Substation Grounding System Research Articles

Numerical calculation of grounding system in low‐frequency domain based on the boundary element method

AbstractA new mathematical model for accurately computing currents flowing along the high‐voltage ac substation's grounding system and nearby floating metallic conductors buried in the multilayer earth model has been developed in this paper, which is a hybrid of the Galerkin‐type boundary element method (BEM) and the conventional nodal analysis method. Only the propagation effect of electromagnetic waves within the substation's limited area has been neglected in this model. The quasi‐static complex image method and the closed form of Green's function are introduced into this model to accelerate the mutual impedance and induction coefficients calculation. The model is then implemented in a computer program, which can be used to calculate current distribution of any configuration of the grounding system, with or without floating metallic conductors'. Copyright © 2007 John Wiley & Sons, Ltd.

Determination of Frequency-Dependent Characteristics of Substation Grounding Systems by Vector Finite-Element Analysis

A three-dimensional finite-element tool was developed to compute time-harmonic electromagnetic fields and impedance of substation grounding systems. The formulation employs edge-based finite elements for the magnetic vector potential A and nodal shape functions for the electric scalar potential V. The method has been applied in several configurations presented in the literature. The results are compared with both analytical and experimental data reported by other authors, with overall good agreement

Investigating the validity of existing definitions and empirical equations of effective length/area of grounding wire/grid for transient studies

There are various definitions for effective length/area of grounding wire/grid for lightning transients [A.S. Farag, T.C. Cheng, D. Penn, Grounding terminations of lightning protective systems, IEEE Trans. Dielectics, Elect. Insul 5(6) (1998) 869–877; B.R. Gupta, B. Thapar, Impulse impedance of grounding grid, IEEE Trans. Power Apparatus Syayem PAS-99(6) (1980) 2357–2362; Y. Liu, N. Theethayi, R. Thottappillil, An engineering model for transient analysis of grounding system under lightning strikes: non–uniform transmission line approach. IEEE Trans. Power Delivery 20 (2) (2005) 722–730; M.I. Lorentzou, N.D. Hatziargriou, Modelling of long grounding conductors using EMTP, in: IPST’99, International Conference on Power System Transients, Budapest, 20–24 June, 1999; L.D. Grcev, M. Heimbach, Frequency dependent and transient characteristics of substation grounding system, IEEE Trans. Power Delivery 12 (1997) 172–178.]. The present work investigates and discusses the validity of those existing definitions. Further, practical methods for estimating the effective length/area of different grounding structures are proposed for engineering applications. The calculations for effective length/area based on non-uniform transmission line approach (Liu et al., 2005) show that, for a single grounding wire, the empirical equation for effective length in Farag et al. (1998) is not valid when the injection current has very fast rise time. Also, the empirical equation for effective length of grid edge in Gupta and Thapar (1980) is not applicable for grids with large inner mesh size.

Evaluation of grounding performance of energized substation by ground current measurement

Substation grounding systems typically build up a large network of various grounding electrodes, which consists of a substation grounding grid, multi-grounded transmission line skywires and distribution line neutral wires. Although the fall-of-potential (FOP) method has been widely used for ground resistance measurement, it is difficult to use the method in case of such a substation grounding system because of the difficulty of interpretation of the measured FOP profile. In this paper, we have presented a practical example of ground resistance measurement in a 154 kV substation under commercial operating condition. Conventional FOP tests and the measurement of ground current splits to each part of the grounding system were conducted simultaneously. A simple interpretation method of FOP profiles, with the measured ground current splits, has been suggested.

Calculation of DC current distribution in AC power system near HVDC system by using moment method coupled to circuit equations

When dc current is injected into the earth through the grounding electrodes of a high-voltage direct current (HVDC) system, transformers in ac system may be under dc bias if the dc currents flowing through the transformers are large enough. In this paper, a numerical method coupling the method of moments (MoM) to circuit equations is presented to calculate the dc current distribution in ac power system caused by an HVDC system. The MoM is used to calculate the electric fields in complex earth structure caused by all the grounding systems including the dc grounding electrodes, the ac substation grounding systems and the long metal pipe lines. The circuit equations are coupled to the moment method to take account of the effects of the transmission lines. By using the method, the dc current distribution in an ac power system caused by an HVDC system is analyzed. Some useful conclusions are drawn from the analyzed results

Current North American Assessment and Refurbishment Practices of Substation Grounding Systems

A survey was undertaken on the current assessment and refurbishment practices of North American utilities regarding substation grounding systems. The information was gathered using standard survey techniques where all interested parties were requested to fill out a short written survey and return it with the understanding that their responses would be kept anonymous. Survey results show that almost all utilities evaluate the integrity of their ground grids at some point, and have maintenance or engineering standards for determining the integrity. Test equipment is frequently used to evaluate integrity, and ground grids not meeting these standards are commonly upgraded. The results also show that most use high resistivity surfacing material, yet very few test the resistivity of the material at the time it is installed. This paper provides guidance on the recommended best practices based on these survey results, which can be used for benchmarking purposes. It also includes a short discussion on the importance of maintaining the substation grounding system.

Comparison of 161/69-kV Grounding Grid Design Between Indoor-Type and Outdoor-Type Substations

This paper presents a practical case for the safety assessment of ground grid design at 161/69-kV indoor-type and outdoor-type substations in the system of Taiwan Power Company (TPC). For a substation grounding system to be safe, the touch and step voltages should be limited below the minimum tolerable voltages. Since the mesh voltage is usually the worst possible touch voltage inside an ac substation (excluding transferred potentials), the mesh voltage is used as the criteria for a safe ground grid design. In other words, safety assessment of a ground grid design at 161/69-kV indoor-type and outdoor-type substations in the TPC system is referred to as a procedure by which the mesh and step voltages at substations are computed and compared to the minimum touch and step voltages. Computation results of safety assessment for humans with 50- and 70-kg body weight are quantified in this paper.

Studies on Grounding Technologies for Power System in China

The problems related to grounding systems of substations and power plants include geological structure analysis of substation site, the precise design of grounding system, the technology to decrease grounding resistance of substation in high soil resistivity area, corrosion diagnosis of grounding system, precise measurement of grounding resistance of substation, and impulse characteristics of grounding devices. Up to present, these six aspects related to grounding technology of power system have been widely studied, and achieved satisfied results in China. These achievements were systematically introduced in this paper.

Safety Assessment of Bulk and Traction Supply Substations in Taipei Rail Transit Systems

This paper presents a practical case for safety assessment of bulk supply substations (BSS) and traction supply substations (TSS) in Taipei Rail Transit Systems (TRTS). For a substation grounding system to be safe, the maximum touch-and-step voltages should not exceed postulated safety criteria. The safety criterion in IEEE Standard 80 is defined in terms of permissible body current. The permissible body current is then translated into the permissible touch-and-step voltages. Since the mesh voltage is usually the worst possible touch voltage inside the substation (excluding transferred potentials), the mesh voltage is used as the safety criteria. In other words, safety assessment of an ac substation grounding system in TRTS is referred to a procedure by which the mesh and step voltages are computed and compared to the maximum permissible (safe) touch-and-step voltages. Computation results for the orange line at O19 station are presented.

Seasonal influences on safety of substation grounding system

The low or high resistivity soil layer formed in raining or freezing season affects the safety of the grounding system, and leads to changes of grounding resistance of the grounding system, step and touch voltages on the ground surface. This paper systematically discusses the seasonal influence on the safety of grounding system by numerical analysis method. If the thickness of the low-resistivity soil layer formed in raining season is smaller than the burial depth of the grounding system, the touch voltage would increase. On the other hand, the raining season leads the limit of touch voltage is smaller than the actual touch voltage, even if a surface granite layer is used. The influences of raining on the safety of grounding system must be comprehensively considered. When the high resistivity soil layer is formed in freezing season, the grounding resistance, step and touch voltages would increase with the thickness or resistivity of the freezing soil layer. When the thickness of high resistivity soil layer exceeds the burial depth of the grounding system, the grounding resistance, step and touch voltages would increase sharply, and the touch voltage would exceed the limit of touch voltage decided by a surface granite layer. In order to ensure the safety of the grounding system in freezing season, the grounding system must be designed carefully, and special measures should be used, long vertical grounding electrodes added to the grounding system can effectively attenuate the seasonal influence, and improve the safety of the grounding system.

Transformer phase coordinate models extended for grounding system analysis

The transformer model in phase coordinates should be extended to enable a proper analysis of the associated grounding system currents and potentials. A separate ground connection node is introduced through which the transformer interacts with the substation grounding system. The model proposed yields the transformer terminal phase variables in their natural form, reflecting the voltage ratios and phase shifts introduced by transformer winding connections.

Transformer Phase Coordinate Models Extended for Grounding System Analysis

The transformer model in phase coordinates should be extended to enable a proper analysis of the associated grounding system currents and potentials. A separate ground connection node is introduced through which the transformer interacts with the substation grounding system. The model proposed yields the transformer terminal phase variables in their natural form, reflecting the voltage ratios and phase shifts introduced by transformer winding connections.

Evaluation of Maximum Ground Grid Currents with the EMTP

This paper describes an application of the EMTP (Electromagnetic Transients Program) for the design of substation grounding systems. The power system circuit model is presented, as well as its EMTP implementation. Several case studies are used to show the suitability of the program for this task.

Grounding Effects of HV and MV Underground Cables Associated with Urban Distribution Substations

The paper derives a model for the evaluation of the performances of composite grounding systems of urban main distribution substations and associated cable networks. The effects of the cables upon the dangerous voltages, transferred potentials, and ground fault current distribution are encompassed, including the conductive and magnetic coupling among grounding system elements as well as the nonlinearity of cables sheaths impedances. An application example is presented and some experimental data.

Novel method for analyzing dynamic behavior of grounding systems based on the finite-difference time-domain method

It is important to analyze the dynamic behavior of grounding systems of substations and power systems in order to study lightning protection. In this paper, the authors propose a novel method in which the dynamic behavior is analyzed based on the finite-difference time-domain (FD-TD) method and offer a comparison between experimental results of the dynamic behavior and analytical results.

A practical method for evaluation of ground fault current distribution on double circuit parallel lines

The paper presents an original analytical procedure for quick and, for practical purposes sufficiently accurate evaluation of the principal components of the ground fault current. The procedure is valid for faults at any of the towers of a double 3-phase circuit line with an arbitrary number of spans. The method is obtained by application of relatively simple and exact equations for uniform ladder circuits of any size (for any number of pis, from one to infinity) and for any terminal conditions. The method can be applied to solve several practical problems: the evaluation of the maximum substation grounding system fault current, the selection of a ground wire capable of withstanding the fault currents and the prediction of step and touch voltages near the transmission towers. In the case of a double circuit 3-phase line, the solution of these problems is additionally complicated by the mutual inductive coupling between the two parallel lines.

A boundary element formulation for the substation grounding design

A Boundary Element approach for the numerical computation of substation grounding systems is presented. In this general formulation, several widespread intuitive methods (such as Average Potential Method (APM)) can be identified as the result of specific choices for the test and trial functions and suitable assumptions introduced in the Boundary Element Method (BEM) formulation to reduce computational cost. While linear and parabolic leakage current elements allow to increase accuracy, computing time is drastically reduced by means of new completely analytical integration techniques and semi-iterative methods for solving linear equations systems. This BEM formulation has been implemented in a specific Computer Aided Design system for grounding analysis developed in the last years. The feasibility of this new approach is demonstrated with its application to a real problem.

Testing and evaluating grounding systems of high voltage energized substations: alternative approaches

The paper presents the results of tests performed in the grounding system of an energized high voltage substation. Three different methods for the grounding evaluation are discussed and compared. Results show the promising capability of the alternative testing methods as well as the data processing methodologies. A new named the transition method, has proved to have some advantages over the currently used methods.

A new method to decrease ground resistances of substation grounding systems in high resistivity regions

Decreasing the ground resistance of a grounding system in high resistivity soil is often a formidable task. A new method is proposed to decrease ground resistances. This method requires three steps: (1) drilling deep holes in the ground; (2) developing cracks in the soil by means of explosions in the holes; and (3) filling the holes with low resistivity materials (LRM) under pressure. Most of the cracks around the vertical conductors will be filled with LRM and a complex network of low resistivity tree-like cracks linked to the substation grid is formed. The principle of decreasing ground resistances using this new method is described and the effectiveness of the new method is examined. Application of this method to power system grounding is presented together with measurement results.

A boundary element numerical approach for grounding grid computation

Analysis and design of substation earthing involves computing the equivalent resistance of grounding systems, as well as distribution of potentials on the earth surface due to fault currents [1,2]. While very crude approximations were available in the 1960s, several methods have been proposed in the last two decades, most of them on the basis of intuitive ideas such as superposition of punctual current sources and error averaging [3,4]. Although these techniques represented a significant improvement in the area of earthing analysis, a number of problems have been reported; namely: large computational requirements, unrealistic results when segmentation of conductors is increased, and uncertainty in the margin of error [4]. A Boundary Element approach for the numerical computation of substation grounding systems is presented in this paper. Several widespread intuitive methods (such as the Average Potential Method) can be identified in this general formulation as the result of suitable assumptions introduced in the BEM formulation to reduce computational cost for specific choices of the test and trial functions. On the other hand, this general approach allows the use of linear and parabolic leakage current elements to increase accuracy. Efforts have been made particularly in getting a drastic reduction in computing time by means of completely new analytical integration techniques, while semi-iterative methods have proven to be specially efficient for solving the involved system of linear equations. This BEM formulation has been implemented in a specific Computer Aided Design system for grounding analysis developed within the last years. The feasibility of this approach is finally demonstrated by means of its application to two real problems.