Influence of Metal Installations Surrounding the Feeding Cable Line ...

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 4, OCTOBER 2008

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Influence of Metal Installations Surrounding the Feeding Cable Line on the Ground Fault Current Distribution in Supplied Substations Ljubivoje M. Popovic´, Senior Member, IEEE

Abstract—This paper presents the method for taking into consideration the influence of metallic installations surrounding the feeding line on the ground fault current distribution in supplied substations. When a ground fault occurs in a substation supplied by a cable line, these installations can significantly participate in reducing the part of the ground fault current emanating through the grounding system of this substation into the surrounding earth. Since only this part of the ground fault current creates potentials on the grounding system, this also means the reduction of all potential differences (touch and step voltages) appearing during the ground fault inside and in the vicinity of the supplied substation. However, current analytical expressions for the determination of the cable line reduction factor do not take into consideration this favorable effect. It is also not possible to determine it by direct measurements. This paper presents a method which enables us to achieve this by using the results of reduction factor measurements and by modeling surrounding metallic installations as a single equivalent conductor, cylindrical in form and parallel with the cable line. Index Terms—Cable sheath, ground fault current, measurements, reduction factor, safety conditions.

I. INTRODUCTION

W

HEN a ground fault occurs in a power network, a ground-fault current leaves a phase conductor at the fault place and its flow towards the sources in the power system continues through different paths. Besides the earth as a conductive medium, this current also uses different metallic conductors in a conductive connection with the grounding electrodes of the substations located at the feeding line ends. These conductors are the ground wire(s) in the case of an overhead line, and the metallic sheath(s) in the case of a cable line. Since all potentials on the grounding systems of the supplied substation are created only by the part of the ground-fault current passing through the earth toward the grounded neutral(s) in the supply network, the correct determination of the groundfault distribution is of prime importance in its estimation of the safety conditions inside and in the vicinity of the supplied substation. Because of that, in professional literature, including technical standards (e.g., [3]), a special parameter of the feeding line is introduced, known as the reduction factor. It is defined as the ratio of the part of the ground-fault current returning through

Manuscript received December 6, 2006; revised July 20, 2007. First published May 7, 2008; current version published September 24, 2008. Paper no. TPWRD00784-2006. The author is with Elektrodistribucija Beograd, Belgrade 11000, Serbia (e-mail: [email protected]). Digital Object Identifier 10.1109/TPWRD.2008.923502

earth and the total ground-fault current. By this, it is assumed that the impedances of the grounding systems of the substations at the ends of the feeding line to the remote earth are negligible (e.g., [1]). Based on such an assumption, the current in the neutral conductors appears solely as a consequence of inductive coupling between these conductors and the phase conductor through which the total ground fault current passes. In the case of a cable feeding line, the reduction factor expresses the influence of the inductive coupling between the phase conductors and the metallic sheath(s) on the reduction of the fault current injecting into the surrounding earth through the grounding system of the supplied substation. Based on this definition, a corresponding analytical procedure is developed for determination of the reduction factor for the lines performed by different type of cables [1], [6]. However, can this definition be considered as correct if we know that cable lines are almost without an exception used in urban conditions? In urban areas, there are normally many other underground metallic installations—different pipelines, reinforced building foundations, etc. Although different in their basic function, all of these installations and metal structures form a unique and large underground metallic network covering the whole urban area. Most of them are laid under street pavement and some of them are in an effective and continuous contact with the earth. Due to such spatial disposition, they are more or less inductively coupled with the neighboring feeding line and spontaneously participate in returning ground fault current toward the power system, or in reducing the fault current injected from the grounding system into the surrounding earth. Thus, the problem can be formulated as follows: How can this favorable effect be determined in the stage of putting the newly built substation in operation? First, in all cases, the surrounding metal installations form a very complex network different in each particular case. Second, this network has a changeable configuration along the whole feeding line. Thus, it is clear that the actual physical model is very large and involves numerous different elements. However, this is not the largest problem. The main difficulty is that the mentioned installations are buried underground and, therefore, many relevant data (dimensions, mutual disposition of individual installations, and their elements, etc.) remain uncertain and, in many cases, even completely unknown. The same can be said for the soil composition along the feeding cable lines. Obviously, a complete and correct insight into the influence of this network could be achieved only by onsite measurements. However, the results of many site measurements performed so

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far appear paradoxical at first sight, namely, the values of the reduction factor obtained by measurements of the simulated ground fault current and its part(s) in the cable sheath(s) are most often significantly higher than those obtained for the same cable line by calculation that does not take the surrounding metal installations into account. This paradox is considered here on the basis of the experimental measurements performed by simulating a ground fault in two substations supplied by the cable lines belonging to the 110-kV network of Belgrade. The problem has been notified and partially considered in [9] and [10]. It can be described in the following manner. Metallic sheaths of all medium-voltage (MV) cable lines leaving the high-voltage (HV)/MV substation participate in forming the grounding system of this substation [4], [12]. In order to decrease the potentials transferred to the public places (for the metal parts exposed to human touches, for example, water pipes, customer installations, etc.), all other metallic installations are intentionally galvanically separated from the grounding electrode of the HV/MV substation. These installations are connected to the substation grounding system only in the nearest MV/low-voltage (LV) substations supplied through the mentioned MV cable lines. However, besides the grounding function, the spontaneously formed grounding system acts as a conductive connection with the metallic installations surrounding the feeding cable line. As a result, during the ground fault, one more fraction of the ground fault current circulates through many of the grounding system elements. This current returns to the power system through the metallic installations encompassed by the magnetic field produced by the ground fault current in the feeding cable line. The proximity of different circuits as well as the interconnection of different grounding system elements in the vicinity of the HV/MV substation produce a different split of two fault current components appearing in individual external ground circuits. Due to all of these reasons, the immediate measurement of the component induced by the magnetic field is practically impossible. Besides that, in urban conditions, the other component—fault current emanating into the earth—also cannot be correctly determined by measurements. The reason for this is the unavoidable influence of the nearby metallic installations to the results obtained by measurements (e.g., [11]). Thus, the component produced by the magnetic influence cannot be determined through the measurement of other components, as has been done in the simple case considered in [10]. The problem of the determination of the fault current passing through the metal installations surrounding a feeding cable line is solved in this paper by introducing one fictitious conductor. The space position, shape, and magnitude of the cross-section of this conductor are determined under the condition that its inductive influence on the ground fault current distribution in the supplied substation is identical to the inductive influence of all really existing installations along the feeding cable line. The fictitious conductor determined under this condition is then treated as an additional cable sheath in the calculation procedure. On the basis of this conductor, it is possible to find the component of the fault current dissipated into the earth and explain the previously mentioned apparently paradoxical results of the site measurements. Generally, by using the results of site measurements and


the method developed in this paper, it is possible to evaluate the adequacy of the grounding system performance and verify the design values for the HV/MV substations built in the urban and suburban areas. II. EXPERIMENTAL ANALYSIS Experimental measurements of the reduction factor were performed on a cable line, which radially supplies in series two substations in the 110-kV network of Belgrade, Serbia. The length of the line to the nearest supplied substation measured from the supply substation is 2320 m, while the total feeding line length to the more distant one is 6590 m. The line is performed with XHLP cables with mutually identical designed characteristics, laid in a triangular formation on the whole line length. The cross-bounding technique for circulating currents elimination was not applied to the line. In the areas through which the line passes through, the specific soil resistivity is estimated to be in the range from 30 to 50 m. The line section between the supply substation and the transit (nearer) one passes through an area with a lower degree of urbanization than the rest of the line. The phase conductors are made of aluminum with a cross-section of 1000 mm , while the metallic sheaths are made of copper strings with a total cross-section of 95 mm and a medium diameter that is equal to 91 mm. The main elements of the grounding system in both of the supplied substations are the station-building foundation and the 44-MV outgoing cable lines built from cables with uninsulated metal sheaths. The grounding impedances of the supply substation and both of the supplied substations are between 0.02 and 0.03 . Since these impedances are very small compared to the other parameters influencing the measured values of the reduction factor, they are completely disregarded. The described line is used to obtain the experimental results for two different feeding cable lines—one 2320 m long and the other is 6590 m long. This was achieved in the following manner. The longer one is obtained as a continuous cable line on the whole line length by disconnecting metallic sheaths mutually and from the grounding electrode to the nearest supplied substation. Since the cable lines pass through areas covered with a dense network of different underground metal structures, it was not possible to obtain the exact value of the equivalent soil resistivity, or the equivalent earth penetration depth by standard geological measurements. These necessary data have been obtained by measurements of the self impedance of the phase conductor for each considered line. On the basis of the simulated ground fault in the supplied substation, the following measurement results were obtained. and for • the 2320-m-long line. and • for the 6590–m-long line. To obtain the necessary data on the equivalent soil resistivity, and the following anathe given values of the impedance lytical expression were used (e.g., [3]):

(1)

POPOVIC´: INFLUENCE OF METAL INSTALLATIONS SURROUNDING THE FEEDING CABLE LINE

where phase conductor resistance per unit length (in km); outer radius of the phase conductor (in meters); angular frequency ( ); magnetic permeability of vacuum ( relative magnetic permeability.

);

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According to [1], the reduction factor for a fault anywhere along the line is determined by the expression (4) where total line length;

denotes the quantities given per unit length. Prime The equivalent earth penetration depth is determined by the following expression: (2) where equivalent soil resistivity along the cable line; nominal frequency. Here, it should be mentioned that these expressions are based on the Carson’s theory of the ground fault current return path through the earth [5], [6]. In this way, it has been obtained that the necessary data are: , or for the shorter line; • , or for the longer line. • The obvious differences between these data for the considered lines can be explained by the fact that the shorter line passes through an area with a lower degree of urbanization. Since we now have all of the relevant data, we can validate the analytical expressions for the reduction factor in the case when single-core cables are laid in a triangular formation. In that case, the reduction factor is, according to [7], given by (3) where distance between two adjacent cables, or the diameter of the single-core cables in the case of triangular formation; medium radius of the cable sheath. By applying the given expression to the considered cable lines, we obtain: for the shorter; • for the longer line. • As can be seen, the results obtained by the given expression and the results obtained by measurements are in good agreement, and actually are practically identical. The analytical expression for the reduction factor for the fault anywhere along the line [1], [7] has been also experimentally tested. The fact that the phase conductors are accessible to only the nearest supplied substation has been used for this purpose. So the continuous feeding cable line has been formed. Its length was 6590 m, and a simulated ground fault was in the transit subm, as seen from the beginning station, at a distance of of the line. The results of measurements show that in that case, the value of the reduction factor is

reduction factor for the fault at the end of the line

.

The result obtained by using (4) is also in good agreement with the given measurement result. Thus, it can be stated that the analytical expressions for the ground fault current distribution for the fault anywhere along the line are experimentally verified. However, the equivalent soil resistivity that takes the influence of the surrounding metal installations into account is correct only for testing the accuracy of the existing analytical expressions [1] and [7]. It will be shown in the basis of the following analysis. The calculation procedure performed using (3) and (4), as well as the data on equivalent resistivity of the actual soil (30 m), or by disregarding the existence of the surrounding metallic structures, gives the following significantly lower values: for both considered lines; • for the fault at a • distance of 2320 m along the longer line. The discrepancies between these and the previous results are obvious and need to be explained. Based on the results obtained by taking into account the influence of the surrounding metal structures through the equivalent (in fact effective) soil resistivity, the following facts can be noted. The effective soil resistivity obtained in this way is drastically decreased in comparison with the realistic one (30 m) and, consequently, the value of the reduction factor is significantly increased in both of the considered cases. It leads to the conclusion that the presence of the nearby underground metallic structures increases the part of the ground fault current flowing through the earth and through the grounding system of the supplied substation. Somewhat later, we will see that this conclusion is erroneous. However, the use of the effective soil resistivity is the only way for an experimental verification of the analytical expressions for the reduction factor of cable lines in the urban conditions. Namely, these expressions are derived under the assumption that the cable line is laid in homogeneous soil with a resistivity equal to the equivalent resistivity of the normally heterogeneous (multilayer with each layer having a different resistivity) soil along the cable line. However, does this assumption correspond to the actual situation, or should the surrounding metal installations be completely disregarded? Many underground metallic structures such as sheaths of other cable lines, neutral conductors of the low voltage network, steel water pipes, building foundations, etc. in urban areas are interconnected and form a very large and complex metallic network. Although this network consists of similar or identical elements, their spatial disposition along a cable line is different in each particular case. Most of them are laid under street pavement and some of them are in effective and continuous contact with the earth. Also, some of them are in a direct conductive connection with the grounding electrode of

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Fig. 2. Ground fault current distribution when only one sheath is grounded.

Fig. 1. Schematic presentation of a fictitious conductor.

the supplied substation (e.g., the metallic sheaths of the MV cables [4]). To obtain a more complete insight into the influence of the metal structure surrounding the feeding line, the currents through the cable sheaths were also experimentally investigated. At first, a ground fault current distribution was observed when only the sheath of the cable with the simulated ground fault current was connected to both of the grounding electrodes at the line ends. Then another situation was considered when one more cable sheath was connected to both of the grounding electrodes at the line ends. Finally, the normal operation conditions were observed when all three of the metal sheaths were grounded at the line ends. The measurement results show that a successive increase of the number of the grounded sheaths reduces not only the current flowing through the earth, but also the relative participation of each of the already connected sheaths in reducing the fault current through the earth. Presented respectively, these relative reductions in the case of the sheath belonging to the cable with simulated ground fault current are from 60.66% to 46.31% and from 46.31% to 37.49%. These results help us understand the influence of the surrounding metallic installations. They show that surrounding metal structures act as an additional metallic sheath of the cable(s). Thus, they can be modeled by a single fictitious conductor parallel to the cable line and either continuous, or in contact with the earth in a finite number of places, only. Naturally, this additional sheath reduces the part of the ground fault current flowing into the earth through the grounding system of the supplied substation. The influence of a single underground conductor (copper wire) in effective and continuous contact with the earth, laid in parallel with the cable line, is considered in [2]. However, the problem considered here is reverse. Based on the measurement results, some effects of the surrounding metal installations on the ground fault current distribution are known. It is necessary to determine the parameters of the corresponding fictitious conductor and its spatial arrangement relative to the cable line. III. DETERMINATION OF THE FICTITIOUS CONDUCTOR According to the previous considerations, the influence of the metallic installations encompassed by the magnetic field produced by the ground fault current in the cable line can be examined using the circuit shown in Fig. 1.

The disconnector shown in the given circuit is in open position only in the case when, because of dangerous transferred potentials, all underground metallic installations except the metallic sheaths of the outgoing HV cables are intentionally galvanically disconnected from the grounding electrode of the supply substation. However, in this case, considered effects can also be very pronounced [2]. For the sake of simplicity, we will use the simplest case from the standpoint of the ground fault current distribution. This is obviously the case when only the metallic sheath of the cable through which the ground fault is circulating, is grounded at the line ends. This case is presented by the equivalent circuit in Fig. 2, together with the fictitious conductor. The notation used in this equivalent circuit has the following meaning: supplied substation (fault place); voltage source (in the supply station); total ground fault current; fault current through the metallic sheath of the cable with faulted phase conductor; fault current through the fictitious conductor; fault current returning through the earth; self-impedance of one cable sheath; mutual impedances between the cable phase conductor and its metallic sheath with a common earth return; mutual impedance between the cable phase conductor and fictitious conductor with common earth return; mutual impedances between the metallic sheath and fictitious conductor with common earth return; impedance of the grounding system of the station A (B); remote ground. The balanced components of the ground fault current circulate only through the phase conductors and, as known, their inductive influence to the nearby parallel circuits is negligibly small. Because of this, these currents and the unfaulted phase conductors are omitted in the presented circuit. In accordance with the previously given definition of the reduction factor, in our further consideration, we will assume that and are negligibly small ( and the impedances ). Also, for further consideration, it is necessary to mention that the current directions shown in Fig. 2 are arbitrarily taken.


By using (5) and the approximation presented in the following form:

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, (10) can be

(11) When we know the value of the measured reduction factor and all of the pertinent parameters of the considered cable line ), the only unknown quantities in (11), according ( and and . Since (11) actually represents to (8) and (9), are two equations—one for the real and the other for the imaginary parts of the complex quantities—using this equation, it is posand . In this way, in accorsible to determine parameters dance with Figs. 2 and 3, the problem of the fictitious conductor determination has been solved. When this conductor has been determined in each concrete case, it is possible to calculate the real value of the reduction factor.

Fig. 3. Cross section of the fictitious conductor.

IV. REAL VALUE OF THE REDUCTION FACTOR Based on the given equivalent circuit and according to Kirchoff’s laws, the following system of equations can be written:

(5)

Now, after determining the relevant parameters of the fictiand ), it is possible to take into account tious conductor ( the influence of the metal installations surrounding the feeding line to the value of the reduction factor. Then, the reduction factor according to (5) is given by

The impedances and are determined, according to, for example, [1] by the following expressions: (6) (7) In order to determine the analytical expressions for the imand , it is necessary to assume a form (shape) pedances of the cross section of the fictitious conductor. Bearing in mind that the surrounding installations exist on both (left and right) sides of the cable line, it is most rational to assume that the fictitious conductor has the cylindrical form with the cable line situated in the center of this imaginary cylinder, as shown in Fig. 3. Then, the mathematical forms of the analytical expressions and are simple and apriori known. for the impedances According to [1], they are given by (8) (9) where medium radius of the cylinder representing the fictitious conductor. The experimentally determined value of the reduction factor in the considered case is obtained based on the measured curin practical conrents and (the measurement of current ditions is not possible). Because of that, the measured reduction factor in the considered case is given by (10)

(12) Or in more compact form (13) Since the reduction factor obtained by measurements in the considered cases has the following values: for the shorter; • for the longer line. • The relevant data of the fictitious conductor, according to (11), are: km and m for the shorter; • km and 12.3 m for the longer line. • Using these data and (13), the following actual values of the reduction factor have been obtained. , or for shorter; • , or for a longer line. • Neglecting the surrounding metal installations, it has been obtained that the reduction factor in both of the considered cases is: , or . • Based on the given results, it is interesting to note that the real part of the complex number representing the actual value of the reduction factor is negative. It means that the angle between and is larger than 90 . Also, the phasors of the currents it is interesting that the effective value of the actual reduction factor is significantly lower (58.4% and 65.7%, respectively), in comparison with the result obtained without taking into account the influence of the surrounding metal installations. The presented analytical procedure for taking into account the influence of the surrounding metal installations is developed for a hypothetical case when the only one metallic sheath of the cable line is grounded at both line ends. However, this procedure

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Fig. 5. Single-core flat formation with spacing.

worst case from the standpoint of the safety conditions in the supplied station and the inductive influence of the line [1]). In the case presented in Fig. 5, the reduction factor according to [7] is determined by the expression

Fig. 4. Ground fault current distribution.

(15) corresponds to the case of the feeding lines performed by the three-core cable. It is quite realistic to assume that the given method can be used if an HV/MV distribution substation is supplied by an overhead transmission line. It is then necessary only to use the parameters for the overhead line ground wire instead of parameters for the cable sheath. In typical urban conditions, the favorable effect of the reduction of the fault current flowing through the earth could be comparable with those achieved by installing an additional copper wire into the earth along the feeding overhead line [14]. In the case of a line consisting of three single-core cables, three metallic sheaths are grounded at the line ends. Because of that, this type of cable line will be separately considered.

V. CASE OF THREE SINGLE-CORE CABLES In the case of a ground fault in substation B supplied by the line consisting of three single-core cables, the components of the fault current can be, according to Fig. 1, determined by the equivalent circuit shown in Fig. 4. According to the given circuit and using Kirchhoff’s laws, we can write the following system of equations: 1. 2. 3. 4. 5.

where represents the mutual impedance between two adjacent • and ) with a common earth return; cable sheaths ( represents the mutual impedance between two outer • with a common earth return. cable sheaths is determined according to, for The mutual impedance example [1], by the following relation: (16) is also determined by (16) only, acThe impedance cording to Fig. 5, a “ ” appears in it instead of “ .” This is the reason why “2” is used in the index of the symbol for this impedance. Analytical (15) can be easily modified for the triangular formation that is frequently used in practice. In this case, the cables are buried in a bundle and their positions are such that their centers are placed on top of an equal-sided triangle (i.e., each from the other two). cable is positioned at a distance is equal to the This means that in this case, the impedance . By introducing it into (15), we impedance obtain

(17) If we eliminate we obtain

and

from this relation using (2) and (15),

(14) (18)

This is a closed system of five equations. In order to solve the problem, it is necessary to determine the unknown quantities , , , , and . However, the whole procedure of calculation may be simplified by using the analytical expressions for the reduction factor of the line consisting of three single-core cables [7]. The most general case regarding the spatial disposition of the cables is when the cables are laid in a flat formation with spacing. This case is shown in Fig. 5. It is assumed at that the ground fault current is in the core of one of the other cables (the

Finally, after certain algebraic manipulations, the given expression assumes the more concise form (3). To simplify the solution of (14), the fact will be used that , , , and are mutually equal. the impedances It means that all of these cable sheaths can be substituted by only one equivalent sheath through which a current equal to circulates. In this way, the whole problem is reduced to the former case of the lines preformed by the three-core cable.


In the case of cables laid in a flat formation, the real and the imaginary part of the complex number representing the self impedance of the equivalent metallic sheath is, according to (15), determined by

(19) (20) When cables are laid in triangular formation, according to (3), instead of (19) and (20), the following expressions can be used: (21) (22) After determining the equivalent metallic sheath parameters and ), we can use the previously developed procedure ( (Part IV) to obtain the reduction factor for this type of cable line. Using the previously described method, we see that the reduction factor taking into account the influence of the surrounding metal installations in the considered cases is: , or for the shorter; • , or for the longer line. • It can be seen that its effective values are 65.9% and 72.6% lower than the value obtained without taking into account the . If we assume that surrounding metal installations the cables in the considered cases are laid in a flat formation and , we obtain: at a distance of , or for the shorter; • , or for the longer • line. The differences are still greater in comparison with the previously given results of measurements; the obtained values are lower in that case—78.0% and 84.2%, respectively. Obviously, disregarding the influence of the metal installations surrounding the feeding line as well as determining the reduction factor only by measuring currents through the cable sheaths give results that are excessively conservative. Bearing in mind that this also means the reduction in the same relationship for all potentials appearing on the grounding systems of the supplied substations, one can conclude the following. The results of this analysis throw a completely new light on the grounding problem of the supplied substations. Also, keeping in mind the similarity of urban conditions all around the world, this conclusion can be treated as generally valid for the safety conditions of the distribution substations supplied by cable line. At the design stage of an HV/MV distribution station, one can use the analytical expressions derived in [7] to determine the reduction factor of the feeding cable line. However, these expressions do not take into account the metal installations surrounding the feeding line. At the design stage, these can be included only numerically [8], [13]. However, even then, because of the mentioned numerous practical difficulties, one can do this only partially [13]. Because of that and according to the results presented in this paper, the safety estimation obtained at the design stage can be too severe. The safety conditions in the sup-

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plied station and in its vicinity can be correctly estimated only by using the results of the onsite measurements and the method presented here. The only problem is the fact that it cannot be done at the design stage. As a result, the following question arises: can the influence of the metal installations surrounding the feeding line be used by the designers? Certainly, the considered favorable effect is more significant in urban areas where the general conditions for solving grounding problems—short-circuit level and soil resistivity—as well as the number and type (with uninsulated, or with insulated metallic sheath) of outgoing MV cable lines are not favorable and where additional measurements are deemed to be necessary [9], [10]. It is a realistic assumption that the lines consisting of the same cables and belonging to the same urban area have similar values of the reduction factor. Because of that, it should be useful to determine the actual reduction factor of the existing feeding lines. These data could be the useful information to the designers of future HV/MV substations and their feeding lines. The main purpose of the developed method is correct testing of the safety conditions in the newly built HV/MV stations and in their vicinity. In other words, the presented method could serve as an addition to the standard site measurements which are obligatory before putting a newly built substation in operation and again after a certain period of time. This can be especially useful in critical cases when excessively conservative results of designer calculations and testing measurements show that the additional measurements are necessary to ensure that dangerous potentials are not transferred. Certainly, it does not mean that these measurements will not be necessary in some extremely unfavorable cases. Since the developed method is based on the measurement results, it could be used for the experimental verification of the methods based on computer modeling [8], [13]. However, this is possible only in some especially simple cases (from the standpoint of the surrounding metal installations) that could be encountered in suburban areas. For the application of the presented method, it is important to notify that the simulated ground fault current should circulate along the whole feeding line length. Since necessary measurements of the reduction factor are performed with a significantly lower ground fault current compared to the actual ones, the following should be said also. The results of several experimental measurements of the reduction factor performed by simulating a real ground fault in the 110-kV distribution network of Belgrade show good agreement with the results obtained by almost 100 times smaller simulated ground fault currents. VI. CONCLUSION The developed method is based on the results of site measurements and enables taking into account the favorable influence of the underground metal installations surrounding the feeding line to the ground fault current distribution in the HV/MV substations. The results of the experimental investigations and the application of the developed analytical method show that this influence essentially improves the safety conditions inside and in the vicinity of the distribution substations located in typical urban areas and supplied by a cable line. The method should serve as an addition to the standard testing measurements for newly built HV/MV substations when these measurements

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show that excessive potentials could be transferred to the LV distribution neutrals and customer installations.

REFERENCES [1] Lj. M. Popovic´, “Determination of the reduction factor for feeding cable lines consisting of three single-core cables,” IEEE Trans. Power Del., vol. 18, no. 3, pp. 736–743, Jul. 2003. [2] Lj. M. Popovic´, “Efficient reduction of fault current through the grounding grid of substation supplied by cable line,” IEEE Trans. Power Del., vol. 15, no. 2, pp. 722–727, Apr. 2000. [3] Short-Circuit Currents in Three-Phase a. c.Systems-Part 3:“Currents During Two Separate Simultaneous Line-to-Earth Short Circuits and Partial Short-Circuits Following Through Earth”, Int. Std., 2003, ref. CEI/IEC 60909-3, 2003. [4] Lj. M. Popovic´, “Comparative analysis of grounding systems formed by feeders in one case with uninsulated and in the other case with insulated metallic sheath,” presented at the 18th Int. Conf. Electricity Distribution, Turin, Italy, Jun. 2005. [5] J. R. Carson, “Ground return impedance: Underground wire with earth return,” Bell Syst. Tech. J., vol. 8, pp. 94–98, 1929. [6] R. Rudenberg, “Fundamental consideration of grounding currents,” Elect. Eng., 1945. [7] Lj. M. Popovic´, “Improved analytical expressions for the determination of the reduction factor of the feeding line consisting of tree single-core cables,” Eur. Trans. Electr. Power, Mar. 12, 2007, 10,1002/etep. 172. [8] S. T. Sobral, V. S. Costa, M. S. Campas, and D. Mukhedkar, “Dimensioning of nearby substations interconnected grounding systems,” IEEE Trans. Power Del., vol. 3, no. 4, pp. 1605–1614, Oct. 1988.

[9] J. Wilas, D. Mukhedkar, V. Fernandes, and A. Magalhaes, “Grounding grid design of a transition station–A typical example of fault transfer,” IEEE Trans. Power Del., vol. 5, no. 1, pp. 124–129, Jan. 1989. [10] J. Fortin, H. G. Sarmieto, and D. Mukhedkar, “Field measurements of ground fault current distribution at LG- 2, Quebec,” IEEE Trans. Power Del., vol. PWRD-1, no. 3, pp. 48–60, Jul. 1986. [11] P. R. Pillai and E. P. Dick, “A review on testing and evaluating substation grounding systems,” IEEE Trans. Power Del., vol. 7, no. 1, pp. 53–61, Jan. 1992. [12] L. M. Popovic, “Practical method for the analysis of earthing systems with long external electrodes,” Proc. Inst. Elect. Eng., vol. 140, no. 3, pp. 213–220, May 1993. [13] J. M. Nahman, V. B. Dordevic, and D. D. Salamon, “Grounding effects of HV and NV underground cables associated with urban distribution substations,” IEEE Trans. Power Del., vol. 17, no. 1, pp. 111–116, Jan. 2002. [14] Lj. M. Popovic´, “Efficient reduction of fault current through the grounding grid of a substation supplied by an overhead line,” Eur. Trans. Electr. Power, vol. 16, no. 3, pp. 247–259, Mar./Apr. 2006. Ljubivoje M. Popovic´ (SM’91) was born in Markovac, Serbia, Yugoslavia, on February 24, 1944. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from the University of Belgrade, Yugoslavia, in 1969, 1983, and 1991, respectively. He has worked on the design of different power system installations with Elektrodistribucija Beograd, Belgrade. For the last 27 years, he has been a leading Research Engineer in the field of grounding problems and short-circuit current.