Design of Earthing System for a Substation : A Case Study

Headnote

Abstract

This paper presents the design of earthing system for 132 KV substation and simulation for calculation of required parameters. This paper is to provide information pertinent to safe earthing practices in ac substation design and to establish the safe limits of potential difference under normal and fault conditions. The grounding grid system of a practical 220 kV substation is calculated by MATLAB program. The supporting data has been obtained from actual field tested at the substation. Standard equations are used in the design of earthing system to get desired parameters such as touch and step voltage criteria for safety, earth resistance, grid resistance, maximum grid current, minimum conductor size and electrode size, maximum fault current level and resistivity of soil. By selection the proper horizontal conductor size, vertical electrode size and soil resistivity, the best choice of the project for safety is performed. This paper mentions the calculation of the desired parameters which are simulated by MATLAB program. Some simulated results are evaluated.

Keywords

component; Safe earthing system, Grounding, resistivity , AC substation, MATLAB program, Flowchart

(ProQuest: ... denotes formulae omitted.)

1. Introduction

In substation earthing system is essential not only to provide the protection of people working in the vicinity of earthed facilities and equipments against danger of electric shock but to maintain proper function of electrical system. Reliability and security are to be taken in considerations as well as adherence to statutory obligations (IEEE and Indian standards on electrical safety [1-2] and environmental aspects). Earthing system thus design must be easily maintained and future expansion must be taken into account while designing the dimensions of earth mat.

This paper is concerned with earthing practices and design for outdoor AC substation for power frequency in the range of 50 Hz. DC substation GIS and lightening effects are not covered in this paper. With proper caution, the method described here is also applicable to indoor portion of such substation. By using proper conductor and electrode size, earthing system may be able to overcome lightening effects.

2. Earthing system for a substation

A. Components of earthing system

An effective substation earthing system typically consists of earth rods, connecting cables from buried earthing grid to metallic parts of structures and equipments, connections to earthed system neutrals, and the earth surface insulating covering material briefly discussed in [1,3]. Current flowing into the earthing grid from lightening arrester operation impulse or switching surge flashover of insulators and line to ground fault current from the bus or connected transmission lines all cause potential differences between earthed points in the substation. Without a properly designed earthing system, large potential differences can exist between different points within the substation itself. Under normal circumstances, it is the current constitutes the main threat to personal.

B. Data needed for design of earthing

* Substation grid area

* Soil resistivity at the site

* Fault clearing time

* Maximum grid current

* Resistivity of soil at surface

C. Factors on which design of earthmat depends

* Materials used for earth electrodes and conductors must be chosen carefully taking into account physical, chemical and economical constraints. Ground conductor must be adequate for fault current (considering corrosion). Basic requirements are thoroughly studied in paper [4].

* Conductor sizing depends on fault current and conductivity as well as mechanical strength of material used.

* Resistivity of soil and surface layer determines the STEP and TOUCH potentials, which determine safe values of operation as described in reference [1, 5-9]. Also the multilayer resistivity has been a subject of continuous attention by the researchers [10-13].

* A good grounding system provides a low resistance in order to minimize GPR (ground potential rise)[14].

* Grid geometry is a major factor in determining the step, touch and mesh potential contours and current distribution in grid. The limitations on the physical parameters of a ground grid are based on economics and the physical limitations of the installation of the grid.

D. Fault Clearing Time

The faults clearing time is governed by system stability consideration and depend on protection and switchgear equipment. Generally a value of 0.5 seconds is assumed.

The size of the conductor is based on a time of 1 second.

E. Determination of Maximum Grid Current

A single line to ground fault is more common and causes more fault current as compared to a double line to ground fault. Therefore , the design is based on single line to

ground fault current. For calculation of this current grid resistance and fault resistance are assumed to be zero. Then Ig=SfIf

Where

Ig= symmetrical grid current , Amp

If= rms value of symmetrical ground fault current, Amp

Sf= current division factor relating the magnitude of fault current to that of its portion flowing in the earthing grid.

Only a part of the total fault current flows through the grid. The remaining current flows through the overhead ground wires and other ground return paths. The factor Sf gives this current division and can be calculated, if the system data is known, from the value of Ig the design value of grid current is found as

...

Where

I= maximum grid current

Gf= A factor to account for increase in fault current due to system growth during life span of grid.

Df= Decrement factor to account for asymmetry of the fault current wave.

Typical values of Gf assumed in design, lie in the range of 1.2 to 1.5 depending on the rate of growth of the system.

F. Selection of Electrode material

The material for grounding grid should have good conductivity, be mechanically rugged and resist fusing and deterioration of joints.

Copper was very commonly used in the past. It has high conductivity and is resistant to underground corrosion.

However, a grid of copper forms a galvanic cell with other buried structure and pipes and is likely to hasten the corrosion of the latter. Aluminium is not used because of corrosion problem.

3. Mathematical Description

G. Safe limits and values of potentials

The safety of a person depends on preventing the critical amount of shock energy from being absorbed. The maximum driving voltage of any accidental circuit should not exceed the limits defined as follows.

The tolerable step voltage criteria is

... (1)

The tolerable touch voltage criteria is

... (2)

Where

Cs = 1 for no protective surface layer

ρs= the resistivity of the surface material in A-m

ts = duration of shock circuit in seconds

The minimum conductor size formula is mentioned below-

... (3)

Where

...

tc = time of current flow in sec

TCAP = thermal capacity factor

For grounding resistance, the following formula is used

... (4)

Where

ρ = soil resistivity

Lt = total length of grid conductor

A = total area enclosed by earth grid

h = depth of earth grid conductor

For calculation of grid current, equation (5) is used

... (5)

Where

Ig = maximum grid current

3Io = symmetrical fault current in substation for Conductor sizing in A

Sf = current diversity factor

Equation (6) is expressed for grid potential rise (GPR)

... (6)

Maximum attainable step and touch voltages are given by: Formula for c-alculation of mesh and step voltage are

... (7)

... (8)

Where

ρ = soil resistivity, ohms-m

Em = mesh voltage at the center of corner mesh

Es = step voltage between point

Km = spacing factor for mesh voltage

Ks = spacing factor of step voltage

Ki = correct factor for grid geometry

... (9)

Where

D = spacing between conductor of the grid

d = diameter of grid conductor

Km = spacing factor for mesh voltage

Kii = 1 for grids with rods along perimeter

Kh = Corrective weighting factor for grid depth

... (10)

Where

D = spacing between conductor of the grid

h = depth of burial grid conductor

n = number of parallel conductor in one direction

The length required to keep mesh voltage within safe limits, can be calculated by

... (11)

If the length of conductor in the Preliminary design is less than that given eqn. (11) a revision in design is necessary.

H. Design procedure

Values of maximum attainable touch voltages are calculated. If the GPR of the preliminary design is below the tolerable touch voltage, no further analysis is necessary. Only additional conductor required to provide access to equipment grounds is necessary.

If the computed mesh voltage is below the tolerable touch voltage, the design may be complete. If the computed mesh voltage is greater than the tolerable touch voltage, the preliminary design should be revised. If both the computed touch and step voltages are below the tolerable voltages, the design needs only the refinements required to provide access to equipment grounds. If not, the preliminary design must be revised.

Further for the safe design we need to compare the length of the electrode in the preliminary design to the length of the electrode required to keep mesh voltage within safe limits

If either the step or touch tolerable limits are exceeded, revision of the grid design is required. These revisions may include smaller conductor spacing, additional ground rods, and use of closer spacing of conductor. In case the length given by eqn. (11) is too small to allow for connections to all equipments, more conductor may be required even though it is not necessary for safety ,etc as discussed in reference [1].

If the length of the conductor in the preliminary design is less than the length required to keep mesh voltage within safe limits. , then the design needs to be revised by entering more number of electrodes or by decreasing spacing between the conductors, otherwise the design is accurate. Further, if the Ground Potential Rise (R*Ig) is less than the tolerable value of touch potential then the design of the electrical grid is considered to be safe otherwise we need to guard personnel and other communication equipment against transferred potential.

Checking of step Potential:-

The tolerable step potential and actual step potential can be calculated from eqn (1) and eqn (8). If actual step potential is more than tolerable, a revision in design is necessary. The final step in design includes provision of closer meshes in the area frequently visited by operating personnel, additional ground rods for surge arrester leads, etc., and provision of risers for connection to equipments.

Impulse behaviour of earthing system:

The degree of protection provided by an earthing system under lightning discharge conditions depends on its impulse impedance (i.e.,impedance under impulse conditions) and not the power frequency resistance. The impulse impedance is different from the power frequency resistance due to effect of 1st soil ionization 2nd electrode inductance.

When the magnitude of impulse current is high, the soil in the vicinity of electrode may break-down causing a decrease in resistance. When the length of electrode is large, its inductance may cause only a part of the earthing system to be effective in dissipating current and, thus result in an increase in impedance.

The relative contributions of these two factors depend on the type of earthing system. The effect of soil ionization is very predominant in driven rods so that the impulse resistance of a driven rod is always less than its power frequency resistance.

The whole procedure can be shown by this flowchart:

4. Result

These results are obtained by a MATLAB program.

5. Significance and Touch Voltage

The step and touch voltages are dangerous for human body. Human body may get electric shocks from step and touch voltages. From the safety point of view it is necessary to calculate step and touch voltage. When high voltage substations are designed step and touch voltages should be calculated and must be in specified standard. The expected shock current caused by touch voltage can be effectively limited by applying inexpensive insulating layers on the earth's surface .The limits of step and touch potentials varies in different seasons because the resistivity of the soil layer is affected by different seasons .The step and touch voltage calculations are very significant while designing substation.

6. Conclusion

This paper has a focus on designing of an AC substation.The results for earthing system design are obtained by MATLAB program for earthing conductor and vertical earth electrode mild steel is used.

ACKNOWLEDGEMENT

The author wish to thank Er. O. P. Rahi for his guidance and moral support provided during this research effort, and Dr. M.G.Sharma for helping with substation data.

References

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