Practical Methods for Analysis and Design of HV Installation Grounding Systems

Ljubivoje M. Popovic

• Publisher: Academic Press
• Year: 2018
• ISBN:
  9780128144619
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

Practical Methods for Analysis and Design of HV Installation Grounding Systems gives readers a basic understanding of the modeling characteristics of the major components of a complex grounding system. One by one, the author develops and analyzes each component as a standalone element, but then puts them together, considering their mutual disposition, or so-called proximity effect. This is the first book to enable the making and analysis of the most complex grounding systems that are typical for HV substations located in urban areas that uses relatively simple mathematical operations instead of modern computers.

Since the presented methods enable problem-solving for more complex issues than the ones solved using National, IEC and/or IEEE standards, this book can be considered as an appendix to these standards.

• Develops general equations of lumped parameter ladder circuits
• Includes the analytical expression for determination of ground fault current distribution for a fault anywhere along a cable line
• Presents measurement and analytical methods for the determination of actual ground fault current distribution for high-voltage substations located in urban areas
• Provides the analytical procedure for the determination of the critical ground fault position for faults appearing in outgoing transmission lines
• Defines testing procedure for the correct evaluation of grounding systems of substations located in urban areas

Chapter 1

Introductory Considerations

Abstract

Chapter 1 presents the introductory and general considerations regarding the grounding and grounding systems of high-voltage (HV) substations. This chapter also gives a brief overview of the historical development and the current stage of the methods available for analysis and design of grounding systems of HV substations. This overview, inter alia, involves a brief comment on the methods presented in comparison with the methods available in the corresponding international standards. Since the analytical methods presented in this book can solve the practical problems that cannot be solved by the methods presented in the standards, the main conclusion is: the methods presented here enlarge the application field in comparison with the methods offered though contemporary international standards.

Keywords

External grounding electrodes; Ground fault current distribution; Ground grid; Ground grid potential rise; Grounding; Grounding resistance; Grounding system; Safety criteria; Touch voltage

1.1. Grounding in General

From the very beginning of massive electrification and the widespread use of electrical energy, i.e., since the first decade of the last century, one of the most serious problems set before engineers/designers was how to connect electrical equipment and installations to the ground safely and economically. The problem of grounding has appeared in almost all areas of electrical engineering, from low-voltage stationary equipment and apparatus to electrical installations of high-installed powers. Because of ever-increasing power consumption and the associated developments of power systems, short-circuit currents have become increasingly dangerous and harmful, so a properly designed grounding system appears as an imperative not only for people working in high-voltage (HV) substations, but also for those working on nearby metal installations, such as water pipes, gas installations, telecommunication lines, railways, etc.

The two most important reasons why the grounding problem at the design stage of an electric power installation deserves a serious approach are because it:

1. • provides protection against electric shocks for the people inside, or in the vicinity, of the designed electric power installation in the case of a ground fault, and
2. • conducts a ground fault current into the surrounding earth without any undesirable consequences affecting the safe and reliable operation of the whole power system.

When a ground fault occurs in an electric power network, the earth as a specific conductive medium inevitably becomes a constituent part of the accidentally formed electric circuit. It acts as one of the available return paths of the ground fault current to its sources within the electric power system. However, from the standpoint of its physical appearance the earth as a conductive medium represents a three-dimensional body. Thus the freely involved space and mode of current flow through the earth is not simple, as it is in the case of well-known linear conductors used for constructing electric circuits for transmission or distribution of electrical energy. In selecting the minimum resistance opposing its flow, the current is dissipated with different intensities in all directions, creating widespread potential fields at the points of sinking to earth or coming to its surface. Certainly, under an unrealistic assumption that the earth is a conductive material, i.e., some kind of metal, the mentioned potential fields would not appear and the whole problem of electric power installation grounding would not matter at all. However, this is not the case in practice.

The electric conductivities of materials prevailing in the structure of the earth are very low compared to those of metals. Two basic constituents of earth, silicon and aluminum oxides, have electrical properties of insulators and the conductivity of the earth is due to the salts and humidity embedded between these two insulators. Although a relatively poor conductor by its structure, the earth is capable of conducting very high currents. Since the cross-section of the soil involved by current flows is freely formed, in view of the enormous dimensions of the earth it can enlarge practically with no limits. Only at the points of sinking or coming to the earth’s surface is this cross-section limited to the contact surface between the grounding electrode and the surrounding earth. This is the reason why along the entire current path through the earth at these places the resistances to its flow are the highest. These resistances represent the grounding electrode resistances. For a given value of the surrounding soil resistivity and the shape and spatial position of a grounding electrode, the value of this resistance depends only on the magnitude of the electrode. Thus when we desire a lower value of this resistance in a concrete case, we must enlarge the dimensions of the considered grounding electrode. The formulas for calculating this resistance of grounding electrodes of different magnitude, geometric shapes, and spatial positions are usually given in the corresponding technical standards.

The forms of equipotential contours of the electric field in the vicinity of an HV substation closely follow the external shape of its grounding electrode. Further away, their forms change and after a certain distance, if conductivity of the soil is homogeneous, their form becomes spherical, irrespective of the size and form of the considered grounding electrode. Since people are normally situated on the surface of the soil, for solving practical problems of significance are only the equipotential curves, i.e., the potential distribution, at the earth’s surface and the potentials of grounded metal constructions that could become the subjects of human touch. Practically, this means that by selecting an adequate magnitude and shape of the substation ground grid, we can obtain the most favorable surface potential distribution for a priori known equipment disposition. As an illustration, the potential distribution on the surface of the soil of a typical ground grid laid in a homogenous soil is shown in Fig. 1.1.

The notation used has the following meaning:

1. • U_t and U_s—touch and step voltages, and
2. • V_g—potential of the substation ground grid, or ground potential rise [1].

From Fig. 1.1 one can obtain an idea of the influence of space position of individual elements of the grid upon potential distribution on the surface of the soil.

Calculations of the potential distribution on the surface of the soil, above and in the vicinity of a ground grid, are very complicated and extensive, even if it is assumed that the soil is homogeneous and the ground grid is of a geometrically uniform shape, i.e., the grounding conductors forming the grid are uniformly spatially disposed.

For the purpose of estimation of the achieved safety conditions, it is necessary to determine the value of potential V_g first. If its value is smaller than the maximal tolerable touch voltage, then the required safety conditions are obviously achieved (Fig. 1.1). However, if this criterion is not satisfied and the value of V_g is higher than the maximal tolerable touch voltage, in such a case the maximal calculated touch and step voltages become relevant for estimation of the achieved safety conditions. For a grounding grid of certain form and size the critical potential differences (the maximal touch and step voltages) can be estimated by using the simplified calculation procedure usually given in the corresponding technical standards, e.g., Ref. [1].

Figure 1.1 Potential distribution on the surface of the soil above the ground grid. (A) Ground grid and a power transformer disposition. (B) Potential distribution for a selected direction.

During a ground fault, the current flows and the corresponding raised potentials appear at places where they do not exist under normal operating conditions. Hazardous voltages appear as a consequence of excessive potential differences between points at such distances that can be simultaneously bridged by the human body. These are caused by potential gradients at various directions along the surface of the soil and potential differences between grounded metal structures and the surrounding soil (Fig. 1.1). For the purpose of establishing unique criteria for the estimation of safety conditions, these potential differences have been standardized by introducing precisely defined terms of the touch and step voltages. At the design stage of an electric power installation, it is required to show by calculations that these voltages, at any place within or in the vicinity of this installation, under conditions of the worst (critical) ground fault, will stay within the prescribed limits. The maximum permissible values of the touch and step voltages are dependent on the time duration of the current exposure and are strictly determined by the corresponding technical standards/guidelines, [1].

The problem of estimation of safety conditions in the case of substations operating in medium voltage (MV) networks having neutral point(s) isolated or grounded through a resistor/reactor, i.e., where the ground fault current that is restricted to an a priori adopted and relatively...