Earth Mat Design for Hv and Ehv Substation

EARTHING MAT DESIGN FOR HV & EHV SUBSTATIONS

1. INTRODUCTIONEarthing is essential wherever electricity is generated, transmitted and distributed

or user, to ensure safety and proper operation of the electrical system. It is well that the

earthing systems are intended to protect equipment and personnel in and around the

substation from the dangerous over voltage. An effective earthing system depends on

various factors like resistivity of surface layer of soil, duration and magnitude of fault

current, maximum safe current that a human body can tolerate and the permissible earth

potential rise that may take place due to fault current. Earthing in a substation effective

means to obtain and maintain low resistance value for providing easy path for flow of

fault currents and unbalance current flow through neutral. Design of proper equipment

for electrical substation grounding is important from the safety consideration of

personnel and equipment.

For the actual design of earth mat for a HV & EHV Sub-stations, a few numbers

of complicated formulae are involved. For arriving at step potential and touch potential to

be well within the safe limit for the given soil condition, area of the substations, fault

current and duration of fault current. An optimum design of earth mat can be arrived at

only by trial and error method repeating the calculation many numbers of times.

2. PURPOSE OF SUB-STATION EARTHING SYSTEMThe object of an earthing system in a sub-station is to provide under and around the

sub-station a surface which shall be at a uniform potential and near zero or absolute

earth potential as possible. The provision of such a surface of uniform potential under

and around the sub-station ensures that no human being in the sub-station is subject to

shock or injury on the occurrence of a short circuit or development of other abnormal

conditions in the equipment installed in the yard. The primary requirements of a good

earthing system in sub-stations are:

a. It should stabilize circuit potentials with respect to ground and limit the overall

potential rise.

b. It should protect life and property from over-voltage

c. It should provide low impedance path to fault currents to ensure prompt and

consistent operation of protective devices during ground faults.

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d. It should keep the maximum voltage gradient long the surface inside and around

the sub-station within safe limits during ground faults.

3. EARTHING SYSTEM3.1 The earthing system meeting the above requirements comprises an earthing mat

buried horizontally at a depth of about half-a metre below the surface of the ground

and ground rods at suitable points. All the non-current carrying parts of the

electrical equipment in sub-station are connected to the earthing mat. Under the

normal conditions, the ground rods contribute little towards lowering the ground

resistance. However, these are helpful in lowering mesh potential and maintaining

low values of resistance under all weather conditions.

3.2 The earth mat is connected to the following in a sub-station:

a. The neutral point of each system through its own independent earth.

b. Equipment framework and other non-current carrying parts.

c. All extraneous metallic frameworks not associated with equipment.

d. The earth point of Lightning Arreasters, Capacitive Voltage Transformers,

Coupling Capacitors and the lightning down conductors in the sub-station

through their permanent independent earth electrode.

e. Sub-station fence.

3.3 The earthing system installation shall strictly comply with the requirements of latest

edition of Indian Electricity Rules, relevant Indian Standards and Applicable Codes of

Practices.

4. PARAMETERS AFFFECTIVE THE DESIGN OF EARTHING MATSeveral variable factors are involved in the design of earthing mat conductor.

Therefore, earthing mat for each sub-station has to be designated individually usually.

The earthing mat has to be designated for the site conditions to have low overall

impedance and a current carrying capacity consistent with the fault current magnitude.

The parameters listed below influence the design of earthing mat:

a. Magnitude of fault current

b. Duration of fault

c. Soil resistivity

d. Resistivity of surface material

e. Shock duration

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f. Material of earthing mat conductor and

g. Earthing mat geometry

5. DESIGN PROCEDURE:The following steps are involved in the design of earthing mat:

a. The sub-station layout plan should be finalized before the design of earthing mat

is taken up. From the proposed layout of the sub-station, determine the area to

be covered by the earthing mat.

b. Determine the soil resistivity at the sub-station site. The resistivity of the earth

varies within extremely wide limits, between 1 and 10,000 ohmmeters. The

resistivity of the soil at many station sites has been found to be non-uniform.

Variation of the resistivity of the soil when depth is more predominant as

compared to the variation with horizontal distances. Wide variation of resistivity

with depth is due to stratification of earth layers. In some sites, the resistivity

variation may be gradual, where stratification is not abrupt. A highly refined

technique for the determination of resistivity of homogeneous soil is available.

To design the most economical and technically sound grounding system for large

stations, it is necessary to obtain accurate data on the soil resistivity and on its

variation at the station site. Resistivity measurements at the site will reveal

whether the soil is homogeneous or non-uniform. In case the soil is found

uniform, conventional methods are applicable for the computation of earth

resistivity. When the soil is found non-uniform, either a gradual variation or a

two-layer model may be adopted for the computation of earth resisivity.

The resistivity of earth varies over a wide range depending on its moisture

content. It is therefore, advisable to conduct earth resistivity tests during the dry season

in order to get conservative results.

6.MEARUREMENT OF EARTH RESISTIVITY:6.1Test Location:

In the evaluation of earth resistivity for sub-station and generating stations,

atleast eight test directions shall be chosen from the center of the station to cover the

whole site. This number shall be increased for very large station sites.

6.2Principle Tests:

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Wenner’s four-electrode method is recommended for these types of field

investigations. In this method, four electrodes are driven into the earth along a straight

line at equal intervals. A current is passed through the two outer electrodes and the

earth as shown in Figure 1 and the voltage difference V, observed between the two inner

electrodes. The current flowing into the earth produces an electric field proportional to

its density and to the resistivity of the soil. The voltage V measured between the inner

electrodes is therefore proportional to the field. Consequently, the resistivity will be

proportional to the ratio of the voltage to current i.e. R. The following equation holds for:

4SπRp = ------------------------------------------------- ..… (1)

2S S1 + --------------- - ---------------

√S2 + √4e2 √S2 + √e2 Where

p = Resistivity of soil in ohm-metre

S = Distance between two successive electrodes in metres

R = Ratio of voltage to current or electrode resistance in ohms

E = Depth of burial of electrode in meters

If the depth of burial of the electrodes in the ground is negligible compared to the

spacing between the electrodes, then

p = 2πSR .…. (2)

6.3Test Procedure:At the selected test site, in the chosen direction, four electrodes are driven into

the earth along a straight line at equal intervals, S. the depth of the electrode in the

ground shall be of the order of 10 to 15 cm. The megger is placed on a steady and

approximately level base, the link between terminals P1 and C1 opened and the four

electrodes connected to the instrument terminals as shown in Figure 1. An appropriate

range on the instrument is thus selected to obtain clear readings avoiding the two ends

of the scale as far as possible. The readings are taken while turning the crank at about

135 rev / min. Resistivity is calculated by substituting the value of R thus obtained in the

Equation (2). In case where depth of burial is more than 1/20th of spacing, Equation (1)

should be used instead of (2).

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7.Determine the Maximum Ground Fault Current:Fault current at the sub-station is determined from the system studies. A

correction factor is applied to the fault current thus determined to take care of the future

growth of the system. Value of this correction factor is usually of the order of 1.2 to 1.5.

However, in practice 40KA for 400 kV system and 31.5KA for 220/132 kV systems are

generally adopted for design purpose.

7.1 Duration of Fault:For the design of earth mat, the practices regarding assumption of duration of

fault differ from country to country. Thus in the USSR, the duration of fault is assumed

as 0.2 second. In the USA, it is assumed as 4.0 seconds, which is equal to the duration

on which the short time rating of the switchgear is based. In India, the short time rating

of most of the equipment is based on 1.0-second duration of fault. Therefore, 1.0

second may be adopted as the duration of fault in the calculations to determine the size

of conductor for earthing mat. For the purpose of determining the safe step and mesh

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potentials duration of 0.5 second may be adopted. However, it may be ensured on the

basis of the protective gear and protective schemes provided in each case that fault is

cleared in the period not exceeding of 0.5 seconds. Where the fault clearing time

exceeds 0.5 seconds, this duration may be taken equal to fault cleaning time.

8.Determine the size of conductor for earth mat:Size of conductor based on thermal stability: The size of conductor for earthing

mat based on thermal stability is determined with help of approximate formula as per

IEEE 80-1986 given below:

For welded joints A=12.30 *Isq.mm, for bolted joints A = 15.13 *I sq.mmm.

Where A = area of conductor

I = rms value of fault current in KA.

Assuming duration of fault current as 1.0 sec.

9.Mechanical Ruggedness of Conductor:From the consideration of mechanical ruggedness, and easy installation. The

maximum width to thickness ratio of steel flats for ground mat conductor should be 7.5

such that thickness of the flat is not less than 3 mm. Ground mat conductor comprising

steel rod having a diameter not less than 5 mm. The standard sizes of conductor as per

IS: 1730 – 1989 are as follows:

a. 10 x 6 mm2

b. 30 x 6 mm2

c. 50 x 6 mm2

d. 50 x 8 mm2

e. 75 x 12 mm2

f. 20 x 6 mm2

g. 40 x 6 mm2

h. 60 x 6 mm2

i. 65 x 8 mm2

9.1Corrosion:On average steel corrodes about six times as fast as copper when placed in soil. The

extent of corrosion depends upon the properties of soil. Many times, soil has conflicting

properties, some of which indicate that the soil is corrosive and other indicates the

opposites. Despite this, a very fair degree of correlation has been found between

electrical resistivity of soil and corrosion.

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10.Determine the Maximum Grid Current:The design value of the maximum grid current (IG) is given by the following equation:

IG = CPDFIg

Where Ig = Maximum grid current in Amperes

DF = Decrement factor for the entire duration of fault

Typical values of DF are given in the following table.

Fault Duration (S) Decrement Factor DF 0.008 1.650.1 1.250.25 1.100.5 or more 1.0

CP : Corrective projection factor for the relative increase of fault currents during the

station life span. For zero future growth of the system, CP = 1.

Ig: Sf(3Io)

Where

3Io: rms. Value of the symmetrical ground fault current in Ampere

Sf : Current division factor relating to the magnitude of fault current to that of its

portion flowing between the earthing mat and surrounding earth.

Sf is dependent on the following parameters:a. Location of fault

b. Magnitude of station earthing mat resistance

c. Buried pipes and cables in the vicinity of or directly connected, or both, to the

station earthing system.

d. Overhead ground wires or neutral conductors.

In the absence of full details regarding exact system configuration of which the

substation form a part, at the design stage, it will be fairly accurate to adopt a value of

0.5 for Sf to determine the fault current that flows through the grid to remote earth.

10.1 Resistivity of Surface Layer (s):Crushed rock is used as a surface layer in substations for the following reasons:

a. It provides high resistivity surface layer

b. It serves as impediment to the movement of reptiles and thereby helps in

minimizing the hazards, which can be used by them.

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c. It prevents the formation of pools of oil in the event of leakage of oil from oil

insulated and oil cooled electrical equipment.

d. It discourages the growth of weeds

e. It helps retention of moisture in the underlying soil and thus helps in maintaining

the resistivity of sub-soil at lower value.

f. It discourages running of persons in the substations and saves them from the risk

of being subjected to possible high step potentials.

In tropical countries like India, where the population of reptiles is large, it is

advantageous to surround the electrical equipment and the structures supporting

conductors by a surface layer of about 10cm of crushed rock up to a distance of about

two meters in all directions. Such surface layer around the metallic equipment and

structures, besides minimizing the hazards caused by reptiles, provides a high resistivity

layer below the feet of human beings approaching the equipment / structures and

enables them to withstand higher touch potentials. If step potential without crushed rock

is well within safe limits, it is not necessary to spread crushed rock over the complete

substation area. However, if it exceeds the safe limits crushed rock of 15 to 20mm size

may be spread to cover the earth in the entire substation area.

If the type of rock to be used is known the lower value of resistivity for that type of rock

may be adopted in the design. Otherwise, in conformity with the design practices

followed by most of the electric utilities, an average resistivity value of 3,000 Ohm-metre

may be adopted for the purpose of earthing mat design.

11.Determine the tolerable touch and step potentialsThe values of these potentials depend on the body weights, thickness and

resistivity of surface layer and duration of shock current.

A preliminary earth mat arrangement is developed on the basis of an assumed

spacing between two parallel conductors. In this arrangement a continuous conductor

should be assumed as surrounding the substation and the conductor within it should be

located at reasonably uniform spacing parallel to each other along the rows of the

structures, equipments etc. From the arrangement so arrived at the number of parallel

and cross conductors and the total length of conductor constituting the earth mat are

determined for use in the further design calculations.

The values of the expected maximum mesh and step potentials are calculated

with the help of the following formulae.

Etouch = (1000 +1.5 Cg (hgK)g) 0.116/ts

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Estep = (1000 +6Cg (hgK)g) 0.116/ts

Where,

Cg =1 for crushed rock resistivity equal to that of soil resistivity. If crushed rock

resistivity does not equal to soil, reference may be made in figure.2 for obtaining the

value of Cg

K = -g/ +g

g = Resistivity of surface layer in ohm – metre

= Resistivity of soil in ohm – metre

ts = Duration of shock current flow in seconds

hg = Surface layer thickness in metre

Cg = reduction factor for derating the normal value of surface layer resistivity determined

as follows. Cg =1 for crushed stone resistivity equal to soil resistivity

h = depth of earth mat conductor in metres

d = diameter of earth mat conductor in metres

Em = Km Ki Ig / L Volts

Ki = Corrective factor which accounts for the increase in current density in the grid

extremities

= 0.656 + 0.172 n

Ig = Maximum grid current in Amperes.

Km = 1/2 {In [D2/16hd + (d+2h)2/8Dd - h/4d] +Kii / Kh – In 8/ (2n-1)}

Kii = 1 for grids with earthing rods along the perimeter or for rods in that mat corners as

well as along the perimeter and throughout the grid area.

Kh = 1 + h/hg

= Soil resistivity in ohm – metre

hg = 1 metre (reference depth of earth mat)

d = Spacing between parallel conductors in metres

n = nAnB for calculating Em

nA = The number of parallel conductors in transverse direction

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nB = The number of parallel conductors in longitudinal direction

L = Lc + Lt for earth mat without earth rods or with only a few rods located within the mat

but away from the perimeter

= Lc+1.15L for earth mat with ground rods predominantly along the perimeter

Lc = Total earth mat conductor length in metre

Lr = Total earthing rod length in metre

Step potential Estep = Ks Ki Lg/L Volts

Where Ki = 0.656 + 0.172n

and K = 1/ {1/2h + 1/D+h + 1/D (1-0.5 n-2)}

L, h and D being the same as defined earlier and n being larger of nA and nB for

calculating Es.The value of expected mesh voltage and step voltage should be

determined for the following conditions in the order indicated below:

a. Without ground rods

b. With uniformly distributed ground rods

c. With ground rods only in the perimeter

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If the computed value of mesh voltage is less than the tolerable touch voltage,

the design of earth mat is completed. However, if the computed mesh voltage is

exceeding the tolerable touch voltage the design will require inclusion of ground rods or

revision. Similarly, the computed step voltage should also be less than the tolerable step

voltage. If either the step or touch voltage is found to exceed the tolerable voltages, the

earth mat design will have to be revised by including additional earthing rods, earth mat

depth reducing spacing etc. Additional earthing rod shoud be provided at the base of

lightning arresters and transformer neutrals.

For ground mat depths less than 0.25 metres: The value of the sub station

grounding resistance in uniform soil can be estimated by means of the following formula.

Rg = /4 /A +/L

Where Rg = station ground resistance in ohms

= Average earth resistivity in ohm metre

A = area under earth mat in square metres

L = the total length of buried conductors in metres

For ground mat depth between 0.25 and 2.5 metres: The station ground resistance for

ground mat with ground rods is determined with the Schwarz formula given below:

Rg = [1/L +1/20A {1+ 1/1+h20/A}]

12.ConclusionFrom this paper, it could be seen that for the proper and an optimum design of

earth mat for an HV/EHV substation can be arrived at only by trial and error method

repeating the calculation many numbers of times. But manual calculation is a time

consuming process, wherein software iterates the calculation by changing parameter like

spacing between earth conductors, number of earth electrodes. In order to arrive at the

optimum design of earth mat so that for a given substation, the attainable level of step

potential, touch potential and earth resistance is well within the tolerable value for the

given soil condition, area of the substations, fault current and duration of fault current.

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