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Please contact SES (info@sestech.com) for the full text of these papers.
AC Mitigation Requirements: A Parametric Analysis
R. D. Southey, W. Ruan, and F. P. Dawalibi
This paper presents a parametric analysis in which required mitigation wire length is determined for a pipeline subjected to magnetic field induction from a nearby high voltage transmission line, during load conditions. The effects of the following variables on induced voltages and mitigation requirements are studied: length of parallelism, separation distance, transmission line cross-sectional configuration, type of static wire, soil resistivity, load current magnitude, and pipeline/transmission line crossings. The goal is to provide the reader with a means to estimate mitigation requirements before a detailed study is performed.
In the scenarios studied, which are based on a reference case consisting of a 10-mile (16.1 km) parallelism between a well coated 24” (61 cm) pipeline and a 230 kV transmission line carrying a load current of 1000 A, the required length of zinc anode ribbon, which provides the mitigation, varies from 0 to 1320 feet (402 m). Note that this does not include the length of ribbon that might be required to protect exposed pipeline appurtenances, although this requirement is discussed in the paper, nor does it include the length of ribbon that would be required to protect the pipeline coating from damage during fault conditions. Fault conditions will be the subject of a subsequent paper.
Computation of Cable Parameters for Pipe-type Cables with Arbitrary Pipe Thicknesses
Y. Yang, J. Ma, and F. P. Dawalibi
Accurate computation of power cable parameters is often required in power system analysis. The cases of underground single-core coaxial cables consisting of a core and a sheath, and of pipe-type cables where the pipe thickness is assumed to be infinite have been well studied in the 1970’s. Pipe-type cables with a finite pipe thickness have also been studied. In the studies carried out for the pipe-type cables with finite pipe thicknesses, the pipe thickness is always assumed to be sufficiently larger than the skin depth. In practice however, cases where the pipe thickness is smaller than the skin depth are often encountered. Under such circumstances, the existing formulae will not be applicable for computing the cable parameters accurately.
In this paper, new formulae have been developed for computing cable parameters of pipe-type cables with arbitrary pipe thicknesses. The formulae are developed using cylindrical wave propagation equations in polar coordinate forms. Numerical results are presented showing the comparison with the results based on existing formulae. It can be seen from the comparison that in the region where the pipe thickness is much larger than the skin depth, the results based on the existing formulae are in good agreement with the new results. When the pipe thickness approaches the skin depth, the results based on the existing formulae begin to deviate from the new results. When the pipe thickness is smaller than the skin depth, the results based on the existing formulae are no longer credible (a negative resistive part is generated for the impedance of the cable core). In this case, the new results asymptotically approach the results for the case without the pipe enclosure when the pipe thickness approaches to zero, which is as expected.
The study presented in this paper has extended the theory for the computation of pipe-type cable parameters. New formulae have been developed which is applicable for pipe-type cables with arbitrary thicknesses. Numerical results for pipe-type cable parameters are readily available using computer codes based on the new formulae.
Safety Analysis of Fence Interconnection to Substation Grounding System
J. Ma, W. Ruan, R. D. Southey, and F. P. Dawalibi
A safety analysis of fence interconnection to the substation grounding system has been carried out. Various scenarios of fence connections to the substation grounding system have been examined and safety-related quantities such as touch and step voltages have been computed and compared in order to assess the advantages and disadvantages of connecting the fence to the grounding system. The conclusions of this study can be used as a guide in deciding whether or not to connect a metallic fence surrounding a substation to the substation grounding system.
Effects of the Length and Angle of Conductors on the Computation Accuracy of Inductive Interference
Y. Li, F. P. Dawalibi, and J. Ma
Conventional methods used to analyze the inductive interference between electric power lines and pipelines are usually based on a circuit model approach. In the computation of the line parameters in the circuit model approach, the lines are assumed to be parallel and infinite in length. When they are not parallel, a piece-wise parallelism approach is employed. Obviously, these assumptions will lead to inaccuracy in the computation of inductive interference. The objective of this paper is to evaluate the level of inaccuracy introduced by the use of these assumptions. To carry out this task, a field approach and a circuit approach are used. The field approach is based on electromagnetic field theory, which does not assume the lines to be parallel and infinite in length. By comparing the results from the two approaches for different line lengths and different angles between conductors, the error caused by the assumptions used in the circuit model are revealed. The results presented in this paper can be used as a guideline to evaluate the accuracy level corresponding to the assumptions made in the computation of inductive interference using the circuit model approach.
Computation of Power Line Structure Surge Impedances Using the Electromagnetic Field Method
F. P. Dawalibi, W. Ruan, S. Fortin, and J. Ma
with W. K. Daily of Austin Energy
The computation and measurement of power line structure surge impedances has been the subject of intense discussions in the lightning protection of transmission lines for several decades. In the past, the tower surge impedances of transmission line structures have been measured and also computed based on formula derived using a geometrical model of the tower and electromagnetic field theory. Recently, electromagnetic field theory has been used to study the surge impedance on a full-scale UHV tower. Electromagnetic field theory has also been used to compare different methods to measure structure surge impedances and to study the influence of lead wires on the measurements. In all these studies, the tower surge impedance is computed by assuming a perfect ground and the grounding systems of the power line structures are not considered.
It is obvious that the measurement of surge impedances on full-scale power line structures is a difficult task. The objective of this study is to simulate realistic lightning strikes on various transmission and distribution structures using an electromagnetic field approach. The surge impedance is computed for various transmission and distribution line structure configurations (ranging from a single wood pole to lattice steel towers) including their grounding systems. The influence of the soil resistivity and different grounding system configurations on the structure surge impedance was studied. The structure surge impedances are computed for three impressed surge current waves: (a) Step current wave; (b) Ramp current wave (c) Typical double-exponential lightning surge current. The computed surge impedances using the electromagnetic field approach was compared with those obtained using formula based on a geometrical model of the power line structure.
It is shown that the computed values based on the electromagnetic field theory are, to various extent, different from the values computed using the formula based on the geometrical model of the structure. The initial study reveals that (a) the surge impedance of a lattice steel tower decreases only slightly when the soil resistivity is increased from 0.1 ohm-m to 500 ohm-m; (b) by maintaining the same footing resistance, the surge impedance of a single pole remains essentially the same.