The International Conference on Electrical Engineering 20091 Fault Current Distribution in Transmission and Distribution Systems with Shield Wires, Neutral Wires and Cable Sheaths J. Liu, F. P. Dawalibi Senior Member, IEEE, J. Ma, Senior Member, IEEE, and S. Fortin, Member, IEEE shield wires, neutral wires, cable sheaths, electric line structure grounds, and substation or power plant grounding systems, is an important factor that affects the design of the grounding system, the induced electromagnetic interference levels along the utility lines, and mitigation measures, if necessary. Several studies of fault current distribution for various situations have been analyzed in recent years [1-2]. The fault current distribution is difficult to determine accurately since it is not easy to account for the large number of variables such as complex electric line networks and grounding systems. Detailed computer methods are usually used to obtain accurate results. IEEE STD 80-2000 [3] suggests a graphical method for determining the maximum fault current injected into the grounding system. The method attempts to correlate the substation zero sequence fault current obtained from a standard short circuit study to the actual current flowing between the grounding systems and surrounding earth. This provides a quick and simple method to estimate the current division, avoid complex computations and give acceptable results for a few simple examples compared with the computer method. However, it is an approximate method that focuses on a few typical line configurations that may not apply in many cases. This paper presents a parametric analysis in which the fault current distribution among various paths is accurately computed during single phase-to-ground fault conditions. The effects of the following variables on the fault current distribution are studied: cross section of power lines, length of power lines, tower footing resistance, grounding system resistance, type of static wires, and soil resistivity. Moreover, this paper presents the results of a recent electromagnetic interference study in a shared corridor occupied by 345 kV transmission lines, railroad tracks and gas pipelines which are parallel to the electric lines along the corridor that consists of a 37 km long underground 345 kV power cable network and a 74 km long overhead transmission line network. The fault current distribution and electromagnetic interference levels are studied and evaluated for the overhead lines and underground power cables respectively. Furthermore, a measure to effectively reduce interference to acceptable levels is presented and discussed in detail. Abstract-- The fault current distribution among various paths, such as shield wires, neutral wires, cable sheaths, electric line structure grounds, and substation or power plant grounding systems, is computed and analyzed for several scenarios. A parametric analysis of the effects of several key variables on the fault current distribution is carried out. The variables include: cross section of power lines, length of power lines, tower footing resistance, grounding system resistance, type of static wires, and soil resistivity. In this paper, the fault current distribution and electromagnetic interference levels have been studied and evaluated for a typical electrical network with overhead and underground cable lines. Furthermore, a measure has been provided to effectively reduce interference to acceptable levels. Index Terms—Electromagnetic interference, fault currents, grounding, power cables, power distribution lines, transmission lines I. INTRODUCTION LECTRICAL engineers are often faced with the necessity of having to design grounding systems and electromagnetic interference mitigation measures to meet specific criteria concerning personnel safety or integrity of equipment. In grounding design, during single phase-to-ground fault conditions, a portion of the fault current will discharge into the soil through the grounding network and cause a potential rise in the grounding system and induce voltages in neighboring conductors. A high potential can result in dangerous step and touch voltages and transferred potentials at remote locations as well. On the other hand, when high voltage transmission and distribution lines share a common corridor with other utility lines such as pipelines and rail tracks, electromagnetic interference caused by the power lines on the non-energized utility lines during single phase-to-ground faults occurring at any location along the entire power lines is a serious concern because it can result in electric shocks and can threaten the integrity of the neighboring utility lines. The fault current distribution among various paths, such as This work was supported by Safe Engineering Services & technologies ltd. The authors are with Safe Engineering Services & technologies ltd, 3055 Blvd. Des Oiseaux, Laval, Quebec, Canada H7L 6E8 (e-mail:
[email protected]). E a generating station feeds a substation located at the other end. and capacitive interference. 2. 1 shows the parameter settings that are considered in the computer models for this study. conductive. Transmission Line Cross Section Different transmission line cross sections are shown in Fig. Fig. a number of typical cases have been selected and presented here in detail. 4. the ratio of the fault current injected into the substation grounding system to the total fault current is 34. 2. This shows that the impact on the fault current distribution of the transmission line length is small when the length exceeds 10 spans. The circuit approach used in this paper accounts for all the key elements necessary to calculate the fault current distribution accurately during normal conditions. COMPUTER MODEL Fig. III. 4. i. However. Length of T/L The computation results for transmission lines of different lengths are shown in Fig. The cross section of the line is shown in Fig. and fault conditions. B. All simulations have been carried out by using the CDEGS® software package [4]. 3. COMPUTATION METHOD A circuit approach and a frequency domain method are used to carry out the fault current distribution and electromagnetic interference computations through accurate simulation of the electric network.1 ohm. Fault Current Split Factors 40 35 Fault Current Split Factor (%) 30 25 20 15 10 5 0 0 5 10 15 20 25 30 35 Length of the Transmission Line (km) 40 45 50 The static wires are Alumoweld 19 No. COMPUTER RESULTS A. The substation grounding resistance is 1 ohm and the generating station grounding resistance is assumed to be 0. The circuit and frequency domain methods together take induction effects fully into account and the computation results contain the combined effects of the inductive. 7 395. We assume that a 10 kA single phase-to-ground fault occurs at the substation. imbalance conditions. Fault current distribution for different T/L lengths The reference case consists of a 10 km long 230 kV transmission line with a span length of 250 m. The tower footing resistance is 15 ohms.5 km. Overhead T/L cross section for the reference case The phase conductors are 2156 kcmil ACSR (bluebird).2 II. from which many computer simulations of representative transmission and distribution lines are created by varying one parameter at a time.e. Little change in the fault current split factor is observed when the length of the transmission line is greater than 2. IV. Structure of the parametric fault current distribution study Fig. Fig. A uniform soil with a 100 ohm-m resistivity is used in the model. Different transmission line cross sections . The computed fault current split factor. It is not possible to present all the results obtained for each analyzed case.6 kcmil. Fig. 1.9% for this reference case. This study is based on the reference computer model. At one end of the line. 3. 6. Fault current distribution for different soil resistivies Fault Current Split Factors 60 Fig. Fault Current Split Factors 40 mutual impedance of the phase conductor and the static wire. 7. This can be easily explained by the fact that less current will come back through the path consisting of the static wires and tower footings when the tower resistance is high. E. When the faulted phase wire is close to the static wires. 8. 5. Soil Resistivitiy Four soil resistivities. It has been found that the fault current distribution is slightly influenced by the soil resistivity. The results indicate that the fault current split factor is increasing with the increase of the tower resistance. Tower Resistance The computation results for different tower resistances along the transmission line are shown in Fig. As a result. As expected. Most of the fault current will return to the source through the static wires once the faulted substation ground resistance reaches a high value (on the order of 5 ohms in this case). Fault current distribution for different types of static wires D. Fault current distribution for different faulted substation resistances . The fault current has a tendency to go back to the source through the static wires when the soil resistivity is high and therefore less current is injected into the grounding grid. F. the mutual coupling between the phase and static wires is stronger. the main factor governing the fault current distribution is always the geometry distance between the faulted phase wire and the static wires rather than the overall configuration of the transmission lines. 9. the materials of the static wires can significantly affect the fault current split factor.3 It can be seen from the computation results shown in Fig. more current flows along the static wires back to its source and less current is injected into the grounding grid. 10. 9 shows what happens when the faulted substation ground resistance changes. less current is discharged by the grounding system at the faulted substation. Type of Static (Ground) Wire The computation results for different types of static wires are shown in Fig. Clearly. 7. However. As we can see. The fault current distribution has been studied as shown in Fig. 1000 and 10000 ohm-m are used to compute the self impedance of the static wire and the Fault Current Split Factor (%) 50 40 30 20 10 0 0 5 10 15 Substation Ground Resistance (ohms) 20 Fig. the fault current split factor decreases when the faulted substation ground resistance is high. 100. Fault current distribution for different transmission line cross sections C. As a result. 5 that some transmission line configurations produce higher fault current split factors than other configurations. 6. More current returns to its source via the static wires when the static wires are more conductive. Substation Ground Resistance Fig. Fault Current Split Factors 60 Fig. this effect will be small when the tower resistance is about 20 ohms or more. Fault current distribution for different tower resistances Fault Current Split Factors 40 35 Fault Current Split Factor (%) 30 25 20 15 10 5 0 Fault Current Split Factor (%) 50 40 30 20 10 Steel Aluminum Copper 1 10 100 Soil Resistivity (ohm-m) 1000 10000 Type of Static Wire Fig. 8. Fault Current Split Factors 50 Fault Current Split Factor (%) 30 20 45 Fault Current Split Factor (%) 40 35 30 25 20 15 10 5 0 0 20 40 60 Tower Resistance (ohms) 80 100 10 Horizontal (Fault is on Phase A) Wide Horizontal (Fault is on Phase A) Vertical (Fault is on Phase A) Vertical (Fault is on Phase C) Transmission Line Cross Section Fig. Fig.8 cm. inner diameter = 9. 10 shows the computation results with respect to different generating station ground resistances. 10.4 G. When the tower resistance is small. V. A. and outer diameter: = 10. every 600 m). A gas pipeline and a railroad share the same corridor with the 345-kV overhead transmission line for about 6.. This is due to the fact that the mutual coupling between the sheath and cable core is very strong because of the small distance between the two conductors. The horizontal and mixed configurations are shown in Fig. H. The underground cables are divided into sections (from one vault location to another). It is interesting to observe that the fault current split factor varies by following different patterns when the generating station ground resistances have different values. On the other hand. The ground resistance is 15 ohms. Two ground rods are installed per cable vault (i. In the case of an underground cable. The complexity of the relationship between the fault current split factor and the generating station ground resistance have been discovered. the fault current split factor will gradually decline with the increase of the generating station ground resistance when the tower resistance is about 100 ohms or more. The computed fault current split factors are 1. The cable is a XLPE insulated cable and the characteristics used to perform the calculation are as follows: • Cable core: 3500 kcmil copper conductor • Cable overall diameter: 12. 12 shows the plan view of the electrical network under study.5 cm • Lead sheath equivalent relative resistivity (with respect to copper) and permeability(with respect to free space): 12 and 1 Two ground wires are installed with the underground cables. The total length of the underground cables is 37 km and a railroad roughly follows the underground cables for about 21 km. The overhead portion of the 345-kV transmission lines between Substation #3 and Substation #7 is approximately 74 km. Fault Current Split Factors 55 50 Fault Current Split Factor (%) 45 40 35 30 25 20 0 1 2 3 4 5 6 7 Generating Station Ground Resistance (ohms) 8 9 10 Tower Resistance-5 ohms Tower Resistance-15 ohms Tower Resistance-50 ohms Tower Resistance-100 ohms Fig. TYPICAL AC INTERFERENCE STUDY A typical AC interference study that involves an electric network consisting of overhead lines and underground cables is presented in this section. Generating Station Ground Resistance Fig. The equivalent relative resistivity and the relative permeability of the sheaths were derived based on typical values. the fault current split factor will remain practically constant or increase slightly.e. Little change is observed when the overall configurations are different. as expected. Plan view of the electrical network under study Fig. the mutual coupling plays a very important role in the current distribution. Underground Power Cable Configuration Two typical cables have been modeled to demonstrate the fault current distribution.8 cm • Copper core diameter: 5. The underground cables are modeled as a stranded conductor core surrounded by a lead sheath.2 cm The underground power cables between Substation #1 and Substation #3 consist of two 345-kV circuits utilizing XLPE insulated cables. The 345-kV transmission line can cause AC interference to . 11.6% and 2.4 km. The radius of the ground wire is 0. respectively. Network Description and Computer Model Fig. 12. 11.2% for the horizontal and vertical configurations. Fault current distribution for different generating station ground resistances • Lead sheath: extruded lead. Different configurations of the underground cable The length of the underground cables is 3 km.4 cm. respectively. track arresters. The low touch voltage is caused by the proximity of the lead sheath to the underground cable core that results in a very strong mutual coupling between them. and railroad equipment house grounds. The complete model consists of the transmission line phase conductors. the touch voltages when a person contacts a rail track or a pipeline when Therefore. the induced EMF on the rail is small due to the cancellation effect of the core current by the sheath current. insulating joints.5 m • The pipeline has a radius of 30 cm and is covered with coal tar coating Figs. 15. In contrast to the underground cable situation. Rail touch voltages along the underground cable Fig. Both of them are above the IEEE Standard 80 safe limit based on various soils along the entire length of the railway and pipeline. a large amount of the fault current is injected into the tower grounds along the commoncorridor due to the high fault current split factor in the case of an overhead line with two Aluminum 7#8 static wires. In this paper. . 16. 15. static wires. Cross section of the underground cable conduit The worst rail and pipe touch voltages under fault conditions for the overhead portion are 2244 V and 1010 V. underground cables. as shown in Fig. Cross section of overhead transmission lines Fig. 13 and 14 show the typical cross sections of the overhead lines and underground cables. the induced voltages and currents on the railway and pipeline facilities can result in a shock hazard and threaten the integrity of both facilities during single phase-to-ground fault conditions.5 the railway system and pipelines. 14. Rail Touch Voltages 700 Rail Touch Voltages 600 Touch Voltage Limit 500 Touch Voltages (V) 400 300 200 100 0 0 5000 10000 15000 20000 25000 Distance along Railway in Parallel with the Underground Cable (m) Fig. Various soil structures have been modeled along the entire common-corridor. Maximum Rail Touch Voltage s 2500 Rail Touch Voltages Touch Voltage Limit 2000 Touch Voltages (V) 1500 1000 500 0 0 1000 2000 3000 4000 5000 6000 7000 Distance along Railway in Parallel with the Overhead T/L (m) Fig.2 kA per phase for the existing 345 kV line • A 130 lb steel rail was selected and the separation distance between rails is 1. standing 1 m away on both sides of the rail track or pipeline are calculated to ensure that safety concerns are addressed. As a result. Especially. the electromagnetic interference under fault conditions has been analyzed and presented. respectively. As a result. rail track circuits.5 kA per phase for the new 345 kV line and 1. The computation results for the touch voltages along the rail are shown in Fig. mitigation measures are required on the railway and pipelines facilities which parallel the overhead transmission lines. The data and characteristics used in the simulations are as follows: • The overhead line consists of the ACSR phase wires and Aluminum 7#8 static wires • The underground cable characteristics are the same as in the previous section • The maximum load current is 1. The worst rail touch voltage under fault conditions for the underground cable portion is 94 V only which is below the IEEE Standard 80 safe limit of 519 V based on a 800 ohm-m native soil. most of the fault current is going back to the remote source through the sheath. rail ballast resistances. Computation Results The computation results are summarized here. 16. rails. B. 13. In this paper. Rail touch voltages along the overhead transmission lines The touch voltage is one of the important quantities that should be computed and examined. The induced EMF caused by the ground conductors paralleling the cable sheath is also small because little current is flowing into the ground conductors. the continuous mitigation wires provide additional grounding for the railway and pipeline and furthermore. Ma has authored and coauthored more than one hundred papers on transient electromagnetic scattering. as Manager of Technical Services and was involved in power system design. BIOGRAPHIES Ms. P. Dr. He received the Ph. and the M. China in December 1956. vol. degrees from Ecole Polytechnique of the University of Montreal. in 1982 and 1984. China. where he is presently serving as manager of the Analytical R & D Department. P. CONCLUSIONS A parametric analysis of the fault current distribution along electric power transmission network has been conducted. Dr.Sc. a company specializing in soil effects on power networks. From 1984 to 1986. he has been with the R & D Dept. She is the author of more than 30 papers on electrical power system safety. J. Montreal. His research interests are in transient electromagnetic scattering. Liu received the B. VII. pp. grounding and electromagnetic fields compatibility related problems. thus decreasing the induced pipe and rail potential rise. modeling. Winnipeg. the computed rail and pipe touch voltages have been reduced to less than 250 V and 300 V. and soil resistivity. he worked as a consulting engineer with the Shawinigan Engineering Company. Daily and F. Tee. a zinc mitigation wire along the exposed pipeline located about 1 m away from the edge of the pipeline is installed. degree in Electrical Engineering in 1985 and 1990. Two continuous mitigation wires bonded to the railway through track arresters at regular intervals and buried 1 m away from the edge of the railway are installed along the exposed railway zone on both sides of the railway. P. tower footing resistance. lightning and electromagnetic interference analysis. EMI and EMC. R. EMC. Power Delivery. which satisfy the safety design limits. F. as a research scientist in 1994. 17. he was a faculty member with the Department of Electrical Engineering. control. and Ph. railway electrification studies and specialized computer software code development. In 1992-1993. K. he was a research assistant with the Nuclear Physics Department at the University of Montreal. 300 200 100 0 0 1000 2000 3000 4000 5000 6000 7000 Distance along Railway in Parallel with the Overhead T/L (m) Fig.. Canada in 1991. in Montreal. a typical study of electromagnetic interference on railways and pipelines caused by electrical overhead transmission lines and underground cables under fault conditions has been carried out as an example. Simon Fortin has published about 40 papers on lighting. He is a senior member of the IEEE Power Engineering Society. IEEE Standard 802000. analysis and design of reflector antennas. respectively. F. Similarly. no.Sc. a manufacturer of high voltage equipment in Montreal. He has written several research reports for CEA and EPRI. July 10 . This measure reduces interference levels considerably. 97103. Sc. Furthermore. respectively. power quality. Effects of the various parameters which influence the fault current split factor have been studied. As a result. P. and J. Jinxi Ma (M'91. Dawalibi. In case of a fault. 8. Dr. In 1979. SM'00) was born in Shandong. Since then he has been responsible for the engineering activities of the company including the development of computer software related to power system applications. length of power lines. Eng. VIII. act as screening conductors. Fortin was born in 1962. degree from Beijing University of Aeronautics and Astronautics. S. He worked on numerous projects involving power system analysis and design. and the M. In 1976. inductive interference and electromagnetic field analysis. degree in electrical and computer engineering from the University of Manitoba. 2006. He received a B. P. respectively. W. and analysis of grounding systems in various soil structures. This real case study shows how modern computational approaches can be used to analyze complex electromagnetic interference issues and produce accurate results. as can be seen in Fig. Liu.14. types of static wires. Maximum Rail Touch Voltage s with Mitigation Wires 500 Rail Touch Voltages Touch Voltage Lim it 400 Touch Voltages (V) [2] [3] [4] casing enclosure. He is the author of more than 200 papers on power system grounding. a member of the IEEE Standards Association. He received the B. "Influence of current distribution in enclosed underground power cables on the overheating of the steel . China. He worked on projects involving design and analysis of reflector antennas and calculations of radar cross sections of aircraft. station grounding system resistance. observations. equipment selection and testing for systems ranging from a few to several hundred kV. He received a Bachelor of Engineering degree from St. and the M.6 C. Canada. Kunming. and management. Her research interests are electrical grounding systems. power system grounding. he founded Safe Engineering Services & technologies. D. 17 for the rail touch voltages. R. of Safe Engineering Services & technologies in Montreal. affiliated with the University of Lyon. Rail touch voltages along the overhead transmission lines with mitigation wires VI. Dawalibi (M'72." IEEE Trans. SM'82) was born in Lebanon in November 1947.Eng. His area of specialization was in the theoretical aspects of high-energy particle physics. Mitigation Measure A mitigation measure consisting of three continuous mitigation wires along the railway and pipeline is presented. Vancouver. Safe Engineering Services & technologies ltd. degree (1991) in Physics from the University of British Columbia. Since September 1990. IEEE Guide for Safety in AC Substation Grounding. Quebec and a Ph. CDEGS Software Package. EMC. degree (1985) in Physics from Laval University. and computation results offer useful clues to better understanding of the fault current distribution. both in electrical engineering. he joined Montel-Sprecher & Schuh. From 1971 to 1976. Dawalibi. The parameters include power line cross section. Beijing University of Aeronautics and Astronautics. Quebec. She is presently serving as scientific researcher at Safe Engineering Services & technologies ltd. and various aspects of electrical power system analysis.Sc." The International Conference on Electrical Engineering (ICEE). and a corresponding member of the IEEE Substations Committee and is active on Working Groups D7 and D9 Dr. 2005. Joseph's University. His research interests include the computation of electromagnetic fields at various frequencies and transient phenomena. Dr. degree from Shandong University. and computer applications. The mitigation wire is connected to the pipeline through PCR (Polarization Cell Replacement) or solid-state decoupler devices. lightning.D. "Cost reduction and minimization of land based on an accurate determination of fault current distribution in neutral conductors. He joined Safe Engineering Services & Technologies Ltd. again specializing in particle physics. All charts. 1. REFERENCES [1] S. January 1993.D.