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Grounding System
Grounding is a connection donethrough a metal link between the body
of any electrical appliance, or neutralpoint, to the deeper ground soil. Themetal link is normally of MS flat, CI
flat, GI wire which should bepenetrated to the ground earth grid.
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Purpose of Grounding
Safety of Personnel
Protection of Installation
Superior performance ofelectrical system
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Design of Grounding System
Touch and step potentialswithin permissible limits
Low value of earthresistance
Objectives:
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Design of Grounding System
Design of grounding systemmainly depends upon:
Soil Resistivity,
Magnitude & duration of fault
current, Area available for earth mat
laying.
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Soil Resistivity
Soil resistivity depends on natureof soil, humidity, temperatureand climatic conditions.
Type of soil Average Resistivity
(-m)
Wet organic soil 10
Moist soil 100
Dry soil 1000
Bedrock 10000
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Soil Resistivity
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Grid Fault Current
The maximum grid current IGwhich is discharged by thegrounding system to the ground,
IG= Df . Sf . If
Df: Decrement factor for entireduration of fault
Sf: Fault current division factorIf : rms symmetrical ground fault
current
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Tolerable Step and Touch
VoltagesEstep= (1000 +6CS )*0.116/tEtouch= (1000+1.5CS)*0.116/tt : Duration of shock current,
S: Resistivity of the top layer of soil,
C : Reduction factor.
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Size of Conductor
depends on :
Magnitude and duration of fault current,
Type of material,
Type of joint and
Corrosion effect.
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Size of Conductor
Cross-sectional area of conductor(in mm2) is determined by,
Amm2= K. If. tc
If : Fault current in kA,
tc: Fault clearing time in sec,
K: Factor dependent on material ofconductor.
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Values of K for Steel, Aluminium andCopper are as follows:
Material Welded joint Bolted joint
Steel 12.15 15.7
Aluminium 8.4 12.0
Copper 4.7 5.8
Size of Conductor
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For steel conductors, corrosionallowance is also considered.
Soil Resistivity(ohm-m)
Increase inconductor size
(%)
Up to 25 30Between 25 to 100 15
>100 0
(Contd)Size of Conductor
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Step and Mesh Voltages
Es=.Ks.Ki.IG/LsEm= .Km.Ki.IG/LM
Factors Km, Ksand Kidepend on
spacing between conductors,depth of earth mat, diameter ofconductor, number ofconductors etc.
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Resistance of Earth Mat
The earth resistance depends on soilresistivity, available area, total
length of mat conductors and depthof mat in soil.
Rg= .( 1/LT + (1/(20.A))x(1+1/(1+ h(20/A)))
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Dif f icul t Condi t ions
Corrosion
High Soil resistivity
Transferred Potential
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Corrosion
When dissimilar metals are in electrical
or physical contact through anelectrolyte, galvanic corrosion can takeplace.
High saline nature of soil
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Soil resistivity and corrosion
Range of Soilresistivity range
(ohm-m)
Class
Less than 25 Severe
25 to 50 Moderate to severe
50 to 100 Mild Above100 Not likely
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Equilibrium Potential- Ref.
Copper electrodeMetal Equilibrium Potential
(volts)
Copper 0
Aluminium -2.0
Magnesium -2.7
Zinc -1.1
Iron -0.7
Lead -0.4
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Methods to reduce the
corrosion effect
Resistive coatingallowances in size selection of
material for corrosion
Cathodic protection.
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Cathodic protection
To reduce corrosion by minimizing thedifference in potential between anode
and cathode. This protection can beaccomplished by two widely usedmethods Sacr i f ic ial anode Method
Impress ing DC Current
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Design sequence
The criterion of selection is based on currentdensity required and soil resistivity. If thesoil resistivity is low (less than 50 ohm-
meters) and the current density requirementis low (less than 1 milli ampere per squarefoot), a galvanic system can be used. How-ever, if the soil resistivity and/or currentdensity requirement exceed the abovevalues, an impressed current system shouldbe used.
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High soil resistivity
The sites of hydroelectric power
plants are located in rockyareas where earth resistivity isvery high and space for layingof earth mat is limited.
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High soil resistivity
Various methods to lower the groundresistance are :
Connection to Penstocks
Laying of conductors in tail racearea
Satellite Earthing Use of Bentonite clay (generallynot recommended)
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Connection to Penstocks
Penstocks which are metallic andburied in soil are connected with the
main earthmat.
Because of large diameter and longlength of penstocks, low overall
ground resistance is easily achieved.
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Laying of conductors in Tail
Race Area
The tailrace area is generally lowresistivity area as earth mass willalways have a fair amount of waterseepage.
A separate earthmat in tailrace area
is laid and it is connected with mainearthmat to reduce the overall earthresistance.
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Transferred Potential
During fault conditions, high voltageequal to Grid Potential Rise (GPR) isattained by the grounding grid.
This voltage shall not be conveyed toplaces outside the power plant where it
can be dangerous to the person and theequipment.
f
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Transferred Potential
The L.T. (415 V, 3-phase) powerrequirements of the power plant areashould be met from the station
transformer located within the groundinggrid area of the power plant.
No L.T. line should either be taken out ofthe grounding area or brought fromoutside to inside the power plant area.
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Transferred Potential
Any telephone line should leave orenter the grounding grid areathrough isolating transformer.
This transformer shall be capable ofwithstanding voltage equal to GPR
between windings and between eachwinding & the ground.
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Transferred Potential
In pipes and metallic conduitsentering / leaving the grounding gridarea, transferred potentials may be
reduced or stopped at the stationboundary by inserting insulatingsections of sufficient length.
If there is any metallic gate within thearea covered by ground mat, theground mat shall be extended to thefull opening position of the gate.
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CONCLUSION
For a power plant having high soilresistivity, interconnection of dissimilar
material grids, high corrosivity, the safeearthmat design can be achieved fromvarious described methods and at the sametime it is very important to take care of
safety of the men and equipments whichmay come in contact with the area whereearthmat is laid and which may attain a highpotential rise.