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Earthing ELECTRICAL EARTHING SYSTEM CONTENTS

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Earthing ELECTRICAL EARTHING SYSTEM CONTENTS Powered By Docstoc
					ELECTRICAL
 EARTHING
  SYSTEM
                CONTENTS

•   1. What is Earthing
•   2. Why Earthing required
•   3. Touch Voltage
•   4. Step Voltage
•   5. Soil Model
•   6. Design Input
                CONTENTS

•   7 . Touch Voltage Calculation
•   8 . Step Voltage Calculation
•   9. Ground Current Calculation
•   10 Types of Earthing
•   11
             What is Earthing
• The process of earthing is to connect all the
  objects which could become charged to the
  general mass of the earth, to provide a path for
  electrical current and to hold the parts as close as
  possible to earth potential.
• For tying the electrical supply system to earth,
  supply transformer neutral conductor (Y point of 3
  phase Supply) is connected to earth using an
  earthing electrode or the metal sheath and
  armouring of a buried conductor.
              Why Earthing required
• The purpose of grounding system is to provide a low impedance
  electrical contact between the neutral of an electrical system and earth
• 1. To provide safety during normal or fault condition.
• 2. To stabilize the voltage during transient conditions and
  therefore to minimize the probability of a flashover during transients.
• 3. To dissipate lightening strokes
• Due to impedance of the system, the potential of the grounding system
  may become different than the potential at various points on earth
  during abnormal operating condition or fault condition.
• Due to this potential difference a touch and step voltages are subjected
  by the persons which generate an electrical current through his body.
  This flow of current is a source of danger.
• Electrical body current below 116/(sqrt(t) mili amps, where t is
  shock duration time
• Vtouch, allowable = 0.116(1.5 + 1000)/ (sqrt(t)
• Vstep, allowable = 0.116(6.0 + 1000)/ (sqrt(t)
                       Touch Voltage
•   A person touching a grounding structure which has a potential that is different
    from that of earth at which the person is standing. In this case the person is
    subjected to a voltage which generate an electrical current through his body.
    The voltage to which the human body is subjected is called the touch voltage.
•   Safe Electrical body current < 116/sqrt(t) mili Amps
•   Vtouch, allowable = 0.116(1.5Cs + 1000)/ sqrt(t) Volt
•   Where
•    = Soil resistivity (- M)
•   t = duration of electric shock in seconds
•   For a regular ground mat: -
•   Vtouch, max = KmKi Ie/L Volt
•   Ie : - Total Current injected in to grid
•   Ki : - Asymmetry Factor
•   Ks, Km : - Geometric Factor
•   L : - Total conductor Length
                       Step Voltage
• A person working on the surface of the earth will experience a voltage
  between his feet. This voltage will generate electric body current. In
  this case the voltage to which the person is subjected is called the “step
  voltage”.
• Safe Electrical body current < 116/sqrt(t) mili Amps
• Vstep, allowable = 0.116(6.0 Cs + 1000)/ sqrt(t) Volt
• Vstep, max = KsKiIe/L Volt
• The aim of design of grounding system is to meet safety criterion. It
  means value of step and touch voltages must be below allowable
  values. The maximum touch voltage occurs at a point located approx.
  above the centre of the outer mesh of the ground mat.
• Maximum step voltage occurs at the periphery of the ground mat near
  the centre.
                         Soil Model
• 1. Determination of soil model :-
• there is two methods of soil resistivity measurement: -
• 1. Wenner Method
• 2. Driven rod method
• From soil resistivity data (taken from the above method) closed curve
  has been plotted jointing all the resistivity points. The area inside the
  curve is measured and equivalent circle of the same area is drawn. The
  radius of the circle will be average resistivity of the site and the same
  is considered for design of grounding system.
• From available power plant data,
• Average soil resistivity = 195.2 M at a depth of 2.5 Meter
• Hence 200 M resistivity is considered for design.
    Factor affecting Soil resistivity
•   Following Factors affect the soil resistivity: -
•   1. Voltage gradient does not affect the soil resistivity unless it exceeds certain
    critical values( touch and step). Because grounding system is designed on the
    basis of the touch and step voltages.
•   2. the effect of current flowing from the electrode in to the surrounding soil is
    to cause significant drying and thus increase the effective soil resistivity.
•   Current is not to exceed 200A/M2 for one second
•   3. The resistivity of soil rises abruptly whenever the moisture content accounts
    for less than 15 % of soil weight.
•   0.08 to 0.15 meter in depth are very useful incorporating the evaporation of
    moisture and this limiting the drying of top soil layers during prolonged dry
    whether periods
•   Depending upon the type of soil, temperature affects resistivity to a greater or
    lesser extent.
            Design Input
• 1. System Voltage
• 11KV, 40 KA for 1 sec. (Max 100 Amp
  ground fault current)
• 6.6 KV, 40 KA for 1 sec. (Max 100 Amp
  ground fault current)
• 415 volt , 50 KA for 1 sec ( solidly
  grounded)
• 2. Soil Resistivity = 200 M
           Size of ground Conductor
•   A = I{(terr * 104)/(TCAP * ln(1+(Tm-Ta)/(k0+Ta))}1/2
•   Where ;-
•   A = Conductor X- section in mm2
•   I = Fault current in KA = 50
•   Tm = Max. allowable temperature
•   Ta = Ambient Temp.
•   r = Thermal co-efficient of resistivity at reference temp. Tr
•   Tr = Reference temp. Tr = 20 0C
•   K0 = (1/r) – Tr
•   Tc = fault current duration
•   TCAP – Thermal capacity factor in J/cm3/‟c
•   r = 20 = 0.00423
•   20 = 13.8*10-6 M
•    K0 = (1/r) – Tr = 1/0.00423 – 20 = 216.4
•   TCAP = 4.184* Sh*Sw where
•   Sh = Specific Heat in Cal/gm/‟C = 0.114
•   Sw = Specific weight gm/cm3 = 7.86
•   TCAP = 4.184 * 0.114* 7.86= 3.75 W/cm3/‟C
•   A = I{(terr * 104)/(TCAP * ln(1+(Tm-Ta)/(k0+Ta))}1/2
•     = 584.0 mm2
•   Taking 15% allowance for corrosion
•   So minimum X – sectional area of the conductor = 1.15 *584 = 671.6 mm2
•   Hence conductor size selected = 75 *10 mm2
            Design of Grid
• Total area covered = 150 * 80 M2
• Max spacing of ground conductor = 12
  meter( selected)
• Estimated length of conductor =
  80*14+50*8 = 2320 Meter
       Ground Resistance Calculation
•   Considering 20 nos. 40 mm dia, 3 meter long MS rod will be used as ground electrode. As per
    Sehawarg‟s Formula
•   Rg =(R1R2-R212)/(R1+R2-2R12)
•   R1 = (1/ l1)*(ln(2l1/h‟) + K1(l1/sqrt(A) – K2)) = 0915
•   R2 = (a/2n l2)*(ln(8l2/d2) - 1 +2 K1(l2/sqrt(A)*(sqrt(n) – 1)2 ) = 3.33
•   R12 = (a/ l1)*(ln(2l1/l2) + K1(l1/sqrt(A) – K2 + 1)) = 0.857
•   Ground Resistance Rg = (R1R2-R212)/(R1+R2-2R12) = 0.912
•   Where
•    R1 = resistance of grid conductor
•   R2 = resistance of all ground rods
•   R12 = mutual resistance between group of grid conductors and group of ground rods
•   1 = Soil resistivity encountered by grid conductor = 200M
•   a = apparent Soil resistivity as seen by ground rod = 200M
•   l1 = Total length of conductor = 2320 meter
•   l2 = length of ground rod = 3 meter
•   n = number of ground rod = 20
•   d1 = eqt. Dia of grid conductor = 0.0309 meter
•   d2 = Dia of grid Rod = 0.04 meter
•   h = depth of buried conductor = 0.6 meter
•   h‟ = sqrt(d1h) = 0.136 meter
•   A = Total Area = 80*150 = 1200M2
•   K1 = 1.33
•   K2 = 5.21
        Touch Voltage Calculation
•   Touch voltage (Permissible)
•   Vtouch, allowable = 0.116(1.5Cs + 1000)/ sqrt(t) Volt = 213.2 Volt
•   Where
•   Cs = reduce factor = 1
•   t = duration of shock
•   Ig = portion of fault current passing through ground = 1282 Amp
•   Mess Voltage Em based on estimated length
•   Em = KmKi Ig/L Volt
•   = 203.9 volt
•   Em < Etouch
•   Hence OK
    Step Voltage Calculation
• Step Voltage : -
• Step voltage permissible
• Vstep, allowable = 0.116(6.0 Cs + 1000)/ sqrt(t)
  Volt = 360.9 Volt
• Step voltage based on estimated length
• Es = KsKiIg/L1 Volt = 106 Volt
• L1 = L + 0.115 Lr
• Es < Estep
• Hence OK
                       Ground Current
                        Calculation
•   Calculation of ground Current
•   Conductor resistance
•   Rc = c*(path length / cross sectional area)
•   c = 13.8 * 10-8 M (at 20 „C) = 15.66* 10-8 M (at 50 „C)
•   Path length (Max) = 150+80 = 230 Meter
•   X – Sectional Area = 750*10-8 M2
•   Rc = 0.048 
•   No. of parallel path in one direction = 14
•   Rc eqt = 0.048/14 = 0.00343 
•   The ground fault current will have 14 parallel paths 1) Through Ground and 2)
    through ground conductor.
•   The current passing through each path will depend upon the resistance of each
    path
•   Current through ground Ig = 50KA *Rc eqt/(Rg + Rc eqt) = 187.34 Amps
•    And rest of current will pass through ground conductor
           Types of Earthing
•   1. Resonant Grounding
•   2. Solid Grounding or effective grounding
•   3. Resistance Grounding
•   4. Reactance Grounding
•   5. Zig – Zag Transformer Grounding
              Resonant Grounding
• If the value of inductive reactance is such that the fault current
  balances exactly the charging current, then the grounding is known as
  resonant grounding or ground fault neutralizer or Peterson coil.
• Inductance connected between neutral and earth – L = 1/(3ω2C)
• The use of the resonant grounding will reduce the line interruption due
  to transient line to ground faults which will not be possible with other
  form of grounding.
• Ground Fault neutralizer should not be used where
• 1) Fully graded insulation transformers are used as the neutral of such
  transformers are not sufficiently well insulated
• 2) Auto transformer having a ratio greater than 0.95 to 1 are used
             Solid Grounding
• In this grounding system the voltage of the healthy
  phases in case of a L-G fault does not exceed 80%
  of the L-L voltage and is much less as compared
  to other form of grounding, the equipment for all
  voltage classes are less expensive.
• On system 115 KV and above additional saving
  are possible because of the transformers with the
  insulation graded towards neutral are less costly.
     Resistance Grounding
• Resistance grounding is normally used where the charging current is
  small( ie.-Generators, low voltage short length overhead line)
• Resistance Required = KV⁄(√3*I) Ω
• For 6.6 KV system = 38.1 Ω
• For 11 KV system = 63.5 Ω
• Advantages:-
• 1. Reduce arcing ground hazardous
• 2. permits ready relaying of ground faults
• 3. Help in improving the stability of the system during L-G faults by
  replacing the power dropped, as a result of low voltage , with an
  approximately equal power loss in the resistor, thus reducing the
  advance in phase of the conductor.
    Reactance grounded System
• A reactance grounded system is one in which the
  neutral is grounded through impedance which is
  highly reactive.
• System is solidly grounded or reactance grounded
  depends upon the ratio of X0/X1
• For reactance grounded system X0/X1 >3.0
• For solidly grounded system X0/X1 < 3.0
• Reactance grounding is used for synchronous
  motors and capacitors and also for circuits having
  large charging currents
      Earthing Transformers
• If a neutral point is required which
  otherwise is not available (delta connection
  and bus bar point etc.) a zig-zag transformer
  is used.
• This type of transformer do not have
  secondary winding. These transformers are
  of short time rating usually 10 second to 10
  minutes.

				
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