2 Earthing Grounding

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GRID

Technical Institute

System Earthing



> System Earthing 2

System Earthing

Earth faults :- 70

90% of all faults.

IF

E A



> System Earthing 3

System Earthing

Earthing method determines :-

Fault current IF

Damage caused

Steady state overvoltages

Transient overvoltages

Insulation requirements

Quantities available to detect faults

Type of Protection



> System Earthing 4

Earthing Method

Solid / Low Z High Z

IF High Low

Overvoltages in Low High

Sound Phases

Damage High Low

Cost of Insulation Low High

Low Voltage Systems For Safety

Medium Voltage Systems To limit currentcost of insulation

acceptable

High Voltage & To limit cost

EHV Systems of insulation



> System Earthing 5

Methods of Earthing In Common Use

Solid or Direct Earthing

Resistance Earthing

Reactance Earthing

Resonant or Petersen Coil Earthing

Insulated Earth



> System Earthing 6

System Earthing

Solid

Lowest System Z0

IF High

- Damage

- Easy E/F Protn.

No Arcing Grounds IF >> ICHARGE

Lowest Overvoltages


http://slidepdf.com/reader/full/2-earthing-grounding 7/70> System Earthing 7

System Earthing

Reactance

Lower IF

Higher Transient Overvoltages

Cheaper than resistance at high volts

Overvoltages during E/Fs

0.8 1 x VØ/Ø

Not often used except as tuned reactor



System Earthing

Petersen Coil

XE XCHARGING

Arcing faults self extinguishing

- Good for transient faults

XE needs changing if XC alters

Overvoltages during E/Fs VØ/Ø

Insulation important

Restricts use of auto-transformersDiscriminative E/F protection difficult

Tuned



System Earthing

Resistance

Reduced IF

Reduced transient overvoltages

Not self extinguishing but E/F easier todetect



System Earthing

Unearthed

Insulated

IF Capacitive

Can be self extinguishing if IF small

Overvoltages during E/Fs = VØ/Ø

Arcing faults likely - high transientovervoltages

Insulation important



System Earthing

660 V Solid - Safety

Insulated - Special cases where continuityof supply required

660 V 33 kV Resistance or reactance normally used

Solid - When IF is low

Resistance - IF limited to IFL

Reactance - IF(E/F) limited to IF(3Ø)

Petersen - Overhead lines. LightningCoil

> 33 kV SolidOvervoltages more important (insulation)

Directly Coupled Resistance - Most common

Generators Solid and - Not recommended

Reactance (High IF

)



System Earthing

Generator - Transformer Units

IF ~ 200 300 A

IF ~ 10 15 A



Low Voltage System Earthing

Safety :-

Power system neutral solidly earthed at transformer.

Metallic tools and appliances solidly earthed.

Sensitive protection by :-

RCD’s :- Residual current devices

ELCB’s :- Earth leakage circuit breakers



Earth Fault Hazard

ZF = Fault impedance

ZP = Human body impedance

ZE = Environmental impedance

VP = Case / earth potential

ZF

ZE

VP

Z

P

Unearthed

Appliance



Earth Fault Hazard

ZF = Fault impedance

ZP = Human body impedance

ZE = Environmental impedance

VP = Case / earth potential

EFP

P/NH

ZZZ

Z . E V

-:earthprotectiveWithout

ZF

ZE

VP

Z

P

Unearthed

Appliance

IF

VH

RCD for High ZF

Fuses for

High IF

Protective Earth Conductor



Unearthed L.V. Winding

NormalConditions

H.V.

V

v

L.V.



Unearthed L.V. Winding

H.V.

xV

yv

L.V.

VF

VF = xV + (1 - y)v

Inter-winding fault ‘F’

causes dangerous risein L.V. voltage



Breakdown Between HV and LV Windings

1730V

A2

B2 C2

N

c2 b2

a2 254V

n

3000 / 440 V Transformer

Normal voltage conditionsNeutrals earthed or unearthed



Breakdown Between HV and LV Windings

Voltage conditions with breakdownbetween HV and LV at point X on phaseLV neutral unearthed

1730V

850V

A2

B2 C2

xH x

n

c2 b2

a2

xL

755V

254V

1009V

95V



Hand to Hand Resistance of Living Body50Hz AC (Freiburger 1933)

6000

5000

4000

3000

2000

1000

0 100 200 300 400 500 600

Volts

R e s i s t a n c e - O h m s

Very Dry Skin

Very Moist Skin



Effects of Body Current

1mA Can be felt

> 9mA Cannot let go

15mA Threshold of cramp

30mA Breathing difficultRise in blood pressure

50mA Heart misses odd beat

50 200mA Heavy shock

Unconsciousness

> 200mA Reversible cardiac arrest

Current marks

Burns

Eff f V i V l f B d C



Current at 50Hz Duration Physiological effects on humans

to 60Hz r.m.s. of shock

value mA

0-1 not Range up to threshold of perception.critical Electrocution not felt.

1-15 not Range up to threshold of cramp.

Critical Independent release of hands from object gripped no longer possible. Possibly

powerful and sometimes painful effects on muscles of fingers and arms.

15-30 minutes Cramp-like contraction of arms. Difficulty in breathing. Rise in blood pressure.

Limit of tolerability.

30-50 seconds Heart irregularities. Rise in blood pressure. Powerful cramp-effect. to minutes

Unconsciousness. Ventricular fibrillation if long shock at upper limit of range.

less than No ventricular fibrillation. Heavy shock.

50 to a cardiac cycle

few hundred

above one Ventricular fibrillation. Beginning of electrocution in relation to heart phase not

cardiac cycle important. (Disturbance of stimulus conducting system?)Unconsciousness. Current marks.

less than Ventricular fibrillation. Beginning of electrocution in relation to heart phase

cardiac cycle Important Initiation of fibrillation only in the sensitive phase.

Above (Direct stimulatory effect on heart muscle?) Unconsciousness. Current marks

few hundred

over one Reversible cardiac arrest. Range of electrical defibrillation. Unconsciousness.

cardiac cycle Current marks. Burns

Effects of Various Values of Body Current

B d C / Ti d S i



> System Earthing 23

Body Current / Time and Security

10,000

1,000

100

10 0.1 1.0 10 100 1000

Current (mA)

Time

(mS)

Thresholdof

Perception

Thresholdof

Let Go

Let Go Hold On

IEC SecurityCurve

Thresholdof

Fibrillation

E thi I d Aff t T h & St P t ti l




Earthing Impedance Affects Touch & Step Potentials

Don’t forget

communicationscables etc.entering S/S !

Surface

True

Earth

RE Touch

VH VH

Step

E

RF IF

RG

IF

IF

True EarthRG

RG' = f(Distance)

d

!

'RRR

'R E V

GFE

GH

I t t d St (Zi Z ) E thi T f




Interconnected Star (Zig-Zag) Earthing TransformerSingle Earthing Resistor

I

2I

I

3I

3I Earth

Fault

I IITransformer Insulated

for Line Voltage

Resistor Insulated

For System Phase

Voltage

3I2II

I II

I t t d St E thi T f




Interconnected Star Earthing Transformer Three Earthing Resistors

2I

I

I

3I

3IEarth

Fault

I II

3I2II

I II

I IIResistors

3I

Note:- Resistors tobe insulated for linevoltage and to have 3times the ohmic valueof a single neutralresistor

Di l t f N t l f E th




Displacement of Neutral from Earthduring an Earth Fault

Va

Vb Vc

ZE

N

Z

G

Z

Z

IF

Va

Vb Vc

G

NZZ

Z . V Z V

E

EaNEFGN

E th F lt S t ith I l t d E th




Earth Fault on System with Insulated Earth

Va

IF

Ic

Ib Vb Vc N

-jXc

G

-jXc -jXc

c

ab

jX-V

c

ac

jX-

V

c

ac

jX-

V

c

ab

jX-

V

E th F lt S t ith I l t d E th




Earth Fault on System with Insulated Earth

G

Vab Vac

Va

Vb Vc N

IF 30 30

bc

ab - jX-

Vc

ac

jX-

V

cc

ac - jX-

V

currentchargingnormalx3 jX-

3V

jX-

V

jX-

V

c

aNc

ac

c

abF

currentchargingnormalx3

X

V . 3

jX-

V

jX-

V -

currentchargingnormalx3 XV . 3

jX-V

jX-V -

c

cc

c

ca

c

acc

cbb

cac

cabb

Earth Fault on System




Earth Fault on Systemwith Resistance Earthed System

Va

IF

Vb Vc

N

-jXc

G a

-jXc -jXc RE

a, G a, G

c

ac

jX-

V

c

ab

jX-

V

E

aN

R

V

Earth Fault on System




Earth Fault on Systemwith Resistance Earthed System

G

Vab Vac Va

Vb

Vc

N

-Ib -Ic

IF

bc

ab - jX-

V

cc

ac - jX-

V

E

aNR

V

normalx3 X

V 3

normalx3 X

V

3

c

cc

c

bb

charging current

charging current

Earth Fault on System with Resonant




Earth Fault on System with Resonantor Petersen Coil Earthing

Va

IF

Vb Vc

N

-jXc

a,G

-jXc -jXc

a, G a, G

-jXL c

ac jX-V

c

ab jX-V

LaN jXV

c

abb

jX-

V -

Earth Fault on System with Resonant or Petersen




Earth Fault on System with Resonant or PetersenCoil Earthing

G

Vab Vac

Va

Vb Vc N -I

c

-Ib c

c

ac - jX-

V

b

c

ab - jX-

V

L

aN jX

V

L

aN jX

V

cbL

aNL

aN

cbF

jX

V if 0

jX

V - -

3

X X

2jX

3.3

2jX

3.3

jX

V tuningperfectFor c

L

ccL

/N

Sequence Impedances




Sequence Impedances

ZL1 X1

X

ZT1 ZG1 P

1

-jX'C1 -jXC1 Ea

N1

Generator

Generator

Transformer

Transmission Line

Fault

Capacitance of

the transmission

system

C'C

ZE Z'E

Positive Phase-Sequence Network :-

Sequence Impedances




Sequence Impedances

Negative Phase-Sequence Network :-

X2 ZL2 ZT2 ZG2 P2

-jX'C2 -jXC2

N2

ZL0

X0

ZT0 ZG0

3ZE

P0

3Z'E -jX'C0 -jXC0

N0

Zero Phase-Sequence Network :-

Fault Currents and Voltages Analysis of Single




Fault Currents and Voltages Analysis of SinglePhase to Earth and Double Phase to Earth Faults

The 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)






For a phase to phase to earth fault :

)KKK(KZ

K.E-

ZZ

Z .

)KKK(KZ

K.E-

ZZ

Z .

KK

KK

1Z

E

ZZ

ZZ Z

E

02021

2a

02

210

02021

0a

02

012

02

02

1

a

02

02

1

a1






Also :

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 + a2I2 + I0; Vc = aV1 + a2V2 + V0

From all these equations it is possible to calculate the faultcurrents and voltages at the fault location in terms of the phasesequence 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 faultcurrent where I3Ø = Ea/Z1

Sequence Connections for Phase to Earth Fault




-jX'C0

X0 3Z'E ZT0 ZL0

Sequence Connections for Phase to Earth Fault

P2 N1

P1

Z0 Z2

Z1

-jX'C1 -jXC1 Ea

X1 ZL1 ZT1 ZG1

-jX'C2

-jXC2

X2 ZG2 ZT2 ZL2 P0 N2 N0

I2 I0

I1

Phase to Earth Fault





302

a1

3

0211a

101100000

121122222

111

02110121021021

. )KK(1

3

Z

E

-:pointsametheatfaultaFor

KK1Z

3E 3

ZK- ZK- Z- V

ZK- ZK- Z- V

Z-E V

KK1ZE

ZKZKZE

ZZZE






a02

022

cc

a02

022

b

0221021

ab

022

11a2

10112111a2

0212

b

E . K K 1

K Ka a

- E V

E . K K 1

K aK a

- E

K aK a Z . )KK(1Z

E - E

K aK aZ - Ea

ZK - ZK-a Z - Ea

V aV Va V






a02

0

a02

0acb

02

02acb

a

02

022

022

cb

caRES

E . )K K 1(

K3

E . )K K 1(

K3 - E E E

K K 1

2K K 1 E E E

E . K K 1

K Ka a K aK a

- E E

V Vb V V

Sequence Connections for Phase




Sequence Connections for Phaseto Phase to Earth Fault

P2

N1

P1

Z0

Z2 Z

1

-jX'C1 -jXC1 Ea

X1 ZL1 ZT1 ZG1

-jX'C2

-jXC2

X2 ZG2 ZT2 ZL2

P0

N2

-jX'C0

X0 3Z'E ZT0 ZL0 N0

I

2

I0

I1

Steady-state Fault Currents and Voltages for




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

a0202

02a

30202

2ares

30202

202

c

30202

20

22b

302

a

E . KK K K

KK3 0 V

KK K K

3K- toEqual

. KK K K

)a-a(K )1(aK 0

. KK K K

a)-a(K )1(aK 0

0 . K K 1

3

e)-c-(bfaultearthtophaseDouble e)-(afaultearthtoPhase

Steady-state Fault Currents and Voltages for




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

E .. K K 1

K3 V

VtoEqual VtoEqual V

0 E K K 1

1 K E V

VtoEqual V-toEqual V

0 E K K 1

K Ka a - E V

0 E K K 1

K Ka a - E V

a02

0res

acca

bc02

2bcbc

ababa02

022

cc

a02

022

bb




122 Z

Z K

Independent of earthing method

Normally K2 = 1

Close to power stations with synchronousgenerators :-

K2 up to 1.4

X2 for cylindrical rotors = Xd"

for salient poles = Xd" to 1.4 Xd"




1

00

Z

Z K

Depends on method of earthing

Relative values of transformer, generator and line impedances

Transformer winding arrangement

Amount and angle of ZLINE

K0 has an angle

K0 ranges from for unearthed system to 0.2 for solid earthing andfault close to a power station.

Line

XS1 = 25%

XS2 = 25%

XT1

= XT2

= XT0

=

7%

0.219

32

7

7 25

7

X

X

1

0

Variation of Healthy Phase Voltages Due to




Variation of Healthy Phase Voltages Due toDifferent System Earthing for an A-E Fault

abab

a/2ba

2

b

ba

2

b

a/5ba

2

b

a/2ba

2

b

2

B0

E E E - E

E E E 411

4aa - E 4

E E 111

1aa - E 1

E E E 0.5110.50a - E 0.5

E E E 11

aa - E 0

1K 2Tableinformula

fromcalculatedVK

Variation of Healthy Phase Voltages Due to




Variation of Healthy Phase Voltages Due toDifferent System Earthing for an A-E Fault

Ea

G

V'b

Eb

V"b

V'c

Ec

V"c

Effectively earthed systems

K = 0

K = 0.5

K = 1

K = 4

K =

Non-effectively earthed

systems

Healthy Phase Voltages during Earth Faults





a0

02

bb

122

a02

022

bb

E K2

Kaa E V

01 /ZZ K Assuming

E KK1

KaKa

- E V

K0 = 0.5

Solid earthing; Fault near power station

VS.P. < VØ/N rated (- 0.95 VØ/N)

K0 = 1.0

Solid earthing; Fault remote from power station VS.P. = VØ/N rated





ea y ase o ages du g a au s

K0 4.5

- Solid earthing; Remote fault; Long line

ZL0 /ZL1 can be 4.5

- Also possible with low reactance earthing

- VS.P. = 0.75 VØ/Ø rated with K0. = 4

K0 > 1.0

VS.P.

> VØ/N

rated





y g g

Effectively Earthed Systems

British definition (BS 162 : 1961) :-

VS.P. > 80% of VØ/Ø rated

Note :- VS.P. > 0.8 VØ/Ø rated

is possible on solidly earthed systems

Overvoltage relays should not be connectedØ/N or may operate for earth faults.

American definition :-

X0 /X1 3 and R0 /X1 1

K0 = high gives non effectively earthed systeme.g. For Resistance

Petersen Coil

Insulated VSP = VØ/Ørated Z0 }

VØ/Ø during Earth Fault




/ g

bc02

2bcbc E

KK1

1 K E V

If generator AVR is not in service :-

Id"

Id'

Id

Few Seconds

I Z2 and Z0 fixedZ1 varies from Xd" to Xd, i.e. increases

If K2 = Z2 /Z1 varies from 1 to 0.2

K0 = Z0 /Z1 varies from 3.0 to 0.6

Vbc = Ebc

bc

bc

bc02

2bcc'b'

E0.556

E0.60.21

1.0-0.2 1

E . KK1

1 K E V

Variation of Healthy Phase Voltages for an Earth Fault due




to changes of K2 and K0 during the Fault

Vc.and Vd are the healthy phase voltages at fault instant with K2 =1.0 and K0 = 3.0.

Vc' and Vb' are the healthy phase voltages a few seconds after faultoccurs with K2. = 0.2 and K0 = 0.6.

Ea

Vb'V

c'

Eb Ec

Vb

Vc

Figure a




Z1 Z2

Ea C1 C0 V0 3Z

E'

Z0 i

g

Figure b




g

Ea

i

V0 arc restrikes

arc extinguishes




VTRANSIENT

Neutral to Earth

Faulted Phase

Resistor kWCharging kVA

(% EØ/N PEAK)

Unfaulted Phase

400

300

200

100

.2 .4 .6 .8 1.0 2.0




Z1 Z2

C1 C0 3RE

Z0 i

Ea




V0

45º

i

0.7E pk

pk0

0E

t/-pk.e0

0e

E0.06 V

-:7.5msafter

w1 C3R where

E0.7 V

wC

1 3RIf

Sound Phase Currents During an Earth Fault (1)




IF

Source

6

6

5

1

1

1

1

1

1

1

1

3

33

63

LoadX Y1

1

1

5





I1

I

2

I0

ZX1

ZX2

ZX0

IX1

IX2

IX0

ZY1

ZY2

ZY0

E





I1 = I2 = I0 = IX0 + I Y0

if ZX0 = Z Y0 then IX0 = I Y0

IF = 3I0 = 6IX0 ; Iya = I Yb = I Yc = IX0 = I Y0 = IF/6

IXa = IX1 + IX2 + IX0 = 5IX0 = 5IF/6

IXb = a2IX1 + aIX2 + IX0 = -I0 + IX0 = -IF/6

IXc = -IF/6

Parallel Generators (1)




G3 G4 G1 G2

3 E/F

Only 1 machine is earthed :-

Parallel Generators (2)




Consider the current in G1

for :- (i) Earth Fault

(ii) 3Ø Fault

Let

ZG11 = ZG21 = ZG31 = ZG41 = 0.244 p.u.

ZG12 = ZG22 = ZG32 = ZG42 = 0.124 p.u.

ZG10 = ZG20 = ZG30 = ZG40 = 0.05 p.u.

Earth Fault (1)




Sequence Networks :-

N1 EG

ZG11

ZG21

ZG31

ZG41

IG11 IG12 IG10 ZG12 ZG10

ZG22

ZG32

ZG42 F1 N2 IF1 F2 N0 IF2 F0 IF0

0.05p.u.

IF1 IF2 F0 IF0

0.244p.u.

0.244p.u.

0.244p.u.

0.124p.u.

0.124p.u.

0.124p.u.N1

1p.u.

0.244p.u. 0.124p.u.

Earth Fault (2)




N1

0.061

1p.u.0.031 0.05

F1 N2 IF1 F2 N0 IF2 F0 IF0

IF1 = IF2 = IF0 = 1 = 1 = 7.04 p.u.0.061 + 0.031 + 0.05 .142

IF = 3IF1 = 21.12 p.u.

IG11 = IF1 = 1.76 p.u. IG12 = IF2 = 1.76 p.u. IG10 = 7.04 p.u.4 4

IG1 = IG11 + IG12 + IG10 = 1.76 + 1.76 + 7.04 = 10.56 p.u.

3Ø Fault




1p.u.

ZG11 = .244

IF1 F1

IG11

ZG21 = .244

ZG31 = .244

ZG41 = .244

N1

IG11 = 1 = 4.1 p.u.

.244

IG1 = 4.1 p.u.

Thermal Stress




Stress3x6.55

Stress3x4.1

10.56

StressE/F

StressThermal

p.u.4.1

p.u.10.56

2

2

)(3G1

(E/F)G1

Methods of Neutral Earthing (1)




Aspect Solid Resistance Resistance & High value Low value Tuned Insulated

reactance reactor reactor reactor

Normal Suitable for Suitable for Suitable for phase Suitable for Suitable for If used for Suitable for line

insulation phase voltage phase voltage voltage line voltage for phase voltage operation with voltage for longcontinuously continuously continuously long periods continuously one line earthed

for long periods

insulation must

be suitable for

line voltage

Over voltages:

(a) Initiated by Not excessive Not excessive Not excessive provi- Can be very high Not excessive Not excessive if Arcing ground

faults, ding all three phases e.g. neutral no mutual coup- can give very

switching, etc are made or broken inversion ling between zero high voltagessimultaneously & positive seq-

uence networks

(b) Travelling Negative In general, “ Full reflection at Full reflection at Full reflection at Full reflection

waves reflection negative neutral neutral neutral at neutral

reflection at

neutral

Protection:(a) Automatic No difficulty No difficulty No difficulty, normal Extremely diffi- No difficulty By using special Extremely

segregation normal methods normal methods methods can be cult if more than normal methods technique can be difficult

of faulty zone can be used can be used used one zone can be used done satisfac-

involved torily

(b) Travelling Diverters rated In general, In general, diverters Diverters rated In general, Diverter rated Diverters rated

waves for phase volts diverters rated rated for line volts for line volts are diverters rated for for line volts are for line volts

are suitable for line voltage are essential essential line volts are essential are essential

are essential essential

Methods of Neutral Earthing (2)



Aspect Solid Resistance Resistance & High value Low value Tuned Insulated

reactance reactor reactor reactor

Earth-fault

Current

(a) Value Highest value High value High value Negligible High value Negligible Capacitive if

small may be

self exting-uished

(b) Duration Few seconds Few seconds Few seconds Long time Few seconds Few seconds or In general long

continuous, time

depending on

method of

application

(c) Effect on Electromagnetic Electromagnetic Electromagnetic Electrostatic Electromagnetic If used for Electrostatic

communica- interference interference interference interference interference may running contin- interference

tion circuits may necessi- depending on depending on necessitate current uously with one

tate current degree of degree of limitation limitation line earthed

limitation limitation requires partic-

ular consideration

Harmonic No limitation Partial limitations Partial limitation of Limits all Appreciably limits Appreciably limits -

currents in of harmonic of harmonic harmonic currents harmonic all harmonic all harmonic

neutral currents currents currents currents currents

Time rating of 30 sec. 30 sec. 30 sec. Continuous 30 sec. 30 sec. or -

neutral apparatus continuous

General remarks Maximum In general use In general use where Confined mainly Cheaper than Best continuity Some applica-

disturbance to a source neutral is to protection of resistor at very of supply. Can tions on short

system not available generator on high voltages be a danger to feeders in

2 Earthing Grounding

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