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Practical Considerations for Fault Current Split Computation in Power System Grounding

Jinxi Ma, Senior Member, IEEE, Xiao Wei Zhao, and Farid P. Dawalibi, Senior Member, IEEE

Abstract—Practical considerations for fault current split calculations are presented. Scenarios involving various transformer types such as delta-star connected, star-star connected, auto-transformer with a delta tertiary, and zigzag grounding transformers are analyzed. It is shown that the fault current going into the grounding system can be overestimated or underestimated if the actual situation is not modeled correctly. Proper examination of various fault current paths involving different transformer types is also presented. The examples given in this paper can be used as a reference by power engineers to carry out fault current distribution calculations accurately and therefore avoiding inappropriate designs in grounding studies.

Index Terms--Fault current, current split, earth current, grounding

I. INTRODUCTION Accurate calculations of the fault current distribution are

one of the most important tasks when carrying out grounding analyses of electric substations and power plants [1]. The purpose of the fault current distribution calculation in grounding analysis is to determine the earth current, i.e., the current discharged to earth by the grounding system at the faulted substation or power plant. Fault current distribution calculations are also called fault current split calculations, since they determine how the current splits between the earth and the overhead ground wires (OHGW) and other return metallic conductors. When determining the earth current, it is important to identify the local and remote fault current contributions. The local fault current contribution is the fault current from transformer banks at the faulted substation. The remote fault current contribution is the fault current from the lines connected to other substations or power plants. Some mistakes are often made when identifying and modeling the local and remote fault current contributions. For example, a local fault current contribution is systematically assumed to be a circulating current problem, since the source location and the fault location are both in the same substation. However, depending on the transformer type, there may be a remote fault current contribution related to the fault current contribution from the local transformer. Not taking into account this remote fault current contribution may result in an optimistic grounding design.

_______________________ This work has been supported by Safe Engineering Services &

technologies ltd, Laval, Canada. Xiao Wei Zhao is with the Department of Medical Image, Shandong

Medical College, 5460 South Erhuan Road, Jinan, Shandong, P. R. China (e-mail: zxwrui@126.com).

Jinxi Ma and Farid P. Dawalibi are with Safe Engineering Services & technologies ltd, 3055 Blvd. Des Oiseaux, Laval, Québec, Canada, H7L 6E8 (e-mail: info@sestech.com).

Another common mistake is the use of the total fault current at the fault location for a grounding study. The common perception is that this is a conservative approach. While this is often the case, there can be exceptions. As will be demonstrated in this paper, in some cases, the remote fault current contribution can actually be larger than the total fault current at the fault location.

There are many publications describing methods for fault current split calculations [2-5]. In this paper, the main objective is to present practical considerations for fault current split calculations used in grounding studies rather than focusing on the computation methods. Scenarios involving various transformer types such as delta-star connected, star-star-connected, auto-transformer with a delta tertiary, and zigzag grounding transformer are analyzed. The cases analyzed demonstrate that the fault current going into the grounding system can be overestimated or underestimated if the actual situation is not modeled correctly. Proper examination of various fault current paths involving different transformer types is also presented. The examples given in this paper can help power engineers to carry out fault current distribution calculations accurately, thereby resulting in appropriate grounding designs.

II. ∆-Y TRANSFORMER Fig. 1 shows a one-line diagram of an actual power

network, with fault current values for a 66 kV single-line-to-ground fault. The high voltage side of the 66 kV / 4 kV transformer is delta-connected and the low side star-connected. Naturally, there is no fault current contribution (3I0) from the transformer banks in this case. The figure also shows that there is no current contribution from the low voltage lines. As a result, modeling only the fault currents from the 66 kV lines is appropriate.

Fig. 2 is similar to Fig. 1 but for a 4 kV single-line-to-ground fault. In this case, there is no remote fault current contribution (3I0) from the 66 kV lines although phase currents of the 66 kV lines are not zero. The currents on Phase A on the 66 kV lines are also shown in Fig. 2. The huge fault current (52665 A) is all from local sources – the transformer banks. It is simply a circulating current problem. To demonstrate the concept of circulating current, Fig. 3 shows a simple model for a fault on the Y-side of a ∆-Y transformer together with a grounding grid. It can be seen that the total fault current (3 I0 ) enters the grounding grid at the fault location and then returns to the transformer neutral point via the ground conductors. Because of the low impedance path provided by the ground conductors, only a small amount of this current is leaking out from the ground conductors to earth, especially for a small grounding grid. As a result, the grid

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The International Conference on Electrical Engineering 2009

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Fig. 1 One-line diagram of a system with ∆-Y transformer banks: fault on the ∆-side (66kV) of a transformer bank.

Fig. 2 One-line diagram of a system with ∆-Y transformer banks: fault on the Y-side (4 kV) of a transformer bank.

Fig. 3 Circulating current for fault on the Y-side of a ∆-Y transformer.

Fig. 4 Current path of a line fault.

GPR (Ground Potential Rise) and the touch and step voltages will be small. For a large substation or power plant, the distance between the fault location and the transformer neutral point can be quite large. As a result, high potential differences due to this large circulating current may exist within the grounding grid.

A question often arises regarding the fault location yielding the worst case (in the sense of having the largest earth current): is it on the high voltage side or on the low voltage side? As discussed above, remote fault contributions usually result in more earth current than local fault contributions. Obviously, in the above example, the 66 kV bus fault in the substation is the worst case because all the fault current contributions are from remote sources. While the total fault current for a fault on the 4 kV bus is much larger, it is from local sources. It should be pointed out that for a fault on the low voltage side, a line fault should be considered to determine the worst case because the local fault current is very large. A line fault is a fault along a power line outside the substation. In this case, a phase conductor is shorted to a neutral conductor outside the substation. The resulting fault current flows into the tower footings and the neutral conductor and then returns to the transformer banks in the substations, as shown in Fig. 4. For such a case, the current flowing into the grounding grid from the earth can be large, potentially larger than for a fault on the high voltage side.

Step-up transformers in a power plant are mostly ∆-Y transformers with the low voltage side ∆-connected. When a fault occurs at the high voltage switchyard adjacent to the power plant, the fault currents from the step-up transformers are local contributions, while the fault currents from the high voltage lines are remote contributions. Fig. 5 shows a single-line diagram depicting the short-circuit currents of a system in a power plant for a fault on the high voltage side. In order to see the fault current on each phase more clearly, Fig. 6 shows the three phase circuit together with the main power transformer with the fault current flowing in each phase identified. It can be seen that the total fault current from both the remote source (SS Substation) and the local source (Main Power Transformer) is 21.90<-85.20 kA, out of which 9.88<-92.20 kA is flowing into the neutral of the transformer. This current, 9.88<-92.20 kA, is the circulating current between the fault location and the transformer neutral location and should be modeled when evaluating the grounding system

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Fig. 5 One-line diagram of the system.

Fig. 6 Illustration of circulating current and remote current.

performance. It is interesting to see that part of this current, 5.16<-90.60 kA, is flowing via Phase A from the transformer to the fault location, while the rest of this current, 2.37<-95.40 kA + 2.36<-92.40 kA, is flowing via Phases B and C from the transformer to the remote source and then back to the fault location via Phase A. But for grounding analysis which is only concerned about how currents entering and leaving the grounding system, the amount of the circulating current is simply the 3I0 from the transformer side, which is 9.88<-92.20 kA.

III. AUTO-TRANSFORMER WITH A DELTA TERTIARY Fig. 7 shows a one-line diagram of a power system with a

500 kV / 230 kV transformer bank. The zero sequence short-circuit currents (3I0) for a 500 kV bus fault are also shown in the figure. Usually, fault current data provided for grounding studies are for the lines directly connected to the faulted bus, i.e., the 500 kV lines in this case. This is due to the incorrect

perception that only lines connected directly to the faulted bus are relevant. Fig. 7 indicates that the total fault current for a 500 kV bus fault is 11808 A, with 4974 A from remote sources and 6857 A from the local transformer bank. The way the fault current contribution from the transformer bank is treated has a significant effect on the final grounding analysis. A common approach is to treat this current as a local contribution. This is based on the thinking that since the transformer bank is local, the fault current contribution must be local. Another approach is to take this current as a remote contribution and apply a similar split factor as for the other transmission lines. This approach is often regarded as conservative. In fact, both approaches are inappropriate: the first approach can be too optimistic and even the second one may not necessarily be conservative. In Fig. 7 the 230 kV zero sequence currents are also shown. The total remote fault current contribution from all the 230 kV lines is 8808 A. It is this current that has to be considered in the fault current distribution calculations because this current will ultimately return to its sources through the earth and the overhead ground wires. Since the remote fault contribution from the 500 kV lines is 4974 A, the total fault current used for the grounding study should be 13782 A. It is surprising to see that this current is actually greater than the total fault current for a 500 kV bus fault at the fault location. Now we can see that when the 6857 A from the transformer bank is taken as a local fault current contribution and the remote contributions from the 230 kV lines are ignored, the grounding analysis will be based on a much smaller fault current than the actual value. Even when the 6857 A from the transformer bank is taken as a remote fault current contribution, the grounding analysis is still based on a smaller current because the 8808 A remote fault current contribution from the 230 kV lines is larger than 6857 A from the transformer bank.

Fig. 8 is similar to Fig. 7 but for a 230 kV bus fault. It can be seen that the remote fault current contribution from the 230 kV lines is 13044 A and the fault current contribution from the transformer bank is 16076 A. In the fault current distribution calculation, the remote fault current contribution from the 500 kV lines must be included. In this case, if the fault current contribution from the transformer bank is modeled as a remote contribution, the grounding analysis based on the fault current distribution calculation will be very conservative, resulting in over design. If it is treated as a local contribution and the 500 kV line currents are omitted, the grounding analysis will be optimistic.

As mentioned before, one question that often arises in grounding analysis is to determine the worst case fault: on the high voltage side or on the low voltage side. Unfortunately there is no simple answer. Take the present case as an example. The low voltage side fault (230 kV) is probably the worst case because the total remote fault current is 15448 A compared to 13768 A for a 500 kV bus fault. In general, however, fault current split calculations should be carried out for faults on both voltage levels to determine the maximum earth current, which corresponds to the worst case for grounding analysis.

IV. Y-Y TRANSFORMER Y-Y transformers are similar to auto-transformers with a

delta tertiary as far as fault current split calculations for

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Fig. 7 One-line diagram of a system including a 500 kV / 230 kV auto-transformer with a delta tertiary: fault on the 500 kV bus.

Fig. 8 One-line diagram of a system including a 500 kV / 230 kV auto-transformer with a delta tertiary: fault on the 230 kV bus.

grounding analysis are concerned. When a fault occurs on one side of a Y-Y transformer, there is always a fault current contribution (3I0) from the other side of the transformer. This is different than for a ∆-Y transformer. Fig. 9 shows a Y-Y transformer when the fault is on the high voltage side. In this case, the zero sequence current from the source feeding the low voltage side is not zero. Because the fault is on the high voltage side, we have . We can see that is actually a remote contribution which must return to its source through the earth and the overhead ground wires. Clearly, considering

as a circulating current and ignoring will be an incorrect approach. Again we can see that the total remote fault current contribution, , is actually larger than the total fault current at the fault location, . As shown in Fig. 9, the remote fault current contribution from the low voltage line, , actually splits into two components: and

. Component flows to the neutral point of the high voltage windings and eventually enters the grounding system at the fault location while component enters the grounding system at the transformer neutral connection point to the grounding system. In this situation, there is no circulating current.

Lp II 00 33 > pI 03

LI 03 pI 03

Rp II 00 33 +RL II 00 33 +

pI 03 LI 03Lp II 00 33 − LI 03

Lp II 00 33 −

If the fault indicated in Fig. 9 is a low voltage side fault (i.e., the windings on the right side is the low voltage windings), then . As shown in Fig. 10, the fault current contribution from the high voltage lines, , is part of the fault current contribution from the transformer. In other

LP II 00 33 <pI 03

Fig. 9 Fault on the high voltage side of a Y-Y transformer.

Fig. 10 Fault on the low voltage side of a Y-Y transformer.

words, the fault current contribution from the transformer, , consists of two parts: the remote fault current

contribution from the high voltage lines, , and a circulating current which is .

LI 03pI 03

PL II 00 33 −

V. GROUNDING TRANSFORMERS Grounding transformers such as zigzag transformers are

normally used together with an ungrounded delta connected winding of a transformer to provide a path for current to flow into earth under fault conditions. Under steady-state conditions, the transformer impedance to balanced three-phase voltages is high so that only a small magnetizing current flows in the transformer winding. The transformer impedance to zero-sequence voltages, however, is low so that it allows high ground currents to flow under fault conditions. Fig. 11 shows a zigzag grounding transformer connected to a delta winding of a transformer. Clearly, without the grounding transformer, the fault current contribution from the delta winding of the transformer will be zero. With the grounding transformer, current enters the ground at the fault location and then returns to the neutral point of the grounding transformer. This is very similar to the case of a star connected winding of a transformer. If a circulating current is generated by a delta winding of a transformer connected to a grounding transformer, the grounding transformer should be considered as the source of this circulating current in the grounding analysis.

5[5] G. Yu, J. Ma, and F. P. Dawalibi, “Computation of return current

through neutral wires in grounding system analysis,” in Proc. of the Third IASTED International Conference on Power and Energy System, Las Vegas, Nevada, pp. 455-459, Nov. 8-10, 1999.

VIII. BIOGRAPHIES Dr. Jinxi Ma (M'91, SM'00) was born in Shandong, P. R. China. He received the B.Sc. degree from Shandong University, P. R. China, and the M.Sc. degree from Beijing University of Aeronautics and Astronautics, both in electrical engineering, in 1982 and 1984, respectively. He received the Ph.D. degree in electrical and computer engineering from the University of Manitoba, Winnipeg, Canada in 1991.

From 1984 to 1986, he was a faculty member with the Department of Electrical Engineering, Beijing University of Aeronautics and Astronautics. He worked on projects involving design and analysis of reflector antennas and calculations of radar cross sections of aircraft. Since September 1990, he has been with the R & D Dept. of Safe Engineering Services & technologies, where he is presently serving as manager of the Analytical R & D Department. His research interests are in transient electromagnetic scattering, EMI and EMC, and analysis of grounding systems in various soil structures. Fig. 11 Fault current path in a system with a grounding transformer.

Dr. Ma is the author of more than one hundred papers on transient electromagnetic scattering, analysis and design of reflector antennas, power system grounding, lightning, and electromagnetic interference. He is a senior member of the IEEE Power Engineering Society, a member of the IEEE Standards Association, and a corresponding member of the IEEE Substations Committee and is active on Working Groups D7 and D9.

VI. CONCLUSIONS Practical considerations regarding fault current split

calculations were presented. Scenarios involving various transformer types such as delta-star connected, star-star connected, auto-transformer with a delta tertiary, and zigzag grounding transformer were analyzed. It was shown that the fault current going into the grounding system can be overestimated or underestimated if the actual situation is not modeled correctly. Proper examination of various fault current paths involving different transformer types was also presented. The examples given in this paper can help power engineers understand the subtle differences in fault current split calculations for various transformer configurations and assist them in carrying out fault current distribution calculations for grounding designs accurately.

Ms. Xiao Wei Zhao is an associate professor and head of the Medical Engineering Group with the Department of Medical Image, Shandong Medical College, Jinan, Shandong, P. R. China.

Dr. Farid P. Dawalibi (M'72, SM'82) was born in Lebanon. He received a Bachelor of Engineering degree from St. Joseph's University, affiliated with the University of Lyon, and the M.Sc. and Ph.D. degrees from Ecole Polytechnique of the University of Montreal.

From 1971 to 1976, he worked as a consulting engineer with the Shawinigan Engineering Company, in Montreal. He worked on numerous projects involving power system analysis and design, railway electrification studies and specialized computer software code development. In 1976, he joined Montel-Sprecher & Schuh, a manufacturer of high voltage equipment in Montreal, as Manager of Technical Services and was involved in power system design, equipment selection and testing for systems ranging from a few to several hundred kV. In 1979, he founded Safe Engineering Services & technologies, a company specializing in soil effects on power networks. Since then he has been responsible for the engineering activities of the company including the development of computer software related to power system applications.

VII. REFERENCES [1] IEEE Guide for Safety in AC Substation Grounding, IEEE Std. 80-

2000. [2] F. P. Dawalibi, “Ground fault current distribution between soil and

neutral conductors,” IEEE Trans. Power App. Syst., vol. 99, no. 2, pp. 452-461, Mar./Apr. 1980. He is the author of more than two hundred papers on power system

grounding, lightning, inductive interference and electromagnetic field analysis. He has written several research reports for CEA and EPRI. [3] F. P. Dawalibi, Transmission Line Grounding, vol. 1, Chapter 6, EPRI

Report EL-2699, Oct. 1982. Dr. Dawalibi is a corresponding member of various IEEE Committee

Working Groups, and a senior member of the IEEE Power Engineering Society and the Canadian Society for Electrical Engineering. He is a registered Engineer in the Province of Quebec.

[4] D. Garrett, J. Mayers, and S. Patel, “Determination of maximum substation grounding system fault current using graphical analysis,” IEEE Trans. Power Del., vol. 2, no. 3, pp. 725-732, Jul. 1987.