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Importance of Grounding in Power System Presented by Mr. H Jayakumar Ex- Joint Director CPRI

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Page 1: Importance of Grounding in Power System - APQIapqi.org/download/bangalore/05-mr-h-jaykumar.pdf · Importance of Grounding in Power System ... IMPORTANCE OF EARTHING ... Calculation

Importance of Grounding in Power System

Presented by

Mr. H Jayakumar

Ex- Joint Director CPRI

Page 2: Importance of Grounding in Power System - APQIapqi.org/download/bangalore/05-mr-h-jaykumar.pdf · Importance of Grounding in Power System ... IMPORTANCE OF EARTHING ... Calculation

OBJECT OF EARTHING•Prime Object of Earthing is to Provide

a Zero Potential Surface in andaround and under the area where theelectrical equipment is installed.electrical equipment is installed.

•Earthing is essential at every stage ofelectricity generation, transmissionand utilization

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IMPORTANCE OF EARTHING

• Personal Safety

• Protection of Equipment :

Prevent or at least minimize damage toequipment as a result of heavy fault currentequipment as a result of heavy fault currentand lightning thus improve the reliability ofequipment

• Protection of System : Improve the reliability of power supply.

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STANDARDS FOLLOWED• IS: 3043 : 1966, 1987 reaffirmed 2006 Code of Practice for

Earthing.• Indian Electricity rules 1956 ( as ammended up to 2000)• IS: 2309 1989 ( reaffirmed 2005 )Protection of Buildings and

allied Structures against lightning – Code of Practice.• Manual on Earthing of AC Power Systems : CBIP Publication

No.302 : 2007• BS: 7430 : 1991, Code of Practice for Earthing.( formerly CP

1013: 1966) British Standard Institution London 1992• BS: 7430 : 1991, Code of Practice for Earthing.( formerly CP

1013: 1966) British Standard Institution London 1992• IEEE :80 : 2000( Revision of IEEE Std 80: 1986) Guide for

Safety in AC Substation Grounding• IEEE :142 :2007(Revision of IEEE Std 142 :1991) Grounding of

Industrial and Commercial Power System.• IEEE 1100 : 2005 (Revision of IEEE Std 1100 : 1999) Powering

and Grounding Electronic Equipment

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TYPE OF EARTHING

1. Plate Earthing

ohmsAA

Rg

πρ=

2. Pipe / Rod Earthing2. Pipe / Rod Earthing

ohmsd

l

lR eg

4log

2

100

π

ρ=

•Doubling the Diameter Decrease the Resistance by 10 to 12%

•Doubling the Length Decrease the Resistance by 40%

Page 6: Importance of Grounding in Power System - APQIapqi.org/download/bangalore/05-mr-h-jaykumar.pdf · Importance of Grounding in Power System ... IMPORTANCE OF EARTHING ... Calculation

TYPE OF EARTHING

3. Strip Earthing

ohmswt

l

lR eg

22

log2

100

π

ρ=

4. Combination of 1 & 3 and 2 & 3 4. Combination of 1 & 3 and 2 & 3

5. Mat Earthing

ohmsLr

Rg

ρρ+=

4

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TYPE OF EARTHING

R = Station ground resistance in ohms

r = is the radius of a circle having the same area or that occupied by grid in meters

L = Total buried length of conductors in meters

ρ = Resistivity of the soil (assumed uniform) in ohm-m

A = Area of both sides of the plates in m²

L = Length of the rod or pipe (in cm)

d = diameter of rod or pipe (in cm)d = diameter of rod or pipe (in cm)

l = length of the strip (in cm)

w = depth of burial of the electrode in cm

t = width (in the case of strip) or twice the diameter (for round conductor) in cm

•In the above formula no where type of material used forgrounding is mentioned. So, as far as grounding isconcerned material is immaterial since the groundresistance is the resistance offered to the nearest groundhence, ground (Soil Resistivity) plays a very important role.

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Type of Earth Tester• Which type of earth tester should be used for measuring the

Soil Resistivity and Ground Resistance Digital meter or Analog meter?

• Digital meter of reputed make. Ground resistance is directly proportional to the soil resistivity, hence measurement of soil resistivity by accurate meter of reputed make is very important. In grounding the value of soil resistivity is most important for estimating the ground resistance and surface potentials (mesh, step and touch).potentials (mesh, step and touch).

Make Model

• Megger DET 2/2 UK

• Fluke 1623 & 1625 USA / UK

• Chauvin Arnoux 6460, 6462 & 6470 FRANCE / UK

• Kew (Kyoritsu) 4106 JAPAN

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Comparison of Analog & Digital Earth Tester

• Why there is always a difference in the measured resistance values if measured with different earth testers even if all are calibrated ones?

• This is not true in case of reputed make digital earth testers. This may be true when compared with analog to digital earth testers of reputed make.

PARAMETERS ANALOG METER DIGITAL METER

Accuracy 5% of full scale and shall be effective

above 25% of full scale. At 25% of

2% of reading in entire range hence

very accurate and high resolutionabove 25% of full scale. At 25% of

reading absolute error is 20%. Not

accurate and low resolution

very accurate and high resolution

Voltage 250V hand cranking Micro processor based 30 to 50 V

automatic reading

Frequency Fixed (60 to 90 HZ) Variable. In automode it selects test

frequency with least amount of noise

High Spike resistance Do not indicate Displays the high spike resistance.

Open circuit Do not indicate Displays current circuit open/potential

circuit open

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Measurement of Soil Resistivity

• How to check the soil resistivity? Minimum how may readings should betaken and at what points for a correct measurement of soil resistivityvalue ?

a) Equally spaced or Dr. F.Wenner Arrangement

Ρ = 2 Π AR Ω-M

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Measurement of Soil Resistivity• b) Unequally spaced or Schelumberger – Palmer Arrangement

Ρ = Π C ( C+d) * R / d Ω-M

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Measurement of Soil Resistivity

• A set of readings taken with various probespacings (2m, 5 m, 10 m, 25 m, 50m) gives a setof resistivity which, when plotted against spacing,indicates whether there are distinct layers ofdifferent soil or rock and gives an idea of theirdifferent soil or rock and gives an idea of theirrespective resistivities and depth. This has to berepeated in all directions of the site i.e., horizontal,vertical and diagonal.

• The mean value is calculated. In case the valueslies within +/- 30% of mean value (70% to 130%)the soil is considered to be uniform (Homogenoussoil).

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CASE STUDY

Ρ = 2 Π AR Ω-MA= Distance between adjacent electrode (M), R = Earth tester reading

SL.NO a R ρMeters Ω Ohm-Meters

1 2 4.33 54.4122 3 2.60 49.009 min3 4 2.35 59.062 4 5 1.986 62.3924 5 1.986 62.3925 6 1.652 62.2796 7 1.462 64.3017 8 1.305 65.593 max

• ρ Average 59.579• 130% of ρ Ave = 1.3 x 59.579 = 77.453• 70% of ρ Ave = 0.7 x 59.579 = 41.705• Minimum & Maximum value lies within 41.705 and

77.453• The Soil is Homogeneous.

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Measurement of Ground Resistance

• What is the principle for measuring the earth resistancevalue?

• The fall of potential method: Consisting of injecting a knownalternating current through the electrode under test and anauxiliary current probe and plotting the ratio of V/I = R as afunction of probe spacing x. The potential electrode isfunction of probe spacing x. The potential electrode ismoved away from the ground under test in steps of 10%. Avalue of resistance is obtained at each step. The resistanceis plotted as a function of distance in ohms at which thisplotted curve appears to level out is taken as the resistancevalue of the ground under test. This level out will bepotential electrode at 61.8% of distance between groundelectrode under test and auxiliary electrode. This wasestablished by E.B.Curdt’s for small hemispherical electrode

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Measurement of Ground Resistance

Page 16: Importance of Grounding in Power System - APQIapqi.org/download/bangalore/05-mr-h-jaykumar.pdf · Importance of Grounding in Power System ... IMPORTANCE OF EARTHING ... Calculation

Measurement of Ground Resistance

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Measurement of Ground Resistance

Distance between station

earth (P1C1) and potential

electrode (P2) mtrs

Earthing tester reading Ω

Earth Pit No 1 Earth Pit No:2 Earth Pit No3

3 5.59 4.46 14.92

6(20% R1) 7.04 5.00 16.12

9 7.64 5.43 16.68

12(40%R2) 8.06 5.77 17.09

15 8.34 6.10 17.19

18(60%R3) 8.69 6.40 17.37

18.54(61.8%of30) 8.78 6.47 17.4018.54(61.8%of30) 8.78 6.47 17.40

21 9.00 6.73 17.60

24 10.08 7.02 17.93

27 11.14 7.84 18.56

Remote Electrode(C2) from

station ground

(C1P1)

30m 30m 30m

Result:

Fall of Potential Method

Ave. Of

8.06+8.34+8.78

+

9.00= 8.545

Ave. Of 5.43+5.77+

6.10+6.40+

6.73+7.02

=6.242

Average of

17.09+17.19+17.37

+17.60=17.46

8.545 6.242 17.46

Page 18: Importance of Grounding in Power System - APQIapqi.org/download/bangalore/05-mr-h-jaykumar.pdf · Importance of Grounding in Power System ... IMPORTANCE OF EARTHING ... Calculation

Measurement of Ground ResistanceE.B Curd’s Method 8.78 6.47 17.40

Slope Method µ=(R3-R2)

(R2-R1)

(R3-R2)

(R2-R1)

(R3-R2)

(R2-R1)

8.69-8.06

8.06-7.04

6.40-5.77

5.77-5.00

17.37-17.09

17.09-16.12

0.69

1.02

0.63

0.77

0.28

0.97

0.6765 0.8102 0.28

From the chart Value = 0.6013 0.5785-

Remote electrode 60.13% of RE(30) 57.85% of RE(30)

-

(RE)30m =60.13% of RE(30) 57.85% of RE(30)

18.039m 17.355m

Value = 8.69Ω 6.40Ω

Analysis of Result

Fall of Potential 8.545Ω 6.242Ω 17.46ΩE.B Curd’ts 8.780Ω 6.470Ω 17.40Ω

Slope Method 8.690Ω 6.400Ω -

Remote electrode (C2) at 30 mtrs from earth pit (C1P1).

Earth Pits are conventional type back filled 2Ft all around the electrodes.

Earth Pit No1: Located at A & B colony 50mm dia 3mtrs Long GI Pipe.

Earth Pit No2: Located in 33KV Substation 100mm dia 3mts Long GI Pipe.

Earth Pit No3: Near DG Set New 60mm dia 2mtrs Long GI pipe.

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Calculation of Earth Resistance of Multiple

Electrodes• How to calculate the resultant earth resistance value of the earth

electrodes if more than one pits are required to be installed?

• Multiple electrodes in parallel yield lower resistance to ground than asingle electrode. Multiple rods are commonly used to provide thelow grounding resistance required by high capacityinstallations. Adding a second rod does not however provide a totalresistance of half that of a single rod, unless the two are several rodlength apart. A useful rule is that grounding systems of 2-24 rodsplaced one rod length apart in a line, hollow triangle, circle or squarewill provide a grounding resistance divided by number of rods andwill provide a grounding resistance divided by number of rods andmultiplied by the factor F.

Multiplying Factors for Multiple Rods:No.of Rods F

2 1.16

3 1.29

4 1.36

8 1.68

12 1.80

16 1.92

20 2.00

24 2.16

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Current carrying Capacity of Earth Electrode

• How to calculate current carrying capacity of an earth electrode?

• Current caring capacity of an earth electrode depends on 1. The total surface area of the electrode in contact with

earth 2. Resistivity of the soil and 3. Duration of fault in seconds.

The formula for current caring capacity (Current The formula for current caring capacity (Current density) is:

Current density = 7.57 X 10³ Amp / Sq-m√ρt

• ρ = Resistivity of the soil (assumed uniform) in ohm-m.• t = Duration of fault in seconds.

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Geometry & No. of Electrodes Required

7. How does earth electrode geometry affects the earthing system?The total surface area coming in contact with earth is the criteria.

8. How to calculate the number of earth electrodes required for any particular application?This depends on the fault level the calculation is as follows:

Calculation of No.of Plates required: For Example:

The total surface area = 0.6 x 0.6 x 2 sq.m = 0.72 sq.mThe total surface area = 0.6 x 0.6 x 2 sq.m = 0.72 sq.mFault current = 6 kilo amperesDuration of Fault = 1 sec.Soil Resisitivity = 100 ohm - m Current density = 7.57 X 10³ Amp / Sq-m

√ρt= 7.57 X 10³ = 757 Amp /Sq-m

√100X1One Plate will carry 757 X 0.72 = 545.04 Amperes

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Geometry & No. of Electrodes Required

• To carry 6 kilo amperes No.of plates required = 6000 / 545.04 =

11 Nos.

Calculation of No.of Pipes required:

For Example:

The total surface area of a 3 mtr long 80 mm dia = π x 0.08 x 3 sq.m= 0.754 sq.m

Fault current = 6 kilo amperes

Duration of Fault = 1 sec.Duration of Fault = 1 sec.

Soil Resisitivity = 100 ohm - m

Current density = 7.57 X 10³ Amp / Sq-m

√ρt

= 7.57 X 10³ = 757 Amp /Sq-m

√100X1

One Pipe will carry 757 X 0.754 = 570.778 Amperes

To carry 6 kilo amperes No.of plates required = 6000 / 570.8 = 10.5 = 11 nos.

Page 23: Importance of Grounding in Power System - APQIapqi.org/download/bangalore/05-mr-h-jaykumar.pdf · Importance of Grounding in Power System ... IMPORTANCE OF EARTHING ... Calculation

Minimum Accepted Earth Resistance• What is the minimum accepted earth resistance value for different

applications? Are these resistance values mentioned in any standard?

• References: US AID INDIA Book• The earth resistance shall be as low as possible and shall not

exceed the following limitsPower stations (generating station) 0.5 ohmsEHT Sub-station 1.0 ohms33 KV Stations 2.0 ohmsD/t Structure 5.0 ohmsD/t Structure 5.0 ohmsTower Foot resistance 10.0 ohms

• Page 92 – Modernisation of power distributions – Same as above.

• IEEE standard 142-2007 chapter 4 page 164 – Resistance in the 1 ohm to 5 ohms range are generally found suitable for industrial plant sub-station and buildings and large commercial installations.

• Lightning arrestors ground resistance for protection of buildings and allied structures – Less than 10 ohms …… Clause 12.3.1 Page 32 IS 2309 : 1989.

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Precaution to Be Taken For Measuring Ground

Resistance10.Precaution to be taken while measuring ground resistance.• Precautions to be taken during resistance measurement.a. Avoid taking measurement during cloudy day.b. There is a possibility of lethal potential existing between a station ground & a

remote Ground. If a system fault involving the station ground occurs whileground resistance is being measured. The use of Rubber gloves is advisablewhile making connections to the test electrode. Under no circumstancesshould the two hands or other part of the body of the testing personal should beallowed to complete the circuit between the points of possible high potentialdifference.

c. An isolated lightening arrester ground should never be tested with the arrester in service , because of the possible high potential gradients around the ground

c. An isolated lightening arrester ground should never be tested with the arrester in service , because of the possible high potential gradients around the ground connection.

d. Since the resistivity of the upper soil layers is greatly influenced by weather , a day test should be chosen which is free from extreme weather conditions.

11.How many times is year we have to check the Earthing point .IS: 3043: 1987. Clause 34.42 (page76): Normally annual measurement of earth resistance of substations shall be carried out but local circumstances in the light of experience may justify increase or decrease in this interval but it should not be less than once in two years. This shall be compared with the internal record.Allthough resistance to ground will change seasonally and over time any increase of the resistance > 20% or more should be investigated and corrective action taken to lower the resistance.