Novel technique to calculate the effect of electromagnetic field of HVTL on the metallic pipelines by using EMTP program
The Authors
Ghada M. Amer, Assistant professor, Benha Higher Institute of Technology, Egypt
Acknowledgements
The author likes to thank Professor Dr Ossama Elsayed Gouda Professor of High Voltage Engineering Cairo University for his support.
Abstract
Purpose – The paper proposes to present the effect of the high voltage transmission lines on the metallic pipelines by calculating the induced voltage due to mutual inductance between the two circuits especially in short circuit conditions of high voltage overhead transmission lines.
Design/methodology/approach – The electro magnetic transient program (EMTP) is used to simulate the high voltage transmission lines in normal case and in different faulty case conditions. A software is built on MATLAB program (M-file) to study the effects of various parameters on the magnitude of the induced voltage such as: separation distance between the high voltage transmission line and the metallic pipeline (horizontal distance), different cases of short circuits and normal operation case, the screening factor, and the soil resistivity.
Findings – The three-phase to ground fault gives the least induced voltage, and phase to ground fault case is the most serious case. The induced voltage decreases with increasing the soil resistivity until 400 Ωm and after this, the induced voltage in the metallic pipeline increases with increasing the soil resistivity for all phase fault types.
Research limitations/implications – It does not deal with all types of interference such as capacitive interference.
Practical implications – This technique helps to know the electrical influence exerted by power line on a pipeline. So it can prevent the pipeline from posing a shock hazard rather than corrosion.
Originality/value – This paper presents the effect of the high voltage transmission lines on the metallic pipelines by calculating the induced voltage due to mutual inductance between the two circuits especially in short circuit conditions of high voltage overhead transmission lines.
Article Type:
Literature review
Keyword(s):
Electromagnetic fields; High voltage; Pipelines.
Journal:
COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering
Volume:
26
Number:
1
Year:
2007
pp:
75-85
Copyright ©
Emerald Group Publishing Limited
ISSN:
0332-1649
1 Introduction
Electromagnetic interference effects of transmission lines upon near by metallic pipelines are real problems especially in faulty cases. Electromagnetic can be induced on a pipeline from overhead power lines in close proximity by:
- capacitance;
- conductance; and
- induction (Southey et al., 1994).
The electrical influence exerted by power line on a pipeline varies with the electrical characteristics and geometry on the individual system. Electromagnetic induced on pipeline poses a shock hazard rather than a corrosion concern. One of the greatest causes of voltage induction on pipelines is line current flow (Dabkowski, 1995). Current flow in an AC conductor creates an electromagnetic field of force, which always lies at right angles to the current that produces it. With alternating current, the field expands away from the conductor and then collapses towards the conductor at a rate, which is a function of the system frequency (Cist and Schutz, 2001). The magnitude of the induced voltage on the pipeline is a function of the following: separation between AC conductors and pipeline, the magnitude of electric system current flow (especially in short circuit cases), resistivity of the soil, and the screening factors (Cist and Schutz, 2001). Many formulas are used to calculate the mutual impedances and the pipeline induced voltages between the power lines and pipeline such as Carson and Pollaczek formula (Dabkowski, 1995; Cist and Schutz, 2001; Jakl, 1992). Carson-Clem formula (Siegfried, n.d.; Dawalibi et al., 2000; Dabkowski and Taflove, 1978) and Haber-Lands formula (Siegfried, n.d.). In this paper, the induced voltage due to mutual inductance between power lines and pipelines especially in short circuit conditions of high voltage overhead transmission lines is investigated under different short circuit cases, the effect of ground conductors on reducing the mutual interface between the power lines and pipelines is discussed also. The soil resistivity variation is taken into consideration. New technique by using electro magnetic transient program (EMTP) and design Matlab M-file program by the author are used to calculate the pipeline induced voltage.
2 Modelling of high voltage transmission line and pipeline by using computer programs
2.1 Modeling of high voltage transmission lines by using EMTP
EMTP is highly accurate digital simulation program for high voltage three-phase transmission line simulation. EMTP is used to produce voltage and current wave forms for different type of faults. The transmission line components, which are modelled by the programs, included:
- resistance, inductance, and capacitance;
- travelling wave models to represent overhead lines or cables; and
- ideal current and voltage simulation.
The inputs to the program are the length of line to be simulated, the model parameters data, the length of time to be simulated, calculation of time step (sampling time) as well as the desired output. The transmission line is represented by nine π models scaled to the parameters of 132 kV. These parameters are GMR = 7.122 mm, conductor outer strand radius = 1.88 mm, number of strands = 19, and continuous current rating is 400 A. We also consider the physical arrangement of the conductors. Figure 1 shows the arrangement of the conductors, of 132 kV transmission line and pipeline. The 132 kV transmission line parameters are calculated from line geometrical characteristics. The calculated parameters are expressed as series impedance and shunt capacitance per unit length by using EMTP. The outputs of EMTP currents as a function of time are used to feed Matlab M-file program.
2.2 Pipeline parameters
The pipeline parameters used in this study are as follows; coating resistivity=833,000 Ωm, outer diameter=0.4064 m, inner diameter=0.39923 m, wall thickness=0.00717 m, and burial depth=0.5 m.
3 Effect of electromagnetic field on the pipelines
In normal conditions the overhead transmission lines circuit carries currents ia, ib, and ic flowing in each phase. During such conditions, these currents are relatively low in magnitude compared to fault conditions and their effects on nearby pipelines tend to cancel one other. AC interface maybe produce due to the difference in the relative distance of each phase from the nearby pipelines due to any phase imbalance in the line. Under fault, condition the currents on the faulty phases of transmission lines are high so this will induce AC voltage on a pipeline poses a shock hazard rather than a corrosion concern. A typical case was considered in this paper. Figure 2 shows a real case of metallic pipeline installation parallel to high voltage transmission line. The results presented in this paper have been obtained using the Matlab M-file software. The software uses the currents obtained from the EMTP in which the short circuit cases are simulated as a function of time to determine the metallic pipeline induced voltage. The computer results have been verified by the following formula (Power line induced AC, 1983; Dommel, H.W., discussion IEEE Working Group, 1974; Huebler, 2002): Equation 1 where E is longitudinal electric field from overhead transmission lines in V/km. Z m is the mutual impedance in Ω/m. The mutual impedance Z m can be expressed as by IEEE formula (H.W. Dommel, discussion IEEE Working Group, 1974; Huebler, 2002): Equation 2 where f is frequency in Hz, μ is free space permeability =4π×10−7 H/m, ρ is earth resistivity in Ωm, D is the distance from transmission line to metallic pipeline in m, ΔR c and ΔX c are only roughly 1 percent of the value of Z m and are neglected in this study (Huebler, 2002).
The induced voltage due to three-phase system can be calculated by the relation: Equation 3 where Zma is the mutual impedance between the metallic pipeline and the phase carrying current i a . Zmb and Zmc are similarly defined for other phase conductors. All power transmission lines have ground wires for shielding the line against direct strokes and induced strokes. These ground wires also provide screening effect against mutual interference (Varma, 2002; Zunec, 2001; Power line induced AC, 1983).
4 Results and discussion
4.1 Effect of changing horizontal distance between OHTL and the pipeline on the magnitude of the induced voltage interface with compensating conductors
In this case, the resultant induced emf of transmission line with compensating conductors on the pipeline is calculated in different fault conditions. The horizontal distance is shown in Figure 1 between the transmission line and pipeline is changed from 0 to 20 m. the vertical distance is constant and equal the height of the tower 11.595 m plus the burial depth of the pipeline 0.5 m. Figure 3 shows the induced voltages of the three phases in case of three phase to ground fault.
The resultant induced voltage on the pipeline in this case at horizontal distance d = 1 m is shown in Figure 4(a) as a function of time. The soil resistivity is assumed constant and equal 150 Ωm. similar results are obtained in the following cases: Figure 4(b) shows the resultant pipeline induced voltage in normal case operation of high voltage transmission line. Figure 4(c) shows the resultant pipeline induced voltage in single line to ground of phase c of high voltage transmission line. The induced voltages in the metallic pipeline in case of ground faults are shown in Figure 5(a). Moreover, the induced voltages in the metallic pipeline in case of phase faults are shown in Figure 5(b). From Figure 5(a), it can be observed that the pipeline induced voltage decreases with increasing the horizontal distance as it is expected.
From this figure it is noticed also that the three-phase to ground fault gives the least pipeline induced voltage, it is less than 160 V/km, and phase to ground fault gives the most serious case as shown in Figure 5(a). The value of the pipeline induced voltage is greatest when the fault at the phase c the nearest conductor to the pipeline as shown in Figure 1. In case of phase faults, it can be observed that the pipeline induced voltage decreases with increasing the distance after 8 m distance starting from phase c for all phase fault types. As shown in Figure 5(b).
Line a to line b fault gives the least pipe line induced voltages it is less than 60 V/km, and three phase fault gives the most serious case and it is less than 160 V/km as shown in Figure 5(b).
4.2 Effect of changing soil resistivity on the magnitude of the induced voltage of the pipeline
The resultant pipeline induced voltages are calculated with the variation of the soil resistivity. The horizontal distance between the transmission line and pipeline is fixed at 1 m. The vertical distance is constant and equals the height of the tower 11.595 m plus the burial depth of the pipeline 0.5 m. The soil resistivity is changed from 40 to 1,000 Ωm. The induced voltages in the metallic pipeline in different faulty cases are shown in Figure 6. It can be seen that the induced voltage decreases with increasing the soil resistivity until 400 Ωm. More than 400 Ωm the induced voltage in the metallic pipeline increases with increasing the soil resistivity for all ground fault types. As shown in Figure 6.
4.3 Effect of the screening factor on the magnitude of the induced voltage of the pipeline
To calculate the pipeline induced voltages similar calculations are carried out taking the absence of the screening factor into considerations (the transmission lines without earth wires). The screening factor of 132 kV high voltage transmission line is calculated according to the relation (Power line induced AC, 1983): Equation 4 where M 12 is the mutual inductance between the earth wires and each power line phase and L 2 is the earth wire inductance. Approximately, it is found that the screening factor equals 0.43. The soil resistivity is fixed at 150 Ωm and burial depth of the pipeline is fixed at 0.5 m Figure 7(a) and (b) give the change of metallic pipe induced voltages versus the horizontal distance between the pipeline and the transmission lines without compensating conductors in different faulty cases. The maximum value of the pipeline-induced voltages reaches to a bout 1,180 V/km.
4.4 Effect of high sensitivity parameters in the induce voltages
Tables I and II show the maximum values of the induce voltage between high voltage transmission line 132 kV and pipelines fixing all parameters except one of them (the height of tower is constant and the depth of pipeline is constant).
From Tables I and II we can see that the worst case is when the fault type is phase c with ground at horizontal distance 1 m and when soil resistivity is 40 Ωm.
5 Conclusion
Inductive interface generated by 132 kV overhead transmission line on metallic pipelines has been analyzed under faulty conditions by using novel technique based on simulation of high voltage transmission line by EMTP and feeding the output to new software (M-file).
The novel technique could be used to calculate pipeline induced voltages as a function of time in different faulty cases (Figure 4).
The three-phase to ground fault gives the least induced voltage, it is less than 160 V/km for horizontal change between 0 and 20 m, and phase to ground fault case is the most serious case.
The phase a to phase b fault is the least induced voltage, it gives pipeline induced voltage less than 60 V/km for horizontal distance varies from 0 to 20 m. The induced voltage decreases with increasing the soil resistivity until 400 Ωm and after this, the induced voltage in the metallic pipeline increases with increasing the soil resistivity for all phase fault types.
In case of transmission line without compensating conductors the value of induced voltage is increases due to the absence of the screening factor, which reaches to 0.43.
Equation 1
Equation 2
Equation 3
Equation 4
Fixed graphic 1
Figure 1Cross section of transmission lines and metallic pipeline
Figure 2Installation of metallic pipeline parallel to HVTL (real case in Egypt)
Figure 3Shows the induced voltage of the three phases in case of three phases to ground fault
Figure 4
Figure 5
Figure 6The change of induced voltage from the TL with compensating conductor in the pipeline versus the soil resistivity for different faulty cases
Figure 7
Table IEffect of type of fault in the value of induced voltage
Table IIEffect of soil resistivity in the induced voltage
References
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About the author
Fixed graphic 1Ghada M. Amer was born in Manama, Bahrain, in 1972. She received the degree of BSc in electrical engineering from Benha Higher Institute of Technology in 1995, the master degree in electrical power engineering from faculty of engineering, Cairo University in 1999. And PhD degree in electrical power engineering from faculty of engineering, Cairo University in 2002. She works an Associated Professor in electrical department in Benha Higher Institute of Technology. Her present interests are the protection system; effect of EMF of high voltage transmission lines, and biomedical engineer. Ghada M. Amer can be contacted at: dr_ghada11@hotmail.com; dr_ghada11@yahoo.com
