B. Thapar

Simplified equations for mesh and step voltages in an AC substation

A large number of grounding grids in substations have shapes other than a square or a rectangle, whereas the simplified formulas for mesh and step voltages available in the literature are derived for square or rectangular grids. Improvements in the simplified equations for determining mesh and step voltages are presented. With the modified simple equations, mesh and step voltages at a substation of any practical shape can be estimated with reasonable accuracy. The results obtained with the improved equations have been compared with accurate results obtained via computer. The error in the values obtained with the derived equations is within 16% for mesh voltage and less than 30% for step voltage. >

Evaluation of ground resistance of a grounding grid of any shape

A large number of grounding grids in substations have shapes other than a square or a rectangle, whereas the formulas and graphs for the ground resistance of a grounding grid available in the literature are applicable only to nearly square or rectangular grids. A new simple equation for the estimation of ground resistance applicable to any practical shape of the grid is presented. The results obtained with this equation have been compared with accurate results obtained via computer and with the results of analog model tests. The value of ground resistance obtained with the proposed method closely approximates that obtained from elaborate and time-consuming computer methods. >

Scale Model Studies of Grounding Grids in Non-Uniform Soils

A new improved scale model to study the behaviour of grounding systems in non-uniform (two layer) soils has been developed. Constructional details and instrumentation has been discussed. To verify the accuracy of the results obtained from the tests conducted on the model, they have been compared with the data available in the literature. The results giving the potential profile, grounding resistance, mesh and step potentials for a number of grids in two layer soils are presented.

Ground resistance of the foot in substation yards

Equations and graphs given in IEEE Std 80 to determine ground resistance are not accurate and give conservative results. More accurate equations and graphs for determining the ground resistance of the human foot in a substation with a thin surface layer of gravel are given. An analysis has been made for models of the foot. In one model the foot is represented by an equivalent circular plate and in the other by an equivalent rectangular plate. Results obtained with these two models and with the IEEE Std 80 method are compared. >

Ground resistance of concrete foundations in substation yards

The reinforced concrete foundations in a substation yard have extensive reinforcing steel in metallic connection with the ground conductor. The foundations may contribute significantly to the dissipation of the fault and leakage currents. Simple-to-use equations and graphs are presented to determine the ground resistance of various types of foundation encountered in substation yards. The validity of the analytical approach has been verified with scale model tests. The ground resistance of the foundations, estimated as suggested in the paper, can be used to evaluate the role of the foundations as a grounding element. >

Finite Expressions for Computation of Potential in Two Layer Soil

New finite formulae to compute potential at any point due to a point current source located anywhere in a two layer earth are developed in this paper. Nonlinear programming is used to evolve the expressions such that the error with respect to a large data obtained from the usual infinite series expressions of potential is minimized. The formulae reported in the paper can significantly save the computational effort in the analysis and design of a grounding system with negligible loss in accuracy.

Reduction factor for the ground resistance of the foot in substation yards

Limited thickness of the surface layer of gravel in a substation yard decreases the ground resistance of the foot from its value for the infinite thickness of the surface layer, by reduction factor C. The foot is usually represented by an equivalent circular plate of radius 8 cm. This paper presents method and equations to accurately determine the reduction factor for the circular plate. The equations are valid for any depth of the upper layer. The data is presented in the form of graphs. A simple empirical equation for determining the value of C is also presented. >

Equivalent resistivity of non-uniform soil for grounding grid design

This paper develops a method to determine the equivalent resistivity of heterogeneous soils to be used in the available expressions for uniform soils employed to calculate the ground resistance, mesh and step voltages of substation grounding grids. The results obtained with the proposed equivalent resistivity are compared with the results obtained from the two layer and the multilayer models of the soil and with the results from a computer program, developed by the authors, directly based on the potential produced by a point source in heterogeneous soil. >

Effective ground resistance of the human feet in high voltage switchyards

The ground resistance of the feet, which is an important factor that determines the current flow in the body of a person exposed to dangerous touch voltage in a high-voltage switchyard, is modified by the proximity of the two feet and the presence of the energized grounding grid below the feet. Based on the Thevenin approach, a general method for determining the effective ground resistance of the feet that takes into account the modifying factors is presented. The method is verified with the help of analog model study. Results show that in most practical situations the effect of the modifying factors can be ignored. The situations in which it is necessary to consider these factors are identified. >

Finite expression and models for footing resistance in substations

Footing resistance in a substation is affected by the limited crushed rock layer usually spread on the surface of the soil. A finite expression for calculation of the footing resistance is given. Two simple and fairly accurate models for determining the footing resistance are developed. The results obtained from the finite expression and the models are compared with the accurate results obtained from the infinite summation series. A method to calculate the decrease in footing resistance because of the accumulation of dirt at the lower portion of the crushed rock layer in old substations is also presented. >