Importance Of Grounding In Power System

Importance of Grounding in PowerSystemPresented byMr. H JayakumarEx- Joint Director CPRI

OBJECT OF EARTHING Prime Object of Earthing is to Providea Zero Potential Surface in andaround and under the area where theelectrical equipment is installed. Earthing is essential at every stage ofelectricity generation, transmissionand utilization

IMPORTANCE OF EARTHING Personal Safety Protection of Equipment :Prevent or at least minimize damage toequipment as a result of heavy fault currentand lightning thus improve the reliability ofequipment Protection of System : Improve the reliabilityof power supply.

STANDARDS FOLLOWED IS: 3043 : 1966, 1987 reaffirmed 2006 Code of Practice forEarthing. Indian Electricity rules 1956 ( as ammended up to 2000) IS: 2309 1989 ( reaffirmed 2005 )Protection of Buildings andallied Structures against lightning – Code of Practice. Manual on Earthing of AC Power Systems : CBIP PublicationNo.302 : 2007 BS: 7430 : 1991, Code of Practice for Earthing.( formerly CP1013: 1966) British Standard Institution London 1992 IEEE :80 : 2000( Revision of IEEE Std 80: 1986) Guide forSafety in AC Substation Grounding IEEE :142 :2007(Revision of IEEE Std 142 :1991) Grounding ofIndustrial and Commercial Power System. IEEE 1100 : 2005 (Revision of IEEE Std 1100 : 1999) Poweringand Grounding Electronic Equipment

TYPE OF EARTHING1. Plate EarthingRg ρ πAAohms2. Pipe / Rod Earthing100ρ4lRg log e2πldohms Doubling the Diameter Decrease the Resistance by 10 to 12% Doubling the Length Decrease the Resistance by 40%

TYPE OF EARTHING3.Strip Earthing100 ρ2l 2Rg log e2πlwtohms4.Combination of 1 & 3 and 2 & 35.Mat EarthingRg ρ4r ρLohms

TYPE OF EARTHINGR Station groundresistance in ohmsr is the radius of a circle having the same area or that occupied by grid in metersL Total buried length of conductors in metersρ Resistivity of the soil (assumed uniform) in ohm-mA Area of both sides of the plates in m²L Length of the 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 cmt 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.

Type of Earth Tester Which type of earth tester should be used for measuring theSoil Resistivity and Ground Resistance Digital meter orAnalog meter? Digital meter of reputed make. Ground resistance is directlyproportional to the soil resistivity, hence measurement of soilresistivity by accurate meter of reputed make is veryimportant. In grounding the value of soil resistivity is mostimportant for estimating the ground resistance and surfacepotentials (mesh, step and touch). MakeMeggerFlukeChauvin ArnouxKew (Kyoritsu)ModelDET 2/2 UK1623 & 1625 USA / UK6460, 6462 & 6470 FRANCE / UK4106 JAPAN

Comparison of Analog & Digital Earth Tester Why there is always a difference in the measured resistancevalues if measured with different earth testers even if all arecalibrated ones? This is not true in case of reputed make digital earthtesters. This may be true when compared with analog todigital earth testers of reputed make.PARAMETERSAccuracyANALOG METERDIGITAL METER5% of full scale and shall be effective 2% of reading in entire range henceabove 25% of full scale. At 25% of very accurate and high resolutionreading absolute error is 20%. Notaccurate and low resolutionVoltage250V hand crankingMicro processor based 30 to 50 Vautomatic readingFrequencyFixed (60 to 90 HZ)Variable. In automode it selects testfrequency with least amount of noiseHigh Spike resistanceDo not indicateDisplays the high spike resistance.Open circuitDo not indicateDisplays current circuit open/potentialcircuit open

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 ArrangementΡ 2 Π ARΩ-M

Measurement of Soil Resistivity b) Unequally spaced or Schelumberger – Palmer ArrangementΡ Π C ( C d) * R / dΩ-M

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 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).

CASE STUDYΡ 2 Π AR Ω-MA Distance between adjacent electrode (M),R Earth tester .9861.6521.4621.305ρOhm-Meters54.41249.009 min59.06262.39262.27964.30165.593 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 and77.453 The Soil is Homogeneous.

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 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

Measurement of Ground Resistance

Measurement of Ground Resistance

Measurement of Ground ResistanceDistance between stationearth (P1C1) and potentialelectrode (P2) mtrsEarthing tester reading ΩEarth Pit No 1Earth Pit No:2Earth Pit 9318.5630m30m30mAve. Of8.06 8.34 8.78 9.00 8.545Ave. Of 5.43 5.77 6.10 6.40 6.73 7.02 6.242Average of17.09 17.19 17.37 17.60 17.468.5456.24217.4636(20% R1)912(40%R2)1518(60%R3)Remote Electrode(C2) fromstation ground(C1P1)Result:Fall of Potential Method

Measurement of Ground ResistanceE.B Curd’s 0.8102From the chart Value 0.60130.5785Remote electrode(RE)30m 60.13% of RE(30)57.85% of RE(30)Value 18.039m8.69Ω17.355m6.40ΩAnalysis of ResultFall of PotentialE.B Curd’tsSlope Method8.545Ω8.780Ω8.690ΩSlope Method µ e 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.

Calculation of Earth Resistance of MultipleElectrodesHow to calculate the resultant earth resistance value of the earthelectrodes 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 tallations. 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 andmultiplied by the factor F.No.of RodsMultiplyingFactors forF Multiple Rods:2348121620241.161.291.361.681.801.922.002.16

Current carrying Capacity of EarthElectrode How to calculate current carrying capacity of an earthelectrode? Current caring capacity of an earth electrode depends on1. The total surface area of the electrode in contact withearth2. Resistivity of the soil and3. Duration of fault in seconds.The formula for current caring capacity (Currentdensity) is:Current density 7.57 X 10³ ρtAmp / Sq-m ρ Resistivity of the soil (assumed uniform) in ohm-m. t Duration of fault in seconds.

Geometry & No. of Electrodes Required7. 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 anyparticular 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.mFault current 6 kilo amperesDuration of Fault 1 sec.Soil Resisitivity 100 ohm - mCurrent 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

Geometry & No. of Electrodes Required To carry 6 kilo amperesNo.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.mFault current 6 kilo amperesDuration of Fault 1 sec.Soil Resisitivity 100 ohm - mCurrent density 7.57 X 10³ Amp / Sq-m ρt 7.57 X 10³ 757 Amp /Sq-m 100X1One Pipe will carry 757 X 0.754 570.778 AmperesTo carry 6 kilo amperes No.of plates required 6000 / 570.8 10.5 11 nos.

Minimum Accepted Earth Resistance What is the minimum accepted earth resistance value for differentapplications? Are these resistance values mentioned in anystandard? References: US AID INDIA Book The earth resistance shall be as low as possible and shall notexceed the following limitsPower stations (generating station)0.5 ohmsEHT Sub-station1.0 ohms33 KV Stations2.0 ohmsD/t Structure5.0 ohmsTower Foot resistance10.0 ohms Page 92 – Modernisation of power distributions – Same as above. IEEE standard 142-2007 chapter 4 page 164 – Resistance in the 1ohm to 5 ohms range are generally found suitable for industrial plantsub-station and buildings and large commercial installations. Lightning arrestors ground resistance for protection of buildings andallied structures – Less than 10 ohms Clause 12.3.1 Page 32IS 2309 : 1989.

Precaution to Be Taken For Measuring GroundResistance10. 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 & aremote 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 arresterin service , because of the possible high potential gradients around the groundconnection.d. Since the resistivity of the upper soil layers is greatly influenced by weather , aday 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 ofearth resistance of substations shall be carried out but local circumstances inthe light of experience may justify increase or decrease in this interval but itshould not be less than once in two years. This shall be compared with theinternal record.Allthough resistance to ground will change seasonally and over time anyincrease of the resistance 20% or more should be investigated andcorrective action taken to lower the resistance.

IEEE :80 : 2000( Revision of IEEE Std 80: 1986) Guide for Safetyin ACSubstation Grounding IEEE :142 :2007(Revision of IEEE Std 142 :1991) Grounding of Industrial andCommercial Power System. IEEE 1100 : 2005 (Revision of IEEE St