13 earthing grounding

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Describes about electrical earthing

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  • System Earthing

  • System EarthingEarth faults :- 70 90% of all faults.IFEA

  • System EarthingEarthing method determines :-

    Fault current IFDamage causedSteady state over voltagesTransient over voltagesInsulation requirementsQuantities available to detect faultsType of Protection

  • Earthing MethodSolid / Low ZHigh Z

    IF High Low

    Overvoltages in Low HighSound Phases

    Damage High Low

    Cost of Insulation Low High

    Low Voltage Systems For Safety

    Medium Voltage SystemsTo limit currentcost of insulationacceptable

    High Voltage &To limit costEHV Systemsof insulation

  • Methods of Earthing In Common UseSolid or Direct EarthingResistance EarthingReactance EarthingResonant or Petersen Coil EarthingInsulated Earth

  • System EarthingSolidLowest System Z0IF High - Damage - Easy E/F Protn.No Arcing Grounds IF >> ICHARGELowest Overvoltages

  • System EarthingReactanceLower IFHigher Transient OvervoltagesCheaper than resistance at high voltsOvervoltages during E/Fs 0.8 1 x V/ Not often used except as tuned reactor

  • System EarthingPetersen CoilXE XCHARGINGArcing faults self extinguishing - Good for transient faultsXE needs changing if XC altersOvervoltages during E/Fs V/ Insulation importantRestricts use of auto-transformersDiscriminative E/F protection difficultTuned

  • System EarthingResistanceReduced IF Reduced transient overvoltagesNot self extinguishing but E/F easier to detect

  • System EarthingUnearthedInsulatedIF CapacitiveCan be self extinguishing if IF smallOvervoltages during E/Fs = V/ Arcing faults likely - high transient overvoltagesInsulation important

  • System Earthing 660 VSolid-SafetyInsulated-Special cases where continuity of supply required

    660 V 33 kVResistance or reactance normally used

    Solid-When IF is lowResistance-IF limited to IFLReactance-IF(E/F) limited to IF(3) Petersen-Overhead lines. Lightning Coil

    > 33 kVSolidOvervoltages more important (insulation)

    Directly CoupledResistance-Most commonGeneratorsSolid and-Not recommendedReactance(High IF )

  • System EarthingGenerator - Transformer UnitsIF ~ 200 300 AIF ~ 10 15 A

  • Low Voltage System EarthingSafety :-Power system neutral solidly earthed at transformer.Metallic tools and appliances solidly earthed.Sensitive protection by :- RCDs :- Residual current devices ELCBs :- Earth leakage circuit breakers

  • Earth Fault HazardZF =Fault impedanceZP =Human body impedanceZE =Environmental impedanceVP =Case / earth potential

  • Earth Fault HazardZF =Fault impedanceZP =Human body impedanceZE =Environmental impedanceVP =Case / earth potentialZFZEVPZPUnearthed ApplianceIFVHRCD for High ZFFuses for High IFProtective Earth Conductor

  • Unearthed L.V. Winding

  • Unearthed L.V. Winding

    H.V.xVyvL.V.VFVF = xV + (1 - y)vInter-winding fault F causes dangerous rise in L.V. voltage

  • Breakdown Between HV and LV Windings3000 / 440 V Transformer1730VA2B2C2Nc2b2a2254VnNormal voltage conditions Neutrals earthed or unearthed

  • Breakdown Between HV and LV WindingsVoltage conditions with breakdown between HV and LV at point X on phaseLV neutral unearthed1730V850VA2B2C2xHxnc2b2a2xL755V254V1009V95V

  • Hand to Hand Resistance of Living Body - 50Hz AC (Freiburger 1933)6000

    5000

    4000

    3000

    2000

    10000 100 200 300 400 500 600VoltsResistance - OhmsVery Dry SkinVery Moist Skin

  • Effects of Body Current1mACan be felt

    > 9mACannot let go

    15mAThreshold of cramp

    30mABreathing difficultRise in blood pressure

    50mAHeart misses odd beat

    50 200mAHeavy shockUnconsciousness

    > 200mAReversible cardiac arrestCurrent marksBurns

  • Effects of Various Values of Body CurrentCurrent at 50HzDuration Physiological effects on humansto 60Hz r.m.s.of shockvalue mA

    0-1notRange up to threshold of perception.criticalElectrocution not felt.

    1-15notRange up to threshold of cramp.CriticalIndependent release of hands from object gripped no longer possible. Possibly powerful and sometimes painful effects on muscles of fingers and arms.

    15-30minutesCramp-like contraction of arms. Difficulty in breathing. Rise in blood pressure. Limit of tolerability.

    30-50seconds Heart irregularities. Rise in blood pressure. Powerful cramp-effect. to minutes Unconsciousness. Ventricular fibrillation if long shock at upper limit of range.

    less thanNo ventricular fibrillation. Heavy shock.50 to a cardiac cyclefew hundredabove oneVentricular fibrillation. Beginning of electrocution in relation to heart phase notcardiac cycle important. (Disturbance of stimulus conducting system?) Unconsciousness. Current marks.

    less thanVentricular fibrillation. Beginning of electrocution in relation to heart phase cardiac cycleImportant Initiation of fibrillation only in the sensitive phase. Above(Direct stimulatory effect on heart muscle?) Unconsciousness. Current marksfew hundredover oneReversible cardiac arrest. Range of electrical defibrillation. Unconsciousness. cardiac cycleCurrent marks. Burns

  • Body Current / Time and Security10,000

    1,000

    100

    100.1 1.0 10 100 1000Current (mA)Time(mS)ThresholdofPerceptionThresholdofLet GoLet GoHold OnIEC Security CurveThresholdofFibrillation

  • Voltage Range for Protection Operation0.1 1.0 10 100 1000Current (mA)V(Hz)ThresholdofPerceptionLet GoRegionHold OnRegionVery Moist Skin1,000

    100

    10

    1.020mAVery Dry SkinVolts to operate protection30V - 80VFibrillationPossible

  • Earthing Impedance Affects Touch & Step PotentialsSurfaceTrue EarthRETouchVHVHStepERFIFRGIFIFTrue EarthRGRG' = f(Distance)d!Dont forget communications cables etc. entering S/S !

  • Interconnected Star (Zig-Zag) Earthing TransformerSingle Earthing Resistor2III3I3IEarthFaultIIITransformer Insulatedfor Line VoltageResistor InsulatedFor System PhaseVoltage3I2IIIII

  • Interconnected Star Earthing TransformerThree Earthing Resistors2III3I3IEarthFaultIII3I2IIIIIIIIResistors3INote:- Resistors to be insulated for line voltage and to have 3 times the ohmic value of a single neutral resistor

  • Displacement of Neutral from Earth during an Earth FaultVaVbVcZENZGZZIFVaVbVcGN

  • Earth Fault on System with Insulated EarthVaIFIcIbVbVcN-jXcG-jXc-jXc

  • Earth Fault on System with Insulated EarthGVabVacVaVbVcNIF3030

  • Earth Fault on System with Resistance Earthed SystemVaIFVbVcN-jXcG a-jXc-jXcREa, Ga, G

  • Earth Fault on System with Resistance Earthed SystemGVabVacVaVbVcN-Ib-IcIFcharging currentcharging current

  • Earth Fault on System with Resonant or Petersen Coil EarthingVaIFVbVcN-jXca,G-jXc-jXca, Ga, G-jXL

  • Earth Fault on System with Resonant or Petersen Coil Earthing

  • Sequence ImpedancesPositive Phase-Sequence Network :-ZL1X1XZT1ZG1P1-jX'C1-jXC1EaN1GeneratorGeneratorTransformerTransmission LineFaultCapacitance ofthe transmissionsystemC'CZEZ'E

  • Sequence ImpedancesNegative Phase-Sequence Network :-X2ZL2ZT2ZG2P2-jX'C2-jXC2N2ZL0X0ZT0ZG03ZEP03Z'E-jX'C0-jXC0N0Zero Phase-Sequence Network :-

  • Fault Currents and Voltages Analysis of Single Phase to Earth and Double Phase to Earth FaultsThe following analysis relates to the system shown in Figure 7.

    Let Z1, Z2 and Z0 be the system sequence impedances in the fault path. Let Z2 = K2Z1 and Z0 = K0Z1.

    For a phase to earth fault :

    I1 = I2 = I0 = Ea/Z1 + Z2 + Z0 = Ea/Z1 (1 + K2 + K0)

  • Fault Currents and Voltages Analysis of Single Phase to Earth and Double Phase to Earth FaultsFor a phase to phase to earth fault :

  • Fault Currents and Voltages Analysis of Single Phase to Earth and Double Phase to Earth FaultsAlso :

    V1 = Ea - I1Z1; V2 = -I2Z2 = -I2K2Z1; V0 = -I0Z0 = -I0K0Z1

    Ia = I1 + I2 + I0; Va = V1 + V2 + V0

    Ib = a2I1 + aI2 + I0; Vb = a2V1 + aV2 + V0

    Ic = aI1 + a2 I2 + I0; Vc = aV1 + a2V2 + V0

    From all these equations it is possible to calculate the fault currents and voltages at the fault location in terms of the phase sequence impedances of the system. The values of these currents and voltages are shown in Table 2.Currents have been expressed in terms of the three phase fault current where I3 = Ea/Z1

  • Sequence Connections for Phase to Earth FaultP2N1P1Z0Z2Z1-jX'C1-jXC1EaX1ZL1ZT1ZG1-jX'C2-jXC2X2ZG2ZT2ZL2P0N2-jX'C0X03Z'EZT0ZL0N0I2I0I1

  • Phase to Earth Fault

  • Phase to Earth Fault

  • Phase to Earth Fault

  • Sequence Connections for Phase to Phase to Earth FaultP2N1P1Z0Z2Z1-jX'C1-jXC1EaX1ZL1ZT1ZG1-jX'C2-jXC2X2ZG2ZT2ZL2P0N2-jX'C0X03Z'EZT0ZL0N0I2I0I1

  • Steady-state Fault Currents and Voltages for Phase-to-Earth and Double-Phase to Earth Faults

  • Steady-state Fault Currents and Voltages for Phase-to-Earth and Double-Phase to Earth Faults

  • Independent of earthing methodNormally K2 = 1Close to power stations with synchronous generators :-K2 up to 1.4X2 for cylindrical rotors = Xd"for salient poles = Xd" to 1.4 Xd"

  • Depends on method of earthingRelative values of transformer, generator and line impedancesTransformer winding

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