1. Introduction

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 (topology and dimensions of the grounding system, burial depth, type and characteristics of the soil structure and material used for the grid’s conductors, whether or not grounding rods are attached to the grid, etc…) that can affect the grounding system performance. The AutoGroundDesign specialized software package offers powerful and intelligent functions that help electrical engineers design safe grounding systems quickly and efficiently without the intervention of the user between various phases of the design. AutoGroundDesign takes into account the full complexity of the system with strategies and techniques that have been developed to allow the automated design to be applied to horizontal arbitrary shape grounding systems. The screen to specify the vertices of any horizontal arbitrary grounding system is shown below.

2. Features

AutoGroundDesign has the following unique features:

• Generates grounding system designs based on a simple description of the substation site. The data entry requirements are reduced to a minimum environment settings, soil data, grounding grid zone, fault current in the grid, safety related data, and automated design parameters and controls.
• Models grounding systems and evaluates their performance; it is suited to analyze and design a grounding grid as long as the longitudinal impedances of the ground conductors can be neglected.
• Analyzes and designs horizontal arbitrarily shaped grounding grids consisting of horizontal and vertical arrangements of bare conductors buried in uniform and multilayered soils. This is achieved with an automatic meshing method that accounts for any polygonal shape. A user-friendly interface is also available to simplify the specification of the horizontal arbitrary grounding zone. The shape of this zone can be defined and displayed dynamically from the interface. Each vertex can be moved by dragging it to a new position and the data is updated immediately in data grid and vice-versa.
• Carries out automated designs with several procedures, such as Automatic, Midpoint, Linear, and User-Defined methods. These procedures will specify the performance and progress of the automated design process appropriately and use ground grid databases, smart search algorithms and techniques, and user-supplied criteria and constraints more efficiently.
• Allow users to specify if ground rods are to be used in the design of the final grid and rod characteristics. If yes, positioning methods are available.
• Computes earth potentials at specific soil locations called observation points which can be defined automatically or by the use.
• Offers three other modes of operations, namely, the Estimator, Configuration and Dimension Predictor modes that allow users to quickly and accurately estimate the resistance of various grounding systems (such as grids, plates array of rods, star electrodes, circular rings, etc.) or predict the size (dimension) or configuration of the grounding system that meets that resistance.
• Intermediate results from the grounding, soil resistivity and fault current distribution analyses are included as an integral part of the final results in order to better understand these results by examining all the steps involved in the automated design process.

3. Program Development History

SES implemented the first automated grounding grid design software in the early 90’s. It had a character-based (DOS-based) menu interface and was called AutoGrid. It was restricted to rectangular grids buried in uniform and two-Layer Horizontal soils. Computation time was the main obstacle to the systematic use of this automated feature.

In 2004, SES developed the first version of a Windows-based specialized package called AutoGroundDesign. This was more pertinent, since SES has developed a new and unique automated design method that promises to reduce considerably the time needed to determine an adequate design for grounding systems. However, AutoGroundDesign was restricted to rectangular grids buried in multilayered soils. This version didn’t have a soil resistivity measurement and fault current distribution analysis modules. The soil structure characteristics and grounding grid fault current had to be specified by the user. These steps were quite inconvenient.

In the 2005 release of AutoGroundDesign, the soil resistivity measurement analysis, fault current distribution analysis and safety computation assessment modules have been integrated into the AutoGroundDesign specialized package. A new iterative approach and improved observation point selection procedure that speed up computation time have been introduced into this package. Moreover, metallic plates have been introduced and a robust and flexible grid and rod creation procedure that allows the specification of unequally spaced grids and rods were added as well. However, AutoGroundDesign was still restricted to rectangular grids.

Finally, SES is pleased to announce that in 2012, AutoGroundDesign can handle horizontal arbitrary shape grounding systems.

4. Grounding System Design Technologies and Procedures

Consider the traditional process of designing a substation grounding system. Based on experience and on the substation ground bonding requirements, a preliminary grounding system configuration is developed and analyzed. The calculated results are examined to determine if all design requirements are met. If all design requirements are not met or if these requirements are exceeded by a considerable margin suggesting possible significant savings, design modifications are made to the grounding system and the design analysis is started again.

To produce an optimized design, better knowledge of the soil structure and the actual fault current flowing into the substation is needed. Also, a large number of factors such as the geometrical proportions of the grid, its depth, the type of grid conductors and whether or not grounding rods are attached to the grid are essential.

A multiple-step approach is used for the automated grounding system design.

• First, a grounding system consisting of a buried metallic plate is used as a reference. This gives the minimum ground impedance achievable with a grid of a given size, and determines whether the desired ground impedance and safety limits can be achieved with a solid plate. If not, then the whole process is stopped and the user is informed that the design process is impossible without additional mitigation measures.

• Second, a grounding system consisting of a minimum number of conductors, for example, the conductors along the grid periphery with or without a few conductors inside the grid is analyzed to see whether the desired ground impedance and safety limits can be achieved with a sparse grid. If yes, the design process is completed quickly without the need to refer to the reference design database and smart iterative techniques.

• Third, an appropriate preliminary grid design is selected based on SES reference database and other intelligent rules or as specified manually by the user. The use of the reference database is based on the input data provided by the user, such as the size and the geometrical proportions of the grid, the soil structure type, the fault current injected, and the required safety criteria.

• Finally, the initial design is refined recursively using rule-based techniques and smart algorithms to improve its performance and meet safety constraints, while reducing the overall cost of the grid.

Some of the unique strategies and techniques used in AutoGroundDesign are as follows:

• A method of automatic meshing a set of a horizontal arbitrary polygonal grounding system was developed. The mesh parameters are then subject to optimization using a technique similar to that previously developed for rectangular grids.

• New positioning methods for the rods were introduced. Among others, it is now possible to specify the maximum spacing between the rods. These methods allow finer control of where the algorithm locates the stems.

• Efficient and intelligent techniques have been developed to generate different types of observation points in order to accurately carry out computations and at the same time minimize the computation time during the iterative design steps and the final results computation steps. During the iterations, three different groups of observation points have been created. The first group of observation points is within the horizontal arbitrary grounding zone and is placed along one horizontal direction. The second group of observation points is along the edge of the grid and inside the zone. The last group of observation points is outside the zone occupied by the grounding system and is placed along its edge. In the final results, the observation points occupy a rectangular area covering the bounding box of the horizontal arbitrary grounding system (i.e., the smallest rectangle that completely encloses the grounding system zone). This provides a complete set of observation points that produces conventional rectangular 3D and spot 2D plots that are useful to explore the computation results.

• The preferred observation point settings (i.e., the preferred point spacing) during the iterations and final computation steps can be defined automatically or by the user. This offers more flexibility to control the time required during the computation process.

5. Automated Grounding System Design Structure

The automated grounding system design software integrates the following modules and has a structure shown below.

Automated Grounding Design Central Module

This core and controlling module has a simple interface that allows a user to establish an automated grounding system design quickly and efficiently. The ultimate objective of this module is to manage and coordinate input data, safety criteria and progress decisions in order to obtain a grid design that meets all requirements. The overall automated design parameters are controlled by this module to select the methodology used to obtain the initial design of the grounding systems, specify which grid database methodology is to be used for the automated design, and specify the maximum number of design iterations as well as the rate at which the design of the grid evolves.

Grounding Analysis Module

This module is used to analyze power system ground networks subjected to AC or DC power frequency currents discharged into various soil structures. It computes the safety performance of the grounding grid, in terms of GPR, touch and step voltages. Since it is assumed that the grid is an equipotential structure, the locations of the current injection points within the ground network do not play a significant role, i.e. the longitudinal impedances of the ground conductors can be neglected.

Soil Analysis Module

This module is dedicated to the development of equivalent earth structure models based on measured soil resistivity data. It can generate models with many horizontal layers, as well as vertically and exponentially layered soil models.

Fault Current Distribution Analysis Module

This module calculates the fault current distribution in multiple terminals, transmission lines and distribution feeders using minimum information and a simple set of data concerning the network. It provides the actual fault current flowing into a grounding grid, as well as currents in the shield wires, tower structures and cable sheaths. Self and mutual impedances of the shield wires and cable sheaths are also computed and available.

Safety Module

This module generates safety threshold values based on IEEE Standard 80, IEC Standard 479, user’s own standard or a hybrid combination of these standards. The computed safety voltage limits are used to decide whether to stop or continue the design process. The parameters to determine the safety voltage limits are: fault clearing time, earth surface covering layer (e.g., crushed rock) resistivity, and thickness, equivalent subsurface layer resistivity (this is the resistivity of the soil beneath the earth surface covering layer), body resistance, optionally specified foot resistance and resistance of protective wear, such as gloves or boots, and fibrillation current threshold computation method.

View, Plot and Report Tools

A CAD-based module is used to view or edit three-dimensional grounding grids consisting of straight-line segments. The line segments represent either metallic conductors or observation profiles. They can be viewed from any direction, in a variety of ways. Another report and graphics module serves as a powerful output processor to display the computation results in various graphical or print formats. This module also has the capability to view the input data and even launch the grounding analysis module.