COMPUTER-BASED MODEL FOR DESIGN OF A 132/33kV SUBSTATION EARTHING SYSTEM
A.O. Ibe, E. Sofimieari, and Uma Uzubi
Department of Electrical/Electronics Engineering, University of Port Harcourt, Port Harcourt
This paper presents the design of earthing system for 132 / 33kV substation and its simulation for required parameters using MATLAB GUI. This paper reviews earthing practices with special reference to safety and also provides guidance and information pertinent to safe earthing practices in AC substation design. Standard Equations are used in determination of design parameters, such as step and touch potential, mesh and grid potential, diameter of required conductor, substation resistance, the number of conductor in x and y axis of the mesh and total length of conductor required.
Keywords:Earthing design, Earth mat and MATLAB program.
Earthing system, sometimes simply called “earthing” is the total set of measures used to connect an electrically conductive part of earth (Gutta, 2007). This is realized by connecting the non current carrying metallic parts in every electrical installation to the underground electrical conductor or electrode placed in intimate contact with the soil some distance below the ground level (IEEE, 1985). The intent of this paper is to provide information pertinent to safe earthing practices in AC substation design. The objective of an earthing system for an electrical installation can be classified into four groups
Ø To ensure that, the non current carrying metal work of the electrical equipment does not attain a dangerous potential with respect to the general mass of earth when fault occurs.
Ø To allow the easily flow of large fault current into the ground mat. So that the protective equipment has time to operate and thus isolate the faulty circuit in the event of fault.
Ø To limit the potential of any part of an installation to a pre – determine value with respect to the general mass of the earth.
Ø Provide earthing for lightning impulses and the surges occurring from the switching of substation equipment, which reduces damage of equipment.
Grounding resistance should be low enough to permit the flow of fault current. The resistance of the mat should not be of such a magnitude as to permit the flow of fatal current in the live body (IEEE, 1976).
This paper briefly shows the procedures for measuring the resistance of the installed earthing system, diameter of conductor required for the design and total length of conductor for the design. The ultimate of goal of this paper is to design earthing system to safety. To evaluate and simulate grid conductor size, vertical and horizontal electrode size, permissible potential difference and required facts for design procedure by using MATLAB Programme.
Basic design requirements
Ø No earth plate should be less than 2ft 2ft in area and inch for copper, so the plates are buried at one foot below the permanent level.
Ø If lower resistance is required for an installation, more than two plates are connected in parallel keeping each plate at least 10 feet away from each other.
Ø If electrical apparatus is present then at least two plates are buried at maximum distance which is never less than 10 feet.
Earthing design equations
Measurement of soil resistivity
The resistivity of the soil is measured by the formula
= resistivity of soil in
s = horizontal distance between two successive spikes in (m)
b = depth of electrode in meters
I = known current passed between electrode
R=resistance in ohms
V = potential drop across electrodes.
For equation (1) becomes
Calculation of step voltage limit
This is the potential difference between the feet of a person standing on the floor of substation, with 0.5m spacing between his feet, during the flow of earthing fault current to the grounding system. It can be calculated by the equation.
= resistivity of the surface material in
t = duration of shock circuit in seconds
Calculation of touch voltage limit
This is potential difference between the fingers of raised hard touching the faulted structure and the feet of the person standing on substation floor. The person should not get shock even if the ground structure is carrying fault current. The expression below can be used for its calculation.
Minimum conductor size
It can be calculated using the equation below;
= conductor sectional size in mm2
= maximum allowable temperature in OC
Ta = ambient temperature for material constants in OC
= thermal coefficient of resistivity at O0C
= thermal coefficient of resistivity at reference temperature