500 kv substation grounding presentation

7/28/2019 500 kv Substation Grounding presentation

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Name: Roll No:Rabbia Khan 2010-EE-13

Sidra Iqbal 2010-EE-17

Anum Shafqat 2010-EE-49


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INTRODUCTION


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Definition:Earthing means a conducting connection by which an electric circuit or

equipment is connected to the earth or some conducting body ofrelatively large extent that serves in place of the earth".

Earthing means the connection of non-current carrying parts toground.

Function:The function of an earthing system for an electrical installation can be

split into three broad bands, To limit the potential of any part of an installation to a pre-determine

value with respect to the general mass of earth.

To permit the flow of current to earth.

To ensure that, if a fault occurs, non-current carrying metal work

associated with the equipment does not attain a dangerous potential

with respect to the general mass of earth.


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Purpose of Earthing:Earthing system has 3 main purposes:

1-Over voltage protection

2-Voltage stabilization

3-Currentt path in order to facilitate the operation of over

current devices.

Reasons:There are a number of good reasons of earthing but primary among

them is to ensure personnel safety. A good earthing system willimprove the reliability of equipment and reduce the likelihood of

damage as a result of lightning or fault currents.


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SAFETY:

Main purpose of earthing is safety.

That is the protection of:1. Personnel Safety

2. Equipment Safety

Personnel Safety:

It is for human life safety.

Personnel protection from electrical shocks, fire etc.

Equipment Safety:

It includes electrical circuit or equipment protection from

failure, over current protection, fire, cable failures etc


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Grid Station Earthing:The sole purpose of substation grounding/earthing is to protect the

equipment from surges and lightning strikes and to protect the

operating persons in the substation.
http://www.amper.com.tr/yukleme/Image/earthing_systems.jpg


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TYPES


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CLASSIFICATION OF EARTHINGEarthing can be classified into the following categories based on the

purpose for which the part of the equipment connected to the general mass

of earth.

System Earthing

Equipment Earthing

SYSTEM EARTHINGEarthing associated with current carrying parts of the equipment is called system

Earthing. The system security, reliability, performance, voltage stabilization, all

relied only on the system Earthing. For example Earthing Neutral of

Transformer, Surge arrester Earthing

System Earthing Methods :

a. Solid Earthing

b. Resistance Earthing

Solid EarthingThis type of grounding system is most commonly used in industrial and

commercial power systems, where grounding conductors are connected to earth

ground with no intentional added impedance in the circuit.


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Resistance EarthingIn resistance earthing the resistance is added along the earthing conductor to

keep fault currents within limits. Thus protecting the insulation of the

conductor.

It has 2 types:

Low resistance Earthing (voltages below 150V)

High resistance Earthing (150-600V)

EQUIPMENT EARTHINGEarthing associated with non-current carrying parts of Electrical Equipment

are called as Equipment Earthing. Safety of operator, consumer, safety of

their property are mainly based on Equipment Earthing.Eg.Body of the Transformer, Body of Motor


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EARTHING DESIGN


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Earthing DesignThe substation ground grid design is based on the substation layout

plan. The following points serve as guidelines to start a earthinggrid design:

The substation should surround the perimeter and take up as much

area as possible. It reduces resistance of the earthing grid.Typically conductors are laid in parallel lines.Typical substation grid systems may include 4/0 bare copper

conductor buried 0.3-0.5 m (12-18 in) below grade and spaced 3-7

m (10-20 ft) apart in a grid pattern.All earth connections are to be made visible as far as possible.


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GROUND RODS (Electrodes)

Materials are selected for

corrosion resistance:

Galvanized steel rods are

cheap but have a relatively

short service life

Solid copper and stainless

steel rods have a long service

life but are considerably

more expensive

Copper bonded earth rods

are less expensive than solid

copper and can be deepdrivenComparison of life

expectancy


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Earthing Conductors:

The earthing conductor is commonly called the earthing lead. It

joins the installation earthing terminal to the earth electrode or

to the earth terminal provided by the Electricity Supply

Company. It is a vital link in the protective system, so care must be taken

to see that its integrity will be preserved at all times.

Mostly copper is used for earthing conductors because it

has the highest electrical conductivity of any of thecommercial metals. Copper is resistant to corrosion, that is,it will not rust. It is malleable, ductile and has long life.


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Earthing Conductors


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Types of earthing conductor

Protective conductor Bonding concuctor

Circuit Protective Conductor CPC

This is a separate conductor installed with each circuit and is present toensure that some, or all, of the earth fault current will flow back to source

along it.

Bonding Conductor

These ensure that exposed conductive parts (such as metal enclosures)

remain at approximately the same potential during electrical faultconditions.


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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 resistance. It is used tomeasure 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 the

same 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-ml 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. Grounding plates should be buried at least 30

inches below grade level. Non-ferrous materials (copper) need only

be .060 inches thick. Grounding plates are typically placed under

poles.


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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

Multiple Electrodes


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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 method is one of the widely used methods for measuring soil

resistivity.

In this method, four test rods are inserted a short distance into the soil in a

straight line with equal spacing between the probes. A test current is applied

to the outer probes and the resulting potential difference between the inner

probes, is measured.

The potential difference divided by the test current give an apparent

resistance in ohms. The apparent soil resistivity is obtained from the

measured resistance.


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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 ()

If b is small compared to a, as is the case of probes penetrating the ground

only for a short distance (as normally happens), the previous equation can

be reduced to:

E= 2aR

S hl b h d


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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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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 andthe earth electrodes put all together form an Earth Grid.

Primary requirement of Earthing is to have a low earth resistance.

Substation involves many individual Electrodes, which will have fairly highresistance. But if these individual electrodes are inter linked inside the soil,

it increases the area in contact with soil and creates number of parallel

paths. Hence the value of the earth resistance in the interlinked state which

is called combined earth value which will be much lower than the individual

value.

These Earth Mat and Earth electrode is connected to the equipment

structures, neutral points for the purpose of Equipment earthing and neutral

point earthing.It keeps the surface of substation equipment as nearly as

absolute earth potential as possible.


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EARTH MAT


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Potential Hazards


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Potential Hazards In electrical engineering, earth potential rise (EPR) also

called ground potential rise (GPR) occurs when a

large current flows to earth. The potential relative to a distant point

on the Earth is highest at the point where current enters the ground,

and declines with distance from the source. Ground potential rise is

a concern in the design of electrical substations because the high

potential may be a hazard to people or equipment.

The change of voltage over distance (potential gradient) may be so

high that a person could be injured due to the voltage developed

between two feet, or between the ground on which the person is

standing and a metal object


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Electrical Shock Situations:There are three main electrical shock situations that can occur when a person is

around a substation.

foot-to-foot shockhand-to-feet shock

hand-to-hand shock or metal-to-metal contact


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Step Potential:Step potential is the potential difference between the feet of a person

standing on the floor of the substation, with 1m spacing between the feet

(one step), through the flow of earth fault current through the grounding

system.


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Touch Potential:Touch potential is a potential difference between the fingers of raised hand

touching the faulted structure and the feet of the person standing on thesubstation floor. The Touch Potential should be very small.


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Point to be noted

Tolerable touch potential of human body is less than tolerable

step potential. Hence Touch potential is more critical for design while

Step potential is usually academic.

Step potential is independent of the diameter ( cross- section)

of the earthing conductor.

For 400% increase in diameter, reduction in Touch potential

is only 35%.

Thus cross- section has minor influence on Touch and Step

potentials.


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Ensuring Proper Grounding

The following steps, when put into practice, will

ensure a reliable, safe and trouble-free substation

grounding system:

1. Size conductors for anticipated faults

2. Use the right connections

3. Ground rod selection

4. Soil preparation5. Attention to step and touch potentials


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1-Size Conductors For Anticipated Faults

Conductors must be large enough to handle any anticipated

faults without fusing (melting).Failure to use proper fault time

in design calculations creates a high risk of melted conductors.

For example, a AWG conductor can withstand 42,700A for 0.5

sec before fusing. However, this same conductor can withstand

only 13,500A for 5 sec.

Two aspects govern the choice of conductor size: the first is the

fault current that will flow through the conductor and the

second is the time for which it can flow.The IEEE 80 suggests using a time of 3.0 s for the design of

small substations.


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2. Use the Right ConnectionsGrounding Connections, Resistance Test and Bonding Test

It is very evident that the connections between conductors and the

main grid and between the conductor and ground rods are asimportant as the conductors themselves in maintaining a permanentlow-resistance path to ground.

Connections must maintain the integrity of the conductor and thesystem as a whole for up to 40 years.

They must be of an appropriate material and mass to:

carry prospective fault currents

be able to resist corrosion

maintain original low resistance

The basic issues here are: The type of bond used for the

connection of the conductor in its run,

with the ground grid and with the ground rod.


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The temperature limits, which a joint can withstand.

The most frequently used grounding connections are mechanical

pressure type. Pressure-type connections produce a mechanical bond between

conductor and connector by means of a tightened bolt-nut or by

crimping using hydraulic or mechanical pressure.

On the other hand, the exothermic process fuses the conductor endstogether to form a molecular bond between all strands of the

conductor.

Temperature limits are stated in standards such as IEEE 80 and IEEE

837 for different types of joints based on the joint resistance

normally obtainable with each type. Exceeding these temperaturesduring flow of fault currents may result in damage to the joint and

cause the joint resistance to increase, which will result in further

overheating.


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3. Ground Rod Selection

In MV and HV substations, where the source and load are connected

through long overhead lines, it often happens that the ground fault current

has no metallic path and has to flow through the groundmass (earth). This

means that the ground rods of both source and load side substations have to

carry this current to or from the groundmass.

The ground rod system should be adequate to carry this current and ground

resistance of the grounding system assumes importance.

The length, number and placement of ground rods affect the resistance of

the path to earth. Doubling of ground rod length reduces resistance by a

value of 45%, under uniform soil conditions. Usually, soil conditions are not

uniform and it is vital to obtain accurate data by measuring ground rod

resistance with appropriate instruments.


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For maximum efficiency, grounding rods should be placed no closer

together than the length of the rod. Normally, this is 10 ft (3 m).

It should be noted that as the number of rods is increased, thereduction of ground resistance is not in inverse proportion. Twenty

rods do not result in 1/20th of the resistance of a single rod but only

reduce it by a factor of 1/10th.

For economic reasons, there is a limit to the maximum distance

between rods.

Normally, this limit is taken as 6 m. At more than 6 m, the cost of

additional conductor needed to connect the rods.


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4. Soil Preparation

Soil resistivity is an important consideration in substation grounding system

design. The lower the resistivity, the easier it is to get a good groundresistance.

Areas of high soil resistivity and those with ground frost need special

consideration. The highest ground resistivity during the annual weather

cycle should form the basis of the design since the same soil will have much

higher resistivity during dry weather when percentage of moisture in theground becomes very low.

One approach to take care of this problem is to use deep driven ground rods

so that they are in contact with the soil zone deep enough to remain

unaffected by surface climate.

The other approach is to treat the soil around the ground rod with chemicalsubstances that have the capacity to absorb atmospheric/soil moisture.

Use of chemical rods is one such solution


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5. Attention to Step and Touch Potentials

Limiting step and touch potential to safe values in a substation is vital topersonnel safety.

Step potential is the voltage difference between a persons feet and is causedby the voltage gradient in the soil. The potential gradient is steepest near thefault location and thereafter reduces gradually.

Touch potential represents the same basic hazard, except the potential exists

between the persons hand and his or her feet. This happens when a personstanding on the ground touches a structure of the substation, which isconducting the fault current into ground .

In both situations, the potential can essentially be greatly reduced by anequipotential wire mesh safety mat installed.

Such an equipotential mesh will equalize the voltage along the workers pathand between the equipment and his or her feet. With the voltage difference(potential) thus essentially eliminated, the safety of personnel is virtuallyguaranteed.

To ensure continuity across the mesh, all wire crossings are brazed with a 35%silver alloy. Interconnections between sections of mesh should be made so as

to provide a permanent low-resistance high-integrity connection.


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CONCLUSION


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CONCLUSION:

For good earthing following considerations must befollowed:

Size conductors

Selection of right connector

Pay attention to ground rod length, number, placement, andspacing

Prepare the soil

Eliminate step and touch potential

Ground the foundation

Ground all disconnects switch handles

Ground all surge arrestors

Pay attention to temporary grounding


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500 kv substation grounding presentation

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