final presentation earthing 22

7/30/2019 Final Presentation Earthing 22

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Earth resistance:

Earth Resistance is the resistance offered by the earth electrode to the

flow of current in to the ground. The fault current is to be cleared as

quickly as possible and this is done by having the earth resistancelow. Persons touching any of the non current carrying grounded parts

shall not receive a dangerous shock during an earth fault. Each

structure, transformer tank, body of equipment, etc, should be

connected to earthing mat by their own earth connection.

Generally lower earth resistance is preferable but for certainapplications following earth resistance are satisfactory

In large power station - 0.5

Major Substation above 110KV - 1.0

Minor Substations Below 110KV - 2.0


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Monitoring the condition of earth

For monitoring the healthiness of earth, the condition monitoring equipment

used is EARTH MEGGER.

The megger is a portable instrument used to measure insulation resistance. Itis used to measure very high resistance of the order of mega ohms.

Checking and testing

The Earthing systems are to be inspected regularly.

Regular checking of joints and broken connections, if any and rectifying thesame will prove to be of immense help in maintenance of earth grid and

equipments.

The condition of the electrodes, joints are also to be checked.

Ifthe electrodes are corroded immediate

steps for replacement are to be taken. The earth resistance is to be measured

periodically.


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Earth Electrodes

A conductor buried in the ground, used to maintain conductors

connected to it at ground potential and dissipate current conducted

to it into the earth, known as earth electrode; grounding electrode.

Why must we have earth electrodes?

The purpose of the earth electrode is to connect to the general mass

of earth.The principle of earthing is to consider the general mass of

earth as a reference (zero) potential. Thus, everything connecteddirectly to it will be at this zero potential.

The effectiveness of an earth electrode in making good contact with

the general mass of earth depends on factors such as soil type,

moisture content, and so on. A permanently-wet situation may

provide good contact with earth, but may also limit the life of theelectrode since corrosion is likely to be greater


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Earth electrode types

Driven Rod

Grounding Plates

Electrolytic Electrode

Driven Rod

The standard driven rod or copper-cladrod consists of an 8 to 10 foot length of

steel with a 5 to 10-mil coating of copper.This is by far the most commongrounding device used in the field today.

Driven rods are relatively inexpensive topurchase, however ease of installation isdependent upon the type of soil and

terrain where the rod is to be installed.The steel used in the manufacture of astandard driven rod tends to be relativelysoft.Purpose of copper on the rod is to

provide corrosion protection for the steelunderneath.


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Grounding Plates

Grounding plates are typically thin copper plates buried in direct

contact with the earth. The National Electric Code requires that

ground plates have at least 2 ft2 of surface area exposed to thesurrounding soil. Ferrous materials must be at least .20 inches thick,

while non-ferrous materials (copper) need only be .060 inches thick.

Grounding plates are typically placed under poles or supplementing

counterpoises.


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Electrolytic Electrode

The electrolytic electrode was specifically engineered to eliminate the

drawbacks of other grounding electrodes. This active grounding

electrode consists of a hollow copper shaft filled with natural earthsalts and desiccants whose hygroscopic nature draws moisture from

the air. The moisture mixes with the salts to form an electrolytic

solution that continuously seeps into the surrounding backfill material,

keeping it moist and high in ionic content. The electrolytic electrode is

installed into an augured hole and backfilled with a special highlyconductive product.

The electrolytic solution and the special backfill

material work together to provide a solid connection

between the electrode and the surrounding soil that

is free from the effects of temperature, environment,

and corrosion


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Earthing ElectrodesA typical earthing electrode (left),

consisting of a conductive rod driven

into the ground


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Soil Resistivity Soil resistivity is a measure of how much the soil resists the flow of

electricity.

It is a critical factor in design of systems that rely on passing current

through the Earth's surface. An understanding of the soil resistivity and

how it varies with depth in the soil is necessary to design thegrounding system in an electrical substation.

In general there is some value above which the impedance of the earth

connection must not rise, and some maximum step voltage which must

not be exceeded to avoid endangering people and livestock.


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The soil resistivity value is subject to great variation, due to moisture,

temperature and chemical content.

Typical values are: Usual values: from 10 up to 1000 (m)

Exceptional values: from 1 up to 10000 (m)

SI unit of resistivity The SI unit of resistivity is the Ohm-meter (m); in the United States

the Ohm-centimeter (cm) is often used instead.

Measurement of soil resistivity

Wenner method

Schlumberger method


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Wenner method

The Wenner four-pin method, as shown in figure above, is the most

commonly used technique for soil resistivity measurements.

Using the Wenner method, the apparent soil resistivity value is:

where E = measured apparent soil resistivity (m)

a = electrode spacing (m)

b = depth of the electrodes (m)

RW = Wennerresistance measured as V/I in Figure ()


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Schlumberger method

In the Schlumberger method the distance between the voltages probe is aand the distances from voltages probe and currents probe are c.

Using the Schlumberger method, if b is small compared to a and c, andc>2a, the apparent soil resistivity value is:

where

E = measured apparent soil resistivity (m)

a = electrode spacing (m)

b = depth of the electrodes (m)

c = electrode spacing (m)

RS = Schlumberger resistance measured as V/I in Figure ()


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Measurement of soil resistivity


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METHODS TO IMPROVE SOIL RESISTIVITY

The following are the methods to lowering the earth

Lengthen the earth electrode in the earth

Use multiple rods.

Treat the Soil-

1. Using Bendonite Powder

2. Using Fly Ash3. Using Safe Earth Electrode(SSE)

4. Using Erico GEM


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EARTH MAT DESIGN Earthing System in a Sub Station comprises of Earth Mat or Grid,

EarthElectrode, Earthing Conductor and Earth Connectors.

Earth Mat or grid: Bonding all metal parts of the system to be earthed, the earth conductor and

the earth electrodes put all together form an Earth Grid.

Primary requirement of Earthing is to have a low earth resistance.Substation involves many Earthings thro individual Electrodes, which will

have fairly high resistance. But if these individual electrodes are inter linkedinside the soil, it increases the area in contact with soil and creates numberof parallel paths. Hence the value of the earth resistance in the interlinkedstate which is called combined earth value which will be much lower thanthe individual value. The inter link is made thro flat or rod conductor whichis called as Earth Mat or Grid. It keeps the surface of substation equipment

as nearly as absolute earth potential as possible. To achieve the primaryrequirement of Earthing system, the Earth Mat should be design properly byconsidering the safe limit of Step Potential, Touch Potential and TransferPotential.


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STEP POTENTIAL- which is the potential difference available betweenthe legs while standing on the ground.

TOUCH POTENTIAL -which is the potential difference between thelegand the hand touching the equipment in operation.

The factors which influence the Earth Mat design are

a. Magnitude of Fault Current

b. Duration of Fault

c. Soil Resistivity

d. Resistivitiy of Surface Material

e. Shock Duration f. Material of Earth Mat Conductor

g. Earthing Mat Geometry


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EARTH MAT

final presentation earthing 22

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