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Transcript of Earthing Practices
Seminar on Earthing Systems organized by C.O.E.T. s P.V.G. College of Engg. Pune Earthing of E.H.V. Sub-station & Transmission line
Earthing PracticesBy Er. J. M. PardhiSuperintending Engineer,Testing Circle, MSEDCL,Nagpur.
11Objectives of EarthingTo dissipate electric currents into the earth without exceeding operating & equipment limits.To ensure human safety from electric shock.To prevent hazardous voltage for reducing risk of fires and personal injuries.
22Earthing SystemsBasically 6 grounding systems in use:Equipment grounds.Static grounds.Systems grounds.Maintenance grounds.Electronic grounds.Lightning grounds.3Equipment GroundsIt is the physical connection to earth of non current carrying metal parts.All metal parts of equipments that personal may come into contact to be at zero potential with respect to ground.All metal parts must be interconnected and grounded to ensure path of lowest impedance for flow of ground fault current.4Equipment Grounds5
Static GroundsConnection made between a piece of equipment and earth for the purpose of draining off electricity charges before a flashover potential is reached.Utilized in dry materials handling, flammable liquid pumps and delivery equipments, plastic piping and explosive storage facilities.6System GroundsRefers to the point in an electrical circuit that is connected to earth through electrical neutral.The sole purpose is to protect equipment. Provides low impedance path for fault currents improving ground fault coordination. Ensures longer insulation life of motors, transformers and other system components.7TEAMWORKnet, Inc.8TYPES OF GROUNDING SYSTEMSUngrounded System:The ungrounded system is one that has no intentional connection between the neutral or any phase and ground. Please note that an ungrounded system is grounded through the concept of capacitively coupling. The neutral potential of an ungrounded system, with balanced loading will be close to ground potential due to the capacitance between each phase conductor and ground.Low ground fault current.Very high voltages to ground potential on unfaulted phases.Sustained faults lead to system line-to-line voltages on unfaulted line.Insulation failure.Failure due to restrike ground faults.TEAMWORKnet, Inc.9TYPES OF GROUNDING SYSTEMSSolidly Grounded System:The solidly grounded system is one that has the neutral connected to ground without an intentional impedance. In contrast to the ungrounded system the solidly grounded system will result in a large magnitude of current to flow (Aids in coordination), but has no increase in voltage on unfaulted phases.
Low initial cost to install and implement, but stray currents then become a possible consequence.Common in low voltage distribution systems, such as overhead lines.typically feeds to transformer primary with high side fuse protection.Not preferred for industrial or commercial facilities due to high magnitude fault currents.10
Normal Pipe type Earthing for trans. line1010Distribution Transformer Double Pole Structure.
- as per REC Construction Standard.11
Typical arrangement of earthing to railway traction system.
HT Metering Cubicle13
TEAMWORKnet, Inc.20TYPES OF GROUNDING SYSTEMSLow Resistance Grounded System:The low resistance grounded system is one that has the neutral connected to ground through a small resistance that limits the fault current. The size of the grounding resistor is selected to detect and clear the faulted circuit..
The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.Limits transient overvoltages during ground faults.Low resistance grounding is not recommended for low voltage systems due to the limited ground fault current. This reduced fault current can be insufficient to positively operate fuses and/or series trip units.Ground fault current typically in the 100 600 Amp range.TEAMWORKnet, Inc.21TYPES OF GROUNDING SYSTEMSHigh Resistance Grounded System:The high resistance grounded system is one that has the neutral connected to ground through a resistive impedance whose resistance is selected to allow a ground fault current through the resistor equal to or slightly more that the capacitive charging current of the system. The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.Limits transient overvoltages during ground faults.Physically large resistor banks.Very low ground fault current, typically under 10 Amps.Special relaying methods utilized to detect and remove ground faults.High resistance grounding is typically applied to situations where it is essential to prevent unplanned outages.Recent trend has been to utilize high resistance grounding methods on 600 volt systems and lower.22Components of earthing system in Sub-stn.
22Sub Station Earth Mesh23
Maintenance GroundsUtilized for safe work practices.It is a temporary ground.24Electronic GroundsEquipment ground and system ground are combined and applied in unity.Must not only provide a means of stabilizing input voltage levels, but also act as zero voltage reference point. Must be able to provide effective grounding and bonding functions well into the high frequency MHz range.TEAMWORKnet, Inc.25System GroundsMethods of System GroundingCharacteristicsUngroundedSolid GroundLow Resistance GroundHigh Resistances GroundSusceptible to Transient overvoltagesWORSTGOODGOODBESTUnder fault conditions (line-to-ground) increase of voltage stressPOORBESTGOODPOORArc Fault DamageWORSTPOORGOODBESTPersonnel SafetyWORSTPOORGOODBESTReliabilityWORSTGOODBETTERBESTEconomics' (Maintenance costs)WORSTPOORPOORBESTPlant continues to operates under single line-to-ground faultFAIRPOORPOORBESTEase of locating ground faults (time)WORSTGOODBETTERBESTSystem coordinationNOT POSSIBLEGOODBETTERBESTUpgrade of ground systemWORSTGOODBETTERBESTTwo voltage levels on same systemNOT POSSIBLEPOSSIBLENOT POSSIBLENOT POSSIBLEReduction in number of faultsWORSTBETTERGOODBESTInitial fault current Into ground systemBESTWORSTGOODBETTERPotential flashover to groundPOORWORSTGOODBESTLightning GroundsDepends upon the structure, equipment to be protected and the level of lightning protection required of desired.26
Straight risersSeparate earth rod per phaseRods connected to each other & to Earth-grid.No G.I. pipe for riser.Earthing of LAEarthing in a EHV SubstationCEA Regulation 2010 (I.E.Rules 1956)27
Earthing in a EHV SubstationCEA Regulation 2010 (I.E.Rules 1956)28
Earth Mat DesignEarthing system in a Sub Station comprises of;Earth mat or grid.Earth electrode.Earthing conductor.Earth connectors.Primary requirement of earthing is to have a low earth resistance.To consider safe limit of step/ touch/ transfer potential .29Surface potentials in a Sub-station1)Step potential: The difference in surface potential experienced by a person bridging a distance of 1 meter with the feet, without contacting any grounded object .
2)Touch potential: The Potential difference between the Ground Potential Rise (GPR) & the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure.
3030Ground Potential Rise (GPR)The maximum electrical potential that a sub-station grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage is equal to: G.P.R.= Ig x Rg where, Ig= Maximum grid current Rg= Grid resistance (Grid means Earth grid i.e. Earthing system ) 3131Step & Touch Voltages
32Tolerable Step & Touch voltagesEstep = 0.116 (Rb + 2Rf) / ts = 0.116 (1000 + 2 x 3 ) / ts = 0.116 (1000 + 6 )/ ts where, = Resistivity of soil.
Etouch = 0.116 ( 1000+ Rf/2)/ ts = 0.116 ( 1000 + 1.5 )/ ts
While designing earthing system, care is taken to ensure that actual Step & Touch voltages are well within tolerable Step & Touch Voltages.Note that tolerable touch voltage is much less than tolerable step voltage. Hence, is critical. 3333METHOD OF MEASURING SOIL RESISTIVITY:Geological information and soil samplesVariation of depth methodTwo point methodFour point method - Equally spaced Wenner arrangement - Unequally spaced or Schlumberger - Palmer arrangement
34WENNER FOUR ELECTRODE METHOD:35
UNEQUALLED SPACED OR SCHLUMBERGER PALMER ARRANGEMENT:36
EQUIPMENT USED FOR MEASUREMENT OF SOIL RESISTIVITY:37
FACTORS AFFECTING THE SOIL RESISTIVITY ():Type of the soil.Moisture.Dissolved salt in water.Temperature.Grain size and its distribution.Seasonal variation.Artificial treatment.
38Effect of temperature on soil resistivity 390 C50C-15 CRESISTIVITYTEMPERATURE39Transferred PotentialIt is a special case of the touch voltage where a voltage is transferred into or out of the sub-station from or to a remote point external to the sub-station site.A person standing in a sub-station coming in contact with say rails/water pipeline/neutral coming from an adjacent sub-station at the time of occurrence of earth-fault at that sub-station gets exposed to the transferred potential which equals difference in GPRs of the two sub-stations. 4040Transferred Potential
41Factors influencing Earth Mat DesignMagnitude of Fault Current.Duration of Fault.Soil resistivity.Resistivity of Surface Material.Shock DurationMaterial of Earth Mat Conductor.Earthing Mat Geometry.42Calculation of size of earth grid conductor.Conductor area A = I . root[ T cap x 10-4 x ln Ko + Tm ] tc r x PvKo + Ta
Simplified formula for steel grid conductorArea A = I x t where; A =area of earth conductor in mm2 K I = short circuit current in kA t = duration of short circuit current in sec. K factor = 80 for steel43Calculation of size of earth grid conductor. If mat is made up of Flat then; A = Area of FlatGive allowance of 35% for corrosion
If it is a conductor; A = r2 , where r = (A) /
STANDARD FLATs in mm10 x 6, 20x6, 30x6, 40x6, 50x6, 60x6, 50x8, 65x10, 75x10
ROD: 40mm mild steel rod
44Maintenance ScheduleWatering of earth pit- DailyMeasurement of earth resistance of individual earth pit- Half yearlyMeasurement of combined earth resistance at all the pits- Half yearlyChecking of inter connectionsbetween earth pits and tightnessof bolts and nuts- Quarterly45Case StudyFatal Electrical Accident to farmer at Kholapur (Amravati) dtd:06/06/2012.Bird fault occurred at 11kV cut point.Support insulator not provided for cut point jumpers.High resistance arcing fault.Sufficient fault current not developed.Feeder protection not operated.High soil resistivity and high impedance.Electric pole, stay and soil acted as load.
Earth fault current- two return paths 51 Parallel path for fault current through Ground Wire of trans. line Grid current Ig through earthSourceFault5152 Electric currents & their effects on Human bodySr.No A.C. Current range (mA)
Called asEffect11.0ThresholdTingling21 to 6Let-goCan hold & release39 to 25 >More than 25 >Painful breathlessness.Difficult to release4116alarmVentricular fibrillation, heart failure.52Tolerable Body currentDr. Dalziel observed that the limiting current a human body weighing 50Kg can tolerate without ventricular fibrillation is given by: Ib = 0.116/ts where, ts=duration of shock currentSimilarly, Ib = 0.157/ts for 70 Kg. weight.If ts = 1second, Ib=116 milli-amps or 157 ma.With numerical relays ts= 240 ms Then tolerable Ib= 0.116/.240= 236milli-amps.5353Earthing of EHV sub-stations- challengesWith advent of transmission voltage of 765KV & ambitious programme of addition of Generation capacities at national level (78700 MW by 2012) fault levels are going up.With higher X/R ratios of lines, D. C. component in fault current takes longer time for decay. Results in higher temp. rise of earthing system.Breaker re-closures increase duration of shock currents needing more efficient earthing system.Switching & lightning surges need good earthing. 5454Earthing system of EHV sub-stations- care to be taken upMeasurement of average resistivity of new s/s by taking numerous readings spread all over the area.Projected fault level at the s/s in next 10 years to be considered while deciding earthing system.Allowance for corrosion of electrode(15% for 12 years) in moderately corrosive soil to be kept.5555Monitoring & maintenance of Earthing systemContinuity of under-ground earth conductors to be checked.Continual observation, tightening, painting of over-ground connections. Measurement of earth resistance at least once a year (I.E. Rules, section 67).Treatment by Bentonite where necessary. Maintaining Stone metal cover 100 mm. Removal of grass, providing earth mat at operating points. 5656Earthing of ehv transmission line5757System of earthingGround-wire strung on tower peaks & bonded to tower body.Earthing at tower bases by Pipe type or Counter-poise type.Surge Arresters provided at wide creek crossings.Electrode line with special earthing station for HVDC line for Single pole earth return mode.Earth resistance below 10 ohms to avoid back flash. 5858 Thank you!5959