dokumen.tips_500-kv-substation-grounding-presentation.pdf

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

INTRODUCTION

Definition:
Earthing means a conducting connection by which an electric circuit or
equipment is connected to the earth or some conducting body of
relatively large extent that serves in place of the earth".
Earthing means the connection of non-current carrying parts to
ground.
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.

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 will
improve the reliability of equipment and reduce the likelihood of
damage as a result of lightning or fault currents.

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

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.

TYPES

CLASSIFICATION OF EARTHING
Earthing 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 EARTHING
Earthing 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 Earthing
This 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.

Resistance Earthing
In 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 EARTHING
Earthing 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

EARTHING DESIGN

Earthing Design
The substation ground grid design is based on the substation layout
plan. The following points serve as guidelines to start a earthing
grid 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.

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 deep
driven
Comparison of life
expectancy

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 the
commercial metals. Copper is resistant to corrosion, that is,
it will not rust. It is malleable, ductile and has long life.

Earthing Conductors

Types of earthing conductor

Protective conductor
 Bonding concuctor
Circuit Protective Conductor CPC
This is a separate conductor installed with each circuit and is present to
ensure 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 fault
conditions.

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 resistance
low. 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 certain
applications following earth resistance are satisfactory
 In large power station - 0.5 Ω
 Major Substation above 110KV - 1.0 Ω
 Minor Substations Below 110KV - 2.0 Ω

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 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 the
same will prove to be of immense help in maintenance of earth grid and
equipment‟s.
 The condition of the electrodes, joints are also to be checked.
 If the electrodes are‟ corroded immediate
steps for replacement are to be taken.
 The earth resistance is to be measured
periodically.

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 connected
directly 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 the
electrode since corrosion is likely to be greater

Earth electrode types
 Driven Rod
 Grounding Plates

Electrolytic Electrode
Driven Rod
 The standard driven rod or copper-clad
rod 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 common
grounding device used in the field today.
 Driven rods are relatively inexpensive to
purchase, however ease of installation is
dependent upon the type of soil and
terrain where the rod is to be installed.
The steel used in the manufacture of a
standard driven rod tends to be relatively
soft. Purpose of copper on the rod is to
provide corrosion protection for the steel
underneath.

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 the
surrounding 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.

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 earth
salts 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 highly
conductive 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

Earthing Electrodes
A typical earthing electrode (left),
consisting of a conductive rod driven
into the ground
Multiple Electrodes

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 the
grounding 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.

 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 (Ωm)
 Exceptional values: from 1 up to 10000 (Ωm)
SI unit of resistivity
 The SI unit of resistivity is the Ohm-meter (Ωm); in the United States
the Ohm-centimeter (Ωcm) is often used instead.
Measurement of soil resistivity
 Wenner method
 Schlumberger method

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.

 Using the Wenner method, the apparent soil resistivity value is:
 where
 ρE = measured apparent soil resistivity (Ωm)
 a = electrode spacing (m)
 b = depth of the electrodes (m)
 RW = Wenner resistance measured as “V/I” in Figure (Ω)
 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:
 ρE = 2πaR

 Schlumberger method
 In the Schlumberger method the distance between the voltages probe is a
and the distances from voltages probe and currents probe are c.
 Using the Schlumberger method, if b is small compared to a and c, and
c>2a, the apparent soil resistivity value is:
where
 ρE = measured apparent soil resistivity (Ωm)
 a = electrode spacing (m)
 b = depth of the electrodes (m)
 c = electrode spacing (m)
 RS = Schlumberger resistance measured as “V/I” in Figure (Ω)

Measurement of soil resistivity

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 individual Electrodes, which will have fairly high
resistance. 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.

EARTH MAT

Potential Hazards

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

Electrical Shock Situations:
There are three main electrical shock situations that can occur when a person is
around a substation.
foot-to-foot shock
hand-to-feet shock
hand-to-hand shock or metal-to-metal contact

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.

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 the
substation floor. The Touch Potential should be very small.

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.

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 preparation
5. Attention to step and touch potentials

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.

2. Use the Right Connections
Grounding 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 as
important as the conductors themselves in maintaining a permanent
low-resistance path to ground.
 Connections must maintain the integrity of the conductor and the
system 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.

 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 ends
together 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 temperatures
during flow of fault currents may result in damage to the joint and
cause the joint resistance to increase, which will result in further
overheating.

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.

 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, the
reduction 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.

 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 ground
resistance.
 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 the
ground 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 chemical
substances that have the capacity to absorb atmospheric/soil moisture.
 Use of chemical rods is one such solution

5. Attention to Step and Touch Potentials

Limiting step and touch potential to safe values in a substation is vital to
personnel safety.
 Step potential is the voltage difference between a person‟s feet and is caused
by the voltage gradient in the soil. The potential gradient is steepest near the
fault location and thereafter reduces gradually.
 Touch potential represents the same basic hazard, except the potential exists
between the person‟s hand and his or her feet. This happens when a person
standing on the ground touches a structure of the substation, which is
conducting the fault current into ground .
 In both situations, the potential can essentially be greatly reduced by an
equipotential wire mesh safety mat installed.

Such an equipotential mesh will equalize the voltage along the worker‟s path
and between the equipment and his or her feet. With the voltage difference
(potential) thus essentially eliminated, the safety of personnel is virtually
guaranteed.
 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.

CONCLUSION

CONCLUSION:
For good earthing following considerations must be
followed:
•Size conductors
•Selection of right connector
•Pay attention to ground rod length, number, placement, and
spacing
•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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