. In general, the greater the distancebetween the generation unit and the substation, the greaterthe ratio of the maximum fault current with the DG to themaximum fault current without the DG is. In the analysedtest network, a significant increase of fault current levels,for both three-phase and single-phase short-circuits, occurswhen the DG is connected far from the substation. For theDG in node 23, the three-phase short-circuit maximumcurrent (in node 33) is two times larger than in the casewithout the DG, whereas with the single-phase fault itincreases four times. It is shown in the application of faultanalysis in four-wire distribution system (on the same testnetwork) presented in [15] that the DG increases the faultcurrent through the neutral wire and ground as well.

The calculated values of arc resistance were in the range of0.4–2.5 V. The largest values of arc resistance are obtained inthe cases of single-line-to-ground fault in the passive networkfar away from the substation HV/MV. The smallest value ofarc resistance is calculated in the active network in the caseof double-line-to-ground fault close to the substation.

For further analysis, let us introduce the difference betweenthe arc resistance in the network without the DGs (Ra) andwith DGs (Rag): delta Ra ¼ |Ra 2 Rag|. The value delta Ra

represents the impact of DG (23) on the arc resistance for

Fig. 5 Fault currents for single-line-to-ground faults in differentnodes

Fig. 6 Fault currents for three-line-to-ground faults in differentnodes

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the fault in the different nodes and for the single-line-to-ground fault presented in Fig. 7. The largest value of deltaRa is obtained at the remote end of the feeder, which meansthat the largest impact of the DG on arcing phenomena isexpected for faults at the end of the feeder.

The error in fault current calculation due to neglecting thearc is expressed in the following way

Error(%) = Ifo − Ifarc

Ifarc

× 100% (4)

where Ifo is the fault current in the head of the feeder(substation) with the arc neglected and Ifarc is the faultcurrent in the head of the feeder with the arc included inthe calculation.

Fig. 8 shows the error calculated according to (4) in thecase of single-line-to-ground faults in different nodes with/without the DG. The error in fault current calculation in thepassive network due to neglecting the arc is in the range of2–12.2%. The largest error is obtained in the case of asingle-line-to-ground fault at the remote end of the feeder.The maximum error due to neglecting the arc resistance inthe single-line-to-ground fault current calculation in thenetwork with the DG is 16.5%. This is an interesting resultsince the single-line-to-ground fault is the most commonly

Fig. 7 Impact of DG (23) on arc resistance for single-line-to-ground fault in different nodes

Fig. 8 Error in fault currents due to neglecting the arc for single-line-to-ground fault in different nodes

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occurring fault in the MV distribution networks. The errorin the fault current calculation increases as the faults occurcloser to the remote end of the feeder. With the arc, theshort-circuit currents are smaller and the fault-time voltagedrops are less than in the case of pure metallic faults.

A practical application of the iterative algorithm for arcresistance calculation might be in calculation of adaptivedistance protection zones. The concept of adaptiveprotection is to determine the best protection settingensuring the most sensitivity of each protection unit, foreach regime of the power system. Distance protection isgenerally applied in the transmission networks. On theother side, modern distribution networks (,69 kV) arebecoming more and more complex (highly loaded andweakly meshed) requiring reliable and accurate protectivefunctions. Therefore it is possible that an adaptive distanceprotection will be used in the distribution networks tofulfil the selectivity protection requirements. Measuredimpedance by the protection device in the case of single-line-to-ground fault, Zf is expressed by (5) [8]

Zf = Zd + Ra

3I01

IL1 + 3kI0

(5)

where Zd is direct impedance of the line, I01 is zero sequenceof the fault current at the place of fault, IL1 is fault current inthe head of the feeder, I0 is the zero sequence of the faultcurrent in the head of the feeder and k is the coefficient ofthe single-phase to ground fault given by (6)

k = Z0 − Zd

Zd

(6)

where Z0 is the zero sequence impedance of the line.According to (5), if there is no arc during the fault, the

impedance measured by the protection device is equal tothe direct impedance of the line. The measured impedanceof the protective relay in the presence of an arc depends onthe direct impedance of the line, as well as the arcresistance and suitable fault currents. From (5), it followsthat apart from the arc resistance, the relationship betweenthe zero sequence fault current I01 and the fault current inthe head of the feeder IL1 has the main impact on theenlargement of the impedance measured by the protectiondevice. The effect of the arc and DG on the performance ofthe protective relay is as follows: the DG increases the faultcurrent and decreases the arc resistance, thereby decreasingthe impact of the arc on the impedance measured by theprotection device.

If the arc resistance is neglected in the single-line-to-ground fault at the DG node 23, the values of impedancesas measured by the distance protection device would beabout 25% smaller. Consequently, the ‘dead-zone’ of thedistance protection would be enlarged, decreasing theefficiency of the protection. Since the largest values of arcresistance are obtained in the passive network in the case ofa single-line-to-ground fault far away from the substationHV/MV (Fig. 3), the enlarging of the ‘dead-zone’ of thedistance protection will be highlighted in such cases. It isclear that the new approach delivers more accurate valuesfor determining the settings of the distance protection.

Other applications of the presented methodology can befound in the maximisation of DG penetration and powerquality assessment, where the renewable energy sources areintegrated into the distribution network (fault-ride through

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requirements of wind generators, nuisance tripping duringvoltage dips) [25].

The amount of DG and its placement obviously determinesthe fault current level at a certain point of the network.Consequently, the amount of DG and its placementinfluences the arc resistance. With the large number of DGsin the network, larger fault currents and smaller arcresistance will be experienced. The work reveals that, withthe presence of a large number of DGs, the error in the faultcurrent calculation due to neglecting the arc can be evenlarger. In general, the error depends on the three-phase short-circuit power of the upstream network, fault location, lengthof the arc, as well as the location and rated power of theconnected DG. The results obtained with the arcing faults inthe network with a single DG show that the distanceprotection zones in the network with a high penetration ofDG might need to be changed. For more accurate short-circuit and voltage dip calculation in the case of high DGpenetration, the dynamic behaviour of the DG during arcingfaults with a complex dynamic arc must be considered.

5 Conclusions

In this paper, an assessment into the impact of DG on arcingfaults in distribution networks is studied. For short-circuitcalculations, the fault analysis method which considersarcing faults is applied. The results show that the DGsignificantly increases the fault currents level for all typesof fault, especially at the DG and neighbouring nodes andconsequently the arc resistance decreases. The largestimpact of DG on the arcing phenomena is expected forfaults at the end of the feeder. Practical applications of thepresented methodology are in the setting of adaptiveprotection (relaying co-ordination). The impact of the arcand DG on the functioning of the protective relay is in thefollowing: the arc increases the impedance measured by theprotection device, whereas the DG increases the faultcurrent and decreases the arc resistance, decreasing theimpact of arc on the impedance measured by the protectiondevice.

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