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Impact of Transmission Lines on Stray Voltage

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Impact of Transmission Lines on Stray Voltage 1 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 transmission line 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, Ground, 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, inductance, and capacitance 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 d V '' 2 = ZYV (1) 2- Transmission line conductors configuration (pole dx configuration and untransposed lines) 2 d I '' 3- Additive phase angles between the induced and load 2 = ZYI (2) related currents in neutral system dx 4- Soil resistivity along the transmission line Where: ''' [Z]=[] R + s[ L] Series Impedance per unit length This paper describes a case study involving induction And [Y'']=[] G + s[ C ' ] Shunt 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 1 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 9 540 A 8 485 A 7 430 A 6 375 A 5 Fig. 3 Generated Stray Voltage RMS at Each of the Transmission 4 Stray Voltage [V] Voltage Stray Line Pole 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 1 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. 8 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.
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