Earthing Concepts

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By my great guru


<ul><li> 1. EARTHING CONCEPTS</li></ul><p> 2. Earthing in a EHV Substation </p> <ul><li>Providing adequate Earthing in a substation is an important safety measure. </li></ul><ul><li>Earthing means connecting the electrical equipment to the general mass of earth of low resistance. </li></ul><ul><li>Objective is to provide under and around the substation a surface of uniform potential</li></ul><ul><li>-- At near zero or absolute earth potential</li></ul><p> 3. Earthing in a EHV Substation </p> <ul><li>Objective: </li></ul><ul><li><ul><li>The touch and step potential shall be within limits under all conditions including fault condition </li></ul></li></ul><ul><li><ul><li>Grounding resistance shall belower. </li></ul></li></ul><ul><li><ul><li>Effective earthing system shall aim at providing protection to life and property against dangerous potentials under fault conditions </li></ul></li></ul><p> 4. Earthing in a EHV SubstationI.E.Rules 1956 </p> <ul><li>Rule 92 </li></ul><ul><li><ul><li>Every substation /generating station exposed to lightning shall adopt efficient means for diverting the electrical surges due to lightning to earth </li></ul></li></ul><ul><li><ul><li>Earth lead of any lightning arrestor shall not pass through any iron or steel pipe. </li></ul></li></ul><ul><li><ul><li>It shall be taken directly, as far as possible, to a separate earth electrode and/or junction of the earth mat. </li></ul></li></ul><ul><li><ul><li>Bends Shall be avoided where ever practicable </li></ul></li></ul><ul><li><ul><li>Earth screen if provided for lightning protection shall be connected to main earth grid.</li></ul></li></ul><p> 5. Earthing in a EHV SubstationI.E.Rules 1956 </p> <ul><li>Functioning of earthing in a substation </li></ul><ul><li><ul><li>It shall be capable of passing maximum earth fault current </li></ul></li></ul><ul><li><ul><li>The passage of fault current does not result in any thermal or mechanical damage to the insulation of connected plant / equipment </li></ul></li></ul><ul><li><ul><li>Every exposed conductor part and extraneous conductive part may be connected to the earth. </li></ul></li></ul><ul><li><ul><li>There is no danger to the personnel </li></ul></li></ul><ul><li><ul><li>Ensure equi-potential bonding within the power system </li></ul></li></ul><ul><li><ul><li>No dangerous potential gradients (step or touch or transfer potentials) shall occur under normal or abnormal operating conditions </li></ul></li></ul><ul><li><ul><li>To minimize electromagnetic interference between power and control/ communication system </li></ul></li></ul><p> 6. Earthing System </p> <ul><li>Points to be earthed in a substation </li></ul><ul><li>The neutral point of each separate system should have an independent earth, in turn interconnected with the station grounding mat. </li></ul><ul><li>Equipment frame work and other non-current parts (two connections) </li></ul><ul><li>All extraneous metallic frame works not associated with equipment ( two connections) </li></ul><ul><li>Lightning arrestors should have independent earths, in turnconnected to the station grounding grid. </li></ul><p> 7. Earthing System Points to be earthed-contd </p> <ul><li>Over head lightning screen shall also be connected to main ground mat. </li></ul><ul><li>Operating handles of Isolators with a auxiliary earth mat underneath, if necessary. </li></ul><ul><li>Peripheral fencing </li></ul><ul><li>Buildings inside the switch yard. </li></ul><ul><li>Transformer Neutrals shall be connected directly to the earth electrode by two independent MS strips </li></ul><p> 8. Earthing and grounding -distinction</p> <ul><li>Grounding:- connection of current carrying parts to ground. Ex :Generator or transformer neutral. </li></ul><ul><li>This is for equipment safety. </li></ul><ul><li>In a resistance grounded system it limits the core damage in stator of rotating machines. </li></ul><ul><li>In solidly grounded system substantial ground fault current flows enabling fault detection and faster clearance.</li></ul><p> 9. Earthing and grounding -distinction </p> <ul><li>Earthing:- connection of non current carrying parts to ground. Ex : Metallic enclosure. </li></ul><ul><li>This is for human safety. </li></ul><ul><li>Earthing system plays no role under balanced power system conditions. </li></ul><ul><li>Under ground fault conditions, enables ground fault current to return back to source without endangering human safety.</li></ul><p> 10. Basics of Earthing Resistivity of earth </p> <ul><li>Resistivity of earth:- </li></ul><ul><li>Mother earth is a bad conductor. </li></ul><ul><li>Resistivity is normally around 100 ohm mt. </li></ul><ul><li>GI of 65x10mm section will have same resistance as copper of 25x4mm section. </li></ul><ul><li>Corresponding figure for earth is 800x800mt (158acres) </li></ul><ul><li>Metallic conductor is a preferred alternative to earth to bring the fault current back to source.</li></ul><p> 11. Electric field Earth resistance </p> <ul><li>Current flows through a series of hemi-spherical shells of earth of continuously increasing cross sections. </li></ul><ul><li>Almost 95% of final resistance is contributed by soil within 5mts of the electrode. </li></ul><ul><li>If current is discharged from a grid towards another grid at B100 km away, only soil with in 5 to10 mts of the electrode contributes maximum resistance. </li></ul><ul><li>Earth beyond, offers very minimum resistance. </li></ul><ul><li>This is the concept of treating the soil around electrode of an earth pit.</li></ul><p> 12. Electric field Earth resistance </p> <ul><li>Earth with its huge mass offers equi-potential everywhere </li></ul><ul><li>A very large charge is requiredto change earth potential everywhere </li></ul><ul><li>Disturbance due to current injection at a point is felt, only locally.</li></ul><p> 13. Substation earthing Design of Earth mat </p> <ul><li>Design depends upon the following parameters </li></ul><ul><li><ul><li><ul><li>Durational andmagnitude of the fault current </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Resistivity of the surface layer of the soil </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Resistivity of thesoil </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Magnitude of current that the human body can safely carry </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Permissible earth potential raise that may take place due to the fault conditions </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Shock duration </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Material of Earth- mat conductor. </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Earth- mat geometry </li></ul></li></ul></li></ul><p> 14. Substation earthing Design of Earth mat </p> <ul><li>Parameters for the calculation of Maximum permissible step and touch potential </li></ul><ul><li><ul><li>Fault duration :- Fault clearing time of back up protection is adopted </li></ul></li></ul><ul><li><ul><li>Modern protection systems provides for fast acting back up protection </li></ul></li></ul><ul><li><ul><li>Considerable saving can be made by optimizing the size of the conductor of earthing grid by considering lesser fault duration. </li></ul></li></ul><ul><li><ul><li>These will change the earth potential raise due to whichStep andTouch potentials arise. </li></ul></li></ul><p> 15. Earth mat parameters Let go current </p> <ul><li>Maximum safe current a person can tolerate and still release grip of an energised object, using muscles affected by the current </li></ul><ul><li>The magnitude of let go current adopted in calculating maximum permissible step and touch potentials (As per IEEE 80 1976) </li></ul><ul><li>for man 9 milli amps </li></ul><ul><li>for woman 6milli amps </li></ul><p> 16. Substation EarthingNon-fibrillation current </p> <ul><li>Developed by Dalziel and approved by AIEE80-1963Magnitudeof power frequency alternating current (mA) that a human body of average weight( 50kgs to 70 kgs) can with stand without ventricular fibrillation,</li></ul><ul><li>I= 0.116for a body of50kgs wt. </li></ul><ul><li> t </li></ul><ul><li>I= 0.157for a body of 7 0kgs wt. </li></ul><ul><li> t </li></ul><ul><li>Av. Value of human body resistance (dry) 8 to 9 K-ohms </li></ul><ul><li>Adopted value for designing Earthing system 1Kohms </li></ul><p> 17. Substation EarthingNon fibrillation current contd </p> <ul><li>Non fibrillating current adopted for earth grid design in India. </li></ul><ul><li>Magnitudeof power frequency alternating current that a human body of average weight( 50kgs to 70 kgs) can with stand without ventricular fibrillation,</li></ul><ul><li>I= 0.165</li></ul><ul><li> t </li></ul><ul><li>I = rms current through human body in amps </li></ul><ul><li>t =durtation of shock in seconds </li></ul><ul><li>Assumption /considerations in deriving the above equation</li></ul><ul><li>--The duration of shockis from 8 milli-seconds to 3 seconds </li></ul><p> 18. Substation EarthingFault duration and magnitude </p> <ul><li>During a line to earth or double line earth fault current through earthing system causes </li></ul><ul><li><ul><li>Heating of earthing conductor </li></ul></li></ul><ul><li><ul><li>Potential gradients in the soil </li></ul></li></ul><ul><li>For earthing design single line to ground fault is considered as </li></ul><ul><li><ul><li>Most of the faults are of this type </li></ul></li></ul><ul><li><ul><li>Current through earth in case of single line to earth fault is higher that in the later case. </li></ul></li></ul><p> 19. Substation Earthing Fault duration and magnitude-contd. </p> <ul><li>For determining maximum permissible step and touch potentials </li></ul><ul><li>Fault duration corresponding to maximum fault clearing time of back up protection relays are considered </li></ul><ul><li>Normally in modern sub station clearance time of primary protection is 0.2 sec, ie., 200 milli sec and clearance time for back up protection is 0.5 sec, ie.,500 milli sec </li></ul><ul><li>A fault duration time of 0.5 sec (500 mill sec) is adopted for design </li></ul><p> 20. </p> <ul><li>Earthing conductor once placed under earth may not be inspected normally. </li></ul><ul><li>Prudent to make it capable of carrying maximum possible current for maximum time. </li></ul><ul><li>If felt necessary and if it is economical,fault duration of 1 sec can be adopted for design. </li></ul><p>Substation Earthing Fault duration and magnitude-contd. 21. Substation Earthing Soil resistivity</p> <ul><li>To design most economically and technicallysound earthing system accurate data of soil resistivity and its variation with in substation soil is essential. </li></ul><ul><li>Resistivity of soil in many substations has been found varying -at times between 1 and 10,000 ohm meters. </li></ul><ul><li>Variation in soil Resistivity with depth is more predominant as compared to variation in horizontal distances. </li></ul><p> 22. Substation Earthing Soil resistivity </p> <ul><li>Large variations in stratification of earth layers will result in large variations in earth resistivity. </li></ul><ul><li>Highly refined techniques for the determination of resistivity of homogeneous soil( non uniform soil) is available. </li></ul><ul><li>As resistivity of soil varies widely based on moisture content earth resistivity readings to be obtained in summer or dry season. </li></ul><ul><li>Weiner's 4 electrode method is generally adopted for testing. </li></ul><p> 23. Substation Earthing-Soil resistivity Weiner's 4 electrode method </p> <ul><li>Earth resistivity tests shall be carried out at least in 8 directions </li></ul><ul><li>If results obtained indicate wide variation, test shall be conducted in more number directions. </li></ul><ul><li>Four electrodes are driven into earth along a straight line at equal intervals. </li></ul><ul><li>Current is passed through two outer electrodes and earth. </li></ul><ul><li>Voltage difference is measured between two inner electrodes. </li></ul><p> 24. Substation Earthing Soil resistivity </p> <ul><li>Current flowing through the earth produces are electric field proportional to current density and resistivity of soil. </li></ul><ul><li>Voltage measured is proportional to the ratio of voltage to the current i.e R </li></ul><ul><li> = 4 s R - __ s __ </li></ul><ul><li>1 +2 s ___s+e </li></ul><ul><li> s + 4e </li></ul><p> 25. Substation Earthing Soil resistivity </p> <ul><li>Where</li></ul><ul><li> = Resistivity of soil inohm-meter </li></ul><ul><li>s= Distance between two successive electrodes in meter </li></ul><ul><li>R= Ratio of voltage to current or electrode resistances in ohm </li></ul><ul><li>e= depth of burial of electrodes in meters </li></ul><ul><li>In case depth of burial of the electrodes in the ground (e) is negligible compared to electrodes spacing. This formula is the adjusted = 2 s R</li></ul><ul><li>(This formula is normally adopted in AP Transco Ltd.) </li></ul><p> 26. Substation Earthing Measurement ofSoil resistivity </p> <ul><li>There point method </li></ul><ul><li>Two temporary electrodes spikes are driven in to the earth at 150ft and 75ft respectively from earth electrode under test. </li></ul><ul><li>Former is for current and the later is for voltage. </li></ul><ul><li>Ohmic values of earth electrode resistances are obtained using earth meager</li></ul><ul><li>R =log 10 (4L/P)where </li></ul><ul><li>2 </li></ul><ul><li>R=Electrode resistance in ohm </li></ul><ul><li>L = Length in cms of the rod driven under ground </li></ul><ul><li>D = Dia in cms of the rod </li></ul><ul><li>= Earth resistivity in ohm-meter</li></ul><p> 27. Resistance of the earthing system </p> <ul><li>R = + </li></ul><ul><li>4r L </li></ul><ul><li>=Soil resistivity in ohm meter </li></ul><ul><li>L =Length of conductor buried in meters </li></ul><ul><li>r= radius in meters of circle having the samearea as that occupied by the earth mat. </li></ul><ul><li>The value of the R should be less than theimpendence to ground values stated below </li></ul><p> 28. Earthing System Permissible resistance of earthing system </p> <ul><li>Primary requirements : Impendence to ground (resistance of earthing system) </li></ul><ul><li>Small substations 2 Ohms </li></ul><ul><li>EHV substations up to 220 kV 1 Ohm </li></ul><ul><li>Power stations and 400 kV substations 0.5 Ohms </li></ul><ul><li>Distribution transformer -5 Ohms. </li></ul><ul><li>In order to avoid abnormal shift of the neutral potential, earth resistance of the station earthing system shall be normally less than or equal to 1ohm. </li></ul><p> 29. Substation EarthingStep and touch potential </p> <ul><li>Step potential - Difference in surface potentials experienced by a man bridging a distance of 1 mt with his feet, with out contracting any other grounded object. </li></ul><ul><li>Touch potential- potential difference between the earth potential raise and the surface potential at the point where a person is standing touching an earthed structure. </li></ul><ul><li>Tolerable touch potential of human body is less than tolerable step potential. </li></ul><p> 30. Substation EarthingStep and touch potential-contd </p> <ul><li>In any switch yard, chances of exposure to Touch potential is higher than that to step potential. </li></ul><ul><li>Resistance offered by the feet of a person against Touch potential is much less compared to that against Step potential. </li></ul><ul><li>Hence Touch potential is more critical for design while Step potential is usually academic. </li></ul><p> 31. Substation EarthingStep and touch potential- contd. </p> <ul><li>Step potential is independent of the diameter ( cross- section) of the earthing conductor. </li></ul><ul><li>For 400% increase in diameter, reduction in Touch potential is only 35%. </li></ul><ul><li>Thus cross- section has minor influence on Touch and Step potentials.</li></ul><ul><li>Length of earthing conductor has significant effect on Touch and Step potentials. </li></ul><p> 32. Substation EarthingStep and touch potential </p> <ul><li><ul><li>Tolerable Step and touch potentials (CBIP Publication no. 223) </li></ul></li></ul><ul><li><ul><li>E step (LMT) = 0. 116 (1000+1.5Cs(hs.K.) s)(volts) </li></ul></li></ul><ul><li> t </li></ul><ul><li>E touch (LMT) = 0. 116 (1000+ 6Cs.(hs.K.) s)(volts) </li></ul><ul><li> t </li></ul><ul><li>Where Cs= Reduction factor for de-rating normalvalue of surface layer resisvity, a function of K. </li></ul><ul><li>K= -- s </li></ul><ul><li> + s </li></ul><ul><li> , s are resistivities of soil and surface layer respectively.</li></ul><ul><li>cs =1 when crushed rock has resistivity equal to that ofsoil . </li></ul><ul><li>Otherwise it is derived from reference graphs ( Cs. vs hs.) </li></ul><ul><li>hs = thickness of surface layer in meter. </li></ul><ul><li>t= Duration of shock current flow in secs. </li></ul><p> 33. Substation EarthingStep and touch potential-contd. </p> <ul><li>Tolerable Step and touch potentials as adopted by certain utilities. </li></ul><ul><li><ul><li>E step (LMT) = IB ( RG+1.5Cs. s)(volts)------ (1) </li></ul></li></ul><ul><li>E touch (LMT) = IB( RG+ 6Cs. s)(volts) ------ (2) </li></ul><ul><li>RG= body resistance in Ohms= 1000</li></ul><ul><li>IB= Permissible body current of human beings. </li></ul><ul><li>Cs=Reduction factor(0 to 1)=1-(k / (2h+0.09) ------(3) </li></ul><ul><li>k=0.09x(1- / s) </li></ul><ul><li> s= surface layer resistivity ( taken as 2000 ohm- mt.) </li></ul><ul><li>h= Thickness of gravel in cm. </li></ul><ul><li> = Soil resistivity ( taken as 100 ohm- mt.) </li></ul><p> 34. Substation EarthingStep and touch potential-contd. </p> <ul><li>Sample calculation forE step (LMT) and E touch (LMT) </li></ul><ul><li>Data </li></ul><ul><li>Weight of the man =70kgs</li></ul><ul><li>Fault duration=0.5 sec </li></ul><ul><li>ResistivitySoil = =100 ohm-mt, Surface layer = s=2000 ohm-mt, </li></ul><ul><li>h= Thickness of gravel in cm.=10cm </li></ul><ul><li>From (3), Cs=0.705 </li></ul><ul><li>From table in slide24for a 70 kgs man and for a shock duration of 0.5 secI B= 222mA </li></ul><ul><li>From (1)E step (LMT)= 691V </li></ul><ul><li>From (2)E touch (LMT) =2100V </li></ul><p> 35. </p> <ul><li>Methodology of design as adopted inAPTransco </li></ul><ul><li>Size of earth mat conductor (steel strip ) Shall be : </li></ul><ul><li>A (Steel) =0.0013 x I t sq. mmfor bolted joints </li></ul><ul><li>= 0.011 x I t sq. mmfor welded joints </li></ul><ul><li>Where A = Area of Cross section </li></ul><ul><li>I= Fault current in Amps. at the station </li></ul><ul><li>=Fault MVA x 1000 </li></ul><ul><li> 3 x system kV </li></ul><ul><li>and t= Time in seconds during which current is </li></ul><ul><li>applied </li></ul><p>Earthing System Size of earth mat conductor 36. Earthing materials </p> <ul><li>Determination of size of conductor for earth mat. </li></ul><ul><li>Based on thermal stability determined by an approximate formula of IEEE - 80-1986 </li></ul><ul><li>A = I/ ( TCAP x10 4 )I n(K o+ T m ) </li></ul><ul><li>t cx i r r(K o+ T a ) </li></ul><ul><li>Where </li></ul><ul><li>In case of steel </li></ul><ul><li>A = I x 12.3tc mm for welded joints </li></ul><ul><li>= I x 15.13 tc mm for bolted joints </li></ul><ul><li>In case tc = Duration of current =1sec </li></ul><ul><li>A = 12.3 x I mm for welded joints </li></ul><ul><li>= 15.3 x I mm for bolted jo...</li></ul>