This chapter contains the following points: design procedure of grounding system according to IEEE 80, methods for calculating the grounding grid resistance (Dwight's formula, Laurent and Niemann, Sverak's equation, Schwarz's Formula, Dawalibi, Mukhedkar's Formula, Chow and Salama's Formula, Nahman's Formula and Heppe's Method). It contains also the design of charts of grid earthing system and application of step and mesh potential in safe grounding system design. This chapter draws attention also to the following points: Grounding resistance of grounding system in non-homogeneous soil, calculations of maximum step and mesh voltages, estimation of minimum buried grid conductor length and finally computerized analysis in grounding design.
TopDesign Criteria
There are two main design goals to be achieved by any substation ground system under normal as well as fault conditions. These are:
- 1.
To provide means to dissipate electric currents into the earth without exceeding any operating and equipment limits
- 2.
To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.
The design procedures described in the following sections are aimed at achieving safety from dangerous step and touch voltages within a substation. It is possible for transferred potentials to exceed the GPR of the substation during fault conditions. The design procedure described here is based on assuring safety from dangerous step and touch voltages within, and immediately outside, the substation fenced area. Since tine mesh voltage is the worst possible touch voltage inside the substation (Excluding transferred potentials).
The mesh voltage will be used as the basis of this design procedure. Since the mesh voltage is the worst possible touch voltage (excluding transferred potentials), the mesh voltage will be used as the basis of design procedure.
Step voltages are inherently less dangerous than mesh voltages. If, however, safety within the grounded area is achieved with the assistance of a high resistively surface layer (crushed rock), which does not extend outside the fence, then step voltages may be dangerous.
In any event, the computed step voltages should be compared with the permissible step voltage after a grid has been designed that satisfies the touch voltage criterion. For equally spaced ground grids, the mesh voltage will increase along meshes from the center to the corner of the grid. The rate of this increase will depend on the size of the grid, number and location of ground rods, spacing of parallel conductors. Diameter and depth of the conductors, and the resistivity profile of the soil. In a computer study of three typical grounding grids in uniform soil resistivity, the data shown in Table (6.1) were obtained. These grids were all symmetrically shaped square grids with no ground rods and equal parallel conductor spacing.
TopCritical Parameters
The following site-dependent parameters have been found to have substantial impact on the grid design: maximum grid current (IG), fault duration (tf), shock duration (ts), soil resistivity (p), high resistivity surface material (ps), and grid geometry. Several parameters define the geometry of the grid, but the area of the grounding system, the conductor spacing, and the depth of the ground grid have the most impact on the mesh voltage, while parameters such as the conductor diameter and the thickness of the surfacing material have less impact. A brief discussion or review of the critical parameters is given below.
Maximum Grid Current (IG)
In determining the maximum current IG consideration should be given to the resistance of the ground grid, division of the ground fault current between the alternate return paths and the grid, the decrement factor, and the future expansion of the power system.