2. Application of Electromagnetic Field Theory to Measure Correct Grounding System Impedance: A Parametric AnalysisR. D. Southey and W. Ruan The 15th Conference on Electric Power Supply Industry (CEPSI) Shanghai, China, October 18-22, 2004.
Abstract: Measurement of the grounding system impedance of a substation or power plant is often required immediately after construction, in order to verify that the design calculations correctly predict the performance of the system. Years later, new measurements are sometimes required to check that the performance of the grounding system has not deteriorated. This type of measurement, however, which is typically carried out using the fall-of-potential method, is plagued by a number of potential problems: conductive coupling between the grid under test and the remote current return electrode, especially for soil structures with low resistivity over high; inductive coupling between current and voltage test leads; inductive coupling between test leads and grounding grid conductors; inductive coupling between test leads and power line static or neutral wires; additional grounding provided by power line static and neutral wires, which lowers the apparent impedance of the grounding grid. This paper presents a methodology for measuring ground impedances that minimizes the effects of inductive coupling, conductive coupling, and power line grounding. A parametric analysis is carried out to illustrate how measurement error changes as a function of grid dimensions, soil structure, test electrode locations, test lead separation distance, locations of test lead connections to the grounding grid, and test signal frequency used. It is found that accurate ground impedances can be obtained by interpreting the test data using electromagnetic field models that determine the expected error as a function of test signal frequency. |
3. Database and Rule Based Automated Grounding System DesignJ. Liu and F.P. Dawalibi The 15th Conference on Electric Power Supply Industry (CEPSI) Shanghai, China, October 18-22, 2004.
Abstract: The design of grounding systems is often based on rough guidelines, derived from engineering experience. It is frequently a trial and error procedure and can be quite time consuming, since it is too difficult to account for the large number of variables (geometrical proportions of the grid, its depth, the nature of the soil and of the grid’s conductors, whether or not grounding rods are attached to the grid, etc…) that can affect the grid’s performance. This paper presents a database and rule based automated grounding system design method to meet design requirements (such as ground potential rise, touch voltage, step voltage, and ground resistance limits), given the soil structure, dimensions of the grid area, characteristics of conductors, configuration of the grid, and fault current discharged by the grid. A three-step approach is used for the automated grounding system design. First, a grounding system which consists of a large number of horizontal conductors forming a very dense grid is modeled. It is almost equivalent to a buried metallic plate. This gives the minimum achievable ground impedance for a grid of the specified size, and verifies whether the desired ground impedance and safety limits can be achieved at all. Second, an appropriate preliminary grid design is retrieved from a database of predefined grids, based on the input data provided. Next, this initial design is refined recursively using rule-based techniques and algorithms to improve its performance and meet the safety constraints, while reducing the overall cost of the grid. The grid database is, optionally, the starting point of any automated design and will cover most grids that are encountered in practice. Extensive collections of predefined grids have been analyzed, constructed and can be easily updated by the user. A strategy has been devised to quickly find an appropriate grid, while at the same time minimizing the size of the database. The techniques presented in this paper can help electrical engineers design safe grounding installations quickly and efficiently. The time devoted to design a safe and cost-effective grounding grid is minimized by the use of the database and rule based automated grounding system design method and technology. |
1. Effects of Current Unbalance and Transmission Line Configuration on the Interference Levels Induced on Nearby PipelinesY. Li and F. P. Dawalibi Corrosion/2004 NACE International Conference, New Orleans, March 28 to April 1, 2004 Abstract: Recent research work on magnetic field induction from nearby high voltage power lines to adjacent pipelines has uncovered several interesting and intriguing results under unbalanced load (steady-state) conditions of a horizontal three-phase transmission line. This paper focuses on this topic, more specifically, on the analysis of the effects of current unbalance ratio and power line the induced interference levels. Three typical transmission line configurations, two horizontal and one vertical, are examined to carry out the study. Various unbalanced currents on a phase conductor are studied to reveal the minimum interference level that can be achieved for a given right-of-way network configuration. This minimum level is caused by a cancellation effect that is introduced by a specific combination of the transmission line geometrical unbalance with respect to the pipelines and the current unbalance introduced by one of the phase conductors. The effect of the soil resistivity on the interference level is also studied under different unbalanced current conditions. It is found that the interference levels decrease with a decrease of the soil resistivity for most unbalanced current conditions. However, under some unbalanced current conditions the interference levels increase with decreasing soil resistivity. Furthermore, significant variations of soil resistivity can have practically no impact on the interference levels during other unbalanced conditions. The results obtained in this paper can be used to develop designs to minimize optimally the interference level on nearby pipelines. |
4. Analysis of A Steel Grounding System: A Practical Case StudyY. Li, F. Dawalibi, J. Ma, and Y. Yang The 15th Conference on Electric Power Supply Industry (CEPSI) Shanghai, China, October 18-22, 2004.
Abstract: This paper presents a thorough analysis of the performance of a large grounding system made of steel instead of copper conductors. The grounding system is located in a relatively low resistivity soil and is interconnected to an extensive network of overhead transmission lines, in low soil resistivity. The extent of the grounding combined with its steel conductors and the low resistivity soil invalidates the equipotential assumption that is usually made when analyzing grounding systems. The presence of a large circulating fault current in the grounding system aggravates this problem further. Obviously, classical grounding analysis methods are no longer applicable and more advanced techniques must be used. This paper presents a detailed study of such problems. The measured soil resistivities and the grounding system impedance are compared to the computed values. Fault current distribution between the grounding system and the other metallic paths are computed to determine the portion of fault current discharged in the grounding system. The performance of the grounding system, including its GPR (ground potential rise), GPDs (grounding potential differences) between the ground conductors and the touch and step voltages have been evaluated accurately, taking into account the impedance of the steel ground conductors and their mutual inductive components. Numerical results are presented and compared to those obtained based on a conventional approach. The paper also examines briefly the electromagnetic coupling between the control cables and the ground conductors to illustrate a typical analysis of the integrity of the electronic equipment connected to the control cables. |
5. Comparison of Computational Methods for the Design and Analysis of Power System Grounding: Parametric AnalysisS. Fortin, W. Ruan and F. P. Dawalibi Proceedings of the Seventh IASTED International Conference on Power and Energy Systems, Clearwater Beach, FL, USA, November 28 - December 1, 2004, pp. 357-363. Abstract: This paper examines three approaches used in grounding analysis and illustrates the advantages, limitations, and the applicability of each approach. The comparisons are carried out using grounding grids of sizes varying from 50 m to 500 m, in uniform and two-layer horizontally layered soil structures. Soil resistivities vary from 1 Ohm-m to 10,000 Ohm-m. The errors in GPR (Ground Potential Rise) and touch voltages, as well as computation time, are compared between the three approaches. The discussions and conclusions given in this paper can be used as a reference when deciding which approach should be used to carry out an accurate and efficient grounding analysis. |
6. A Parametric Analysis of Fault Current Division between Overhead Wires and Substation Grounding SystemsC. Li, X. Wei, Y. Li and F. P. Dawalibi Proceedings of the Seventh IASTED International Conference on Power and Energy Systems, Clearwater Beach, FL, USA, November 28 - December 1, 2004, pp. 48-53. Abstract: This paper presents a parametric analysis of the fault current distribution between a substation grounding system and its overhead ground wires and their effects on the grounding systems performance. The influence of the following variables are studied: the resistance of the substation grounding system, the characteristics of the shield wires, the number and the length of the transmission lines connected to the substation, the terminal types (i.e., power source and non-power source terminals), soil resistivity, and power line configurations. It is concluded that the fault current division factor is dramatically affected by the substation grounding resistance, the shield wires conductor characteristics, and the number of transmission lines while it is less influenced by the soil resistivity along the transmission lines and tower resistances. It is difficult to estimate the current division factor for a practical case. The only way to compute accurately the current division factor is to analyze individual cases based on the existing conditions with proper software tools. |
9. Steel Casing Overheating Analysis of Operating Power Pipe-Type CablesF. P. Dawalibi, J. Liu, S. Fortin, S. Tee, and Y. Yang Proceedings of the Seventh IASTED International Conference on Power and Energy Systems, Clearwater Beach, FL, USA, November 28 - December 1, 2004, pp. 376-380. Abstract: This paper discusses the analysis of an overheating problem which has been observed within a 140’ (43 m) long steel pipe casing containing 115 kV three-phase electric power cables. A steel casing electromagnetic field model has been built to determine the induced current (eddy currents) distribution along the radial, transverse, and longitudinal directions of the casing caused by the energized cables under different operating conditions. This model takes the combined effects of the inductive, conductive, and capacitive interference into account. This study involved a circuit model to determine the voltage and current distributions in the conductors of the buried power cables. Furthermore, a detailed analytical model of the cylindrical steel casing (assuming an infinitely long casing) was conducted to determine the actual paths of eddy current flow and their density throughout the cross section from the inner surface to the outer surface of the steel casing. The computation results show that induced currents in the steel casing can cause significant heat losses and that the exact distribution of the induced current density within the steel casing plays a crucial role in the heat losses generated by such currents. |
7. Computation Stability of Grounding Systems in Soils Containing Heterogeneous VolumesF. P. Dawalibi, N. Mitskevitch and S. Fortin Proceedings of the Seventh IASTED International Conference on Power and Energy Systems, Clearwater Beach, FL, USA, November 28 - December 1, 2004, pp. 364-369. Abstract: This paper studies the performance of grounding systems in soil structures containing heterogeneous soil volumes. This type of soil is crucial for the investigation of a range of practical problems that cannot be approximated by a layered soil structure. This type of problems involves grounding systems that are either close to, partially or totally immersed in one or several finite volumes of soil materials that have resistivity values quite different from that of the bulk volume of surrounding soil (native soil). This paper focuses on the stability of the algorithm used to compute the response of grounding grids for different scenarios. It also describes and discusses the computed results that pertain to a number of typical grounding scenarios, comparing them to some known limiting case solutions. |
8. Detailed Parametric Analysis of Grounding System Performance in Two-Layer Soil StructuresJ. Liu and F. P. Dawalibi Proceedings of the Seventh IASTED International Conference on Power and Energy Systems, Clearwater Beach, FL, USA, November 28 - December 1, 2004, pp. 370-375. Abstract: An extensive parametric analysis of rectangular grounding grids in two-layer soils has been carried out to determine their overall performance. A large number of variables that can affect the grid’s performance (such as the geometrical proportions of the grid, the characteristics of the soil including the reflection coefficient and top layer thickness, and the density of conductors) have been analyzed in details. Ground resistances, touch voltages, and step voltages of grounding systems have been computed and compared for various scenarios. The ultimate objective of this parametric study is to provide detailed evaluations of the grounding system performance. The computed results are summarized in charts; these results will be useful for quick estimates of the expected performance of practical grounding designs. |
12. Grounding Analysis of A Substation Connected to A Nearby Power PlantJ. Ma and F. P. Dawalibi Proceedings of the IASTED International Conference on Power and Energy Systems, Krabi, Thailand, April 18 - 20, 2005. Abstract: The performance of a substation grounding system which is connected to a nearby power plant via overhead ground wires is analyzed. Three methods have been used in the study, namely the conventional analysis, the conservative analysis, and the accurate one. Results obtained using all three methods have been compared using a practical example. Effects of different soil structures on grounding performance using the three methods are also examined. It is shown that the conventional method may underestimate touch and step voltages significantly, while the conservative method may overestimate them significantly. Therefore, accurate method should be used in the analysis to prevent significant underdesign or overdesign of grounding systems in these cases. Results from this paper can be used as a reference for the analysis of grounding systems which are connected to nearby grounding systems of power plants or substations. |
10. Integrated Method in Electromagnetic Interference StudiesJ. Ma and F. P. Dawalibi Proceedings of the IASTED International Conference on Power and Energy Systems, Krabi, Thailand, April 18 - 20, 2005. Abstract: This paper studies the electromagnetic interference problems arising in corridors shared by transmission lines, pipelines, and railways, etc. A new circuit model method for analyzing interference problems is introduced. This method can be used to compute the combined inductive, capacitive, and conductive interference level efficiently and accurately. Practical examples are examined and results obtained using the new method are presented and compared with those obtained using the conventional circuit model method and the exact electromagnetic field method. It is shown that the new circuit model can be applied to problems which cannot be solved using the conventional circuit model. |
11. Analysis and Mitigation of Current Unbalance Due to Induction in Heavily Loaded Multi-Circuit Power LinesJ. Ma, S. Fortin and F. P. Dawalibi IEEE Transactions on PWRD, Vol. 19, No. 3, July 2004, pp. 1378-1383. Abstract: High current unbalance may exist due to induction between circuits in a multi-circuit right-of-way, especially when some of the circuits have heavy loads. This paper studies the problem and presents a mitigation technique to reduce the current unbalance. A brief analytical derivation is carried out to show the principle of the mitigation technique, which is based on transposition of the lines. The mitigation technique is then applied to an actual right-of-way with six circuits. Comparisons have been made between the untransposed circuits and the transposed circuits. It is shown that the unbalance level can be reduced significantly by appropriate phase transpositions based on the proposed principle. The mitigation technique presented in this paper can be applied to other right-of-way configurations to reduce current unbalances, therefore ensuring stable operation of the heavily loaded power lines. |
14. AC Interference of Transmission Lines on Railways: Influence of Track-Connected EquipmentR. Southey, J. Liu, F.P. Dawalibi and Y. Li AREMA 2005 C&S Technical Conference, Omaha, Nebraska, May 24, 2005. Abstract: In joint-use corridors, AC electric power lines can induce objectionable voltages onto nearby railways. Excessive rail-to-rail voltages can interfere with the proper operation of grade crossing control equipment and track-connected signal equipment. The interference mechanism has been well known for many years and a sizeable body of literature exists on the subject. Nevertheless, the relationship between rail-to-rail voltages, track-connected equipment impedances, and ballast resistance has not been studied in a comprehensive manner. This study demonstrates that trackconnected equipment impedances must be modeled in an electromagnetic compatibility study, in order not to overestimate rail-to-rail voltages. Furthermore, the maximum rail-to-rail voltage does not necessarily occur when the ballast resistance is extremely large, but rather at some intermediate value, which is a function of the impedances of the track-connected equipment. This paper provides a series of curves illustrating these points for a simple example, along with a case study showing what impact this can have in practice. |
13. Effects of Power Line Phase Transpositions on Induced AC Voltages To Nearby Pipeline Railway UtilitiesY. Li, F.P. Dawalibi, J. Ma and R. Southey Proceedings of the IASTED International Conference on Power and Energy Systems, Krabi, Thailand, April 18 - 20, 2005. Abstract: This paper analyzes the effects of power line phase transpositions on induced AC voltages to nearby utilities. It is demonstrated that phase transposition can be used to minimize inductive coupling and thus reduce interference on nearby metallic utilities, such as railways and pipelines, under load conditions. The computerized analyses are based on both circuit and field approaches and are carried out using a realistic railway interference case. This analysis ensures that the optimum transposition scenario on a power system network is obtained. The effects of the induced voltages caused by the phase transpositions are examined and quantified. |
