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Impact of Transmission Lines on Stray Voltage

Nagy Abed, Member, IEEE, Sasan Salem, Member, IEEE, and Jim Burke, Fellow, IEEE

to simulate and measure the stray voltage. Figure 1 shows the Abstract-- The purpose of this paper is to study the effect of schematic diagram of the 115 kV transmission system used for transmission system parameters and operating conditions on the stray voltage study. This system was modeled using stray voltage levels. This includes the conductor EMTP-RV to evaluate the stray voltage level and the impact configurations, line loading levels, grounding system parameters, of various system parameters on these generated voltages. and unbalance loading. Excessive stray voltages levels may have a negative effect on dairy farm cows and endanger personnel safety. EMTP-RV was used to model the coupled electromagnetic- power circuit system. EMTP models of the poles and wires were built to represent the transmission line electromagnetic behavior and the stray voltage generation mechanism. The parameters of the proposed models were obtained from the technical literature. Different simulations were conducted by varying the system parameters and operating conditions. Calculations and field tests, which included the effect of earth contact resistance, indicated that most measured values of stray voltage may be incorrect and that the safety hazard to humans and animals may be greatly exaggerated. A discussion of these results is presented. Fig. 1 Schematic Diagram of the Modeled 115 KV Transmission Line System

The transmission line model utilized in the study is a Index Terms—Stray Voltage, Induction, Transmission Line, Earth, Earth Current, , Step Potential, Touch Potential distributed Constant Parameter (CP) model. The model is based on the Bergeron's traveling wave method [6]. In this I. INTRODUCTION model, the wave equation is solved to obtain the line operating characteristics (current, and voltage). Figure 2 shows the tray voltage in power systems has been studied prior to model circuit diagram. The transmission line parameters S the 1970’s. The term stray voltage typically means the resistance, , and per unit length were voltage between the neutral conductor and earth, which calculated using the transmission line conductor’s usually results from unbalanced loading. It was typically configurations (arrangement), the distances between the considered normal, with some issues arising from the dairy conductors, earth resistivity, the tower height, and the industry and pool owner. In the case of transmission lines, conductor’s parameters. however, stray voltage is normally the result of induction. For multiphase system the wave equations are written in the The following factors contribute to induced stray voltages matrix form: on transmission lines: 1- Unbalanced currents in the transmission line 2 conductors Vd '' 2 = VYZ (1) 2- Transmission line conductors configuration (pole dx configuration and untransposed lines) 2 Id '' 3- Additive phase angles between the induced and load 2 = IYZ (2) related currents in neutral system dx 4- Soil resistivity along the transmission line Where: ''' [ ] ][ += [LsRZ ] Series Impedance per unit length This paper describes a case study involving induction And [ ] '' ][ += [CsGY ' ] Branch per unit length related stray voltage concerns, simulation, measurement, and With eigenvalue theory, it becomes possible to transform mitigation. the above two coupled equations from phase quantities to modal decoupled quantities. The multiphase line is II. SYSTEM MODELING transformed into a decoupled set of modal circuits. The This section deals with the modeling methodology utilized equations are then solved to obtain the transmission line terminal response. In this study, a three wire untransposed Nagy Abed, Sasan Salem , and Jim Burke are with Quanta Technology, transmission line with a static wire and grounded at each ,Raleigh, NC, USA ([email protected], ssalem@quanta- tower, is modeled. technology.com, [email protected] ) 2

Generated Stray Voltage In order to evaluate the effect of substation grounding on the stray voltage, a series of cases were conducted in which the substation ground resistance was changed and stray voltage levels were recorded. Figure 4 shows the generated stray voltage for different substation grounding values. By increasing the substation resistance from 0.1 ohms to 1.0 Ohm the stray voltage will increase significantly. Most substation grounds are generally Fig. 2 The Distributed Constant Parameters Line Model assumed to be on the order of 1 ohm (or higher in distribution substations). The stray voltage rises particularly on the poles A transmission system with 25 poles was modeled. in the vicinity of the substation, which is consistent with the Appropriate data was obtained to model poles, lines, shield results of other papers on this topic. wires, ground rods, and the substation grounding. 12

Unbalanced currents on transmission lines, caused by 11 unbalanced load and/or un-transposed lines induce a voltage 10 on parallel lines including static wires, communication lines, 1.0 Ohms 9 and other transmission or distribution wires. The study was 8 conducted for balanced and unbalanced loading and with 7 uniform pole configuration. The induced voltages are 0.75 Ohms 6 considered to be steady state and 60 Hz so; they manifest 0.5 Ohms similar characteristics to “stray voltage 5

4 0.25 Ohms Stray Voltage [Vrms] 3

2 0.1 Ohms

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0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Pole number Fig. 4 Impact of Substation Grounding on SV level 10

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540 A 8 485 A 7

430 A 6

375 A 5

Fig. 3 Generated Stray Voltage RMS at Each of the Transmission 4 Line Pole [V] Voltage Stray 3 215 A 2 III. SIMULATIONS AND RESULTS 260 A A transmission system with 25 poles was modeled to 1 325 A determine the impact of various system parameters on the 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 generated stray voltage level. For this purpose the following Pole Number system parameters were studied: Fig. 5 Impact of Line Loading on the Generated SV Level • • Equivalent of substation grounding mat resistance • • Line loading b. Impact of Line Loading on the Generated Stray Voltage • • Line Span length In order to evaluate the effect of transmission line current • Pole ground rod resistance loading on the generated stray voltage levels, different loading • Unbalanced line loading cases were simulated and the generated stray voltage levels were measured. Figure 3 shows the stray voltage level on each pole of the Figure 5 shows the stray voltage levels for various line 25 modeled transmission line poles. From the graph we can currents. As was expected, higher line currents induce higher see that the minimum stray voltage in the middle (was zero), voltages in the shield wire and consequently stray voltages while its maximum value, of 7 volts, exists in the vicinity of will increase. the substation. a. Impact of Substation Grounding Resistance on the c. Impact of Line Span on the generated Stray Voltage 3

In order to evaluate the line span effect on the generated currents unbalance one of the phases. The loading unbalance stray voltage, a series of simulations was conducted in which was varied from 4% to 20%. the line span was changed. Figure 6 shows the impact of the Figure 8 shows the stray voltage levels results for different line span length on the stray voltage. The line spans were unbalance loading. The results show that stray voltage increased and then reduced by 20% in order to evaluate the increases with the increase in the unbalance (the zero impact on the stray voltage level. The simulation results sequence current). demonstrated the line span length has a little impact on the 10 stray voltage level. 9

8 8 7 7 100 Meters 6 20% 120 Meters 11% 5 6 6% 4 4% 5 3 Balanced load 0 Stray[Vrms] Voltage 2 4 1 80 Meters 3 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Stray Voltage [Vrms] Stray Voltage Pole number 2 Fig. 8: Impact of the Current Unbalance on the Generated SV Level

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0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Pole Number

Fig. 6 Impact of Line Span on the Generated SV Level d. Impact of Pole Grounding Resistance on the Generated Stray Voltage In this part of the study, the pole grounding resistance is varied between 5-200 ohms to study the relationship between the stray voltage and the pole grounding.

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5.0 Ohm 7 10.0 Ohm 20.0 Ohm 6 30.0 Ohm 200 Ohm 50.0 Ohm 100 Ohm 5 200 Ohm Fig. 9 Neutral-to-Earth Measurement without a 500Ω Resistor 4 IV. IMPACT OF TRANSMISSION LINE STRAY VOLTAGE ON 3 HUMANS

Stray Voltage [Vrms] 5.0 Ohm 2 There are a number of references used in the industry that discuss the resistance of the human body. It is common to use 1 1,000Ω from one hand to the other, hand to foot, etc... (See

0 IEEE Std. 80, which gives a range of 500 to 5,000Ω). Recent 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 published tests, performed by the authors, show human Pole number Fig. 7 Impact of Pole Grounding Resistance on the SV Level resistance values for hand-to-hand as follows: Dry skin ≈172kΩ The impact of pole ground rod resistance on stray voltage Wet skin ≈10kΩ is shown in Figure 7. The simulation results demonstrate that Wet (salt water) ≈5kΩ ground rod resistances above 50 Ohm have a slight impact on Added to this is the contact resistance of the earth itself, the stray voltage. In many areas, it is very difficult to drive which can be very large. The earth is actually a pretty good ground rods to attain values less than 50 ohms, so the impact conductor—if you can make good contact with it. It is the of grounding, to reduce stray voltage, might be considered problem of making contact with the earth that negates the minimal for most transmission lines. impact of grounding. When the fact is considered that an 8 e. Impact of load unbalanced on the Stray Voltage foot copper ground rod driven into the earth typically To evaluate the impact of unbalanced loading on the stray measures 100 to 1,000Ω and a downed conductor typically voltage the transmission line loading was changed to create 4 measures from 100Ω to many thousands of ohms even in wet without a 500 ohms resistor, as soon as a human being is soil, the impact of this contact resistance can be appreciated. placed in series with this circuit, the voltage across the human For the purposes of this study a contact resistance of 10,000 will collapse to a very low level (not necessarily ohms was assumed. This is not particularly large but imperceptible) due to the contact resistance of the circuit. illustrates the technical point, shown below. V. CONCLUSIONS Figure 9 shown above, illustrates testing performed without Transmission lines induce stray voltage levels for virtually all a resistor. The circuit is basically between the neutral voltages and configurations. Also, transmission lines will conductor (assumed at 4 volts) and true earth (assumed at 0 usually cause stray voltage that, measured with a voltmeter, volts). The circuit of importance is that which involves the may exceed the threshold limits of regulatory bodies. meter impedance and the earth contact resistance in series. As These voltages, however, when applied to humans and shown, a typical digital meter has an input impedance of about animals will collapse due to the contact resistance of the earth. 10MΩ or even higher. With 4V driving the circuit, virtually Finally, there are several conclusions to this analysis which all the drop is across the meter and the reading is 4V. the authors wish to share with others in the industry: • Highest SV is found near the substations • Substation grounding has a significant impact on SV levels • Line loading plays a significant role in SV levels • Span length does not have much impact on SV • Tower ground rod resistance does not have a major impact on SV levels • Unbalanced transmission line current loading increases the stray voltage levels. • SV levels collapse when a human is in contact with the tower (down lead) due to the contact impedance of the earth.

VI. REFERENCES [1] J. Burke, “The Confusion Surrounding Stray Voltage”, 2007 IEEE Rural Fig. 10 Neutral-to-Earth Measurement with a 500Ω Resistor Electric Power Conference, P. C1-C5. [2] D. J. Ward, J. F. Buch, T.M. Kulas, and W. J. Ros, “An Analysis of the

Five-Wire distribution system” IEEE Transaction on power delivery, TABLE I Vol. 18, No.1 Jan 2003 ACTUAL FIELD MEASURED DATA [3] T. C. Surbrook, N. D. Reese, A. M. Kehrle,” Stray Voltage: Sources and

Solutions”, IEEE Transactions on Industry Applications, Vol. IA-22, Example Measured Voltage Measured Voltage No. 2, March 1983. Locations Without Resistor With 500 Ω [4] M. E. Galey, “Benefits of performing unbalanced voltage calculations,” Resistor IEEE Transactions on Industry Applications, Vol. IA-24, No. 1, Jan-Feb 1988, pp. 15-24. A 5 0.01 [5] J. Burke, Power Distribution Engineering: Fundamentals and B 0.6 0.003 Applications, Marcel Dekker, INC., 1994. C 0.4 0.08 [6] A. Greenwood, Electrical Transients in Power Systems, Wiley- Interscience, 2 edition, 1991. When a 500Ω resistor is placed across the meter, the total [7] J. Burke, C. Untiedt, “Stray Voltage: Two Different Perspectives” – impedance of the circuit decreases significantly as shown in IEEE REPC 2008

Fig. 10. As shown below, the 500Ω resistor is small when Nagy Abed is with Quanta Technology. He received his B.Sc. (The first Rank compared to the contact resistance of the earth (assumed to be on the class) and M.Sc. from Mansoura University, Egypt, and his PhD 10K ohms); therefore, most of the 4V drop now occurs in the from Florida International university, Miami. His research interests include power system modeling, fault diagnosis, power quality, FACTS earth itself. The voltage shown by the meter probes now devices, Application of Finite Element in power system and real time measure less than 0.2V (for the assumptions given). The control with HIL. voltage collapses across the resistor but there is still 4V on the neutral. This is what would actually happen to a 500Ω human Sasan Salem is a Principal Engineer with Quanta Technology. He received his BS in Electrical Power Engineering form Iran university of Science being standing on the earth and touching the tower or neutral and Technology and his M.Sc. from Concordia University in Montreal. down lead. Also, it can be expected that this phenomena will His main research interests include power system analysis and control occur at all locations, similar to the actual field results (and and FACTS applications in power systems. computer simulations) shown in the table above. Digital Jim Burke is an Executive Advisor with Quanta Technology. He has been in simulations were run to confirm these field results. The the industry over 43 years. He is the former chair of the IEEE conclusion being (based on actual field measurements and Distribution Subcommittee as well as the Working Group on digital simulations) that while a digital voltmeter measurement Distribution Neutral Grounding. He is a Fellow of the IEEE may read values of stray voltages of 10 volts or even higher