advanced generator ground fault protections

7/21/2019 Advanced Generator Ground Fault Protections

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Advanced Generator

Ground Fault Protections

Wayne Hartmann

68th Annual Conference for Protective Relay EngineersMarch 31-Aril !" !#1$ College %tation" &'

61(#-118th Avenue ) *argo" +lori,a 333-3!. /0%0A0PH2E !4 $..-!3!6 ) +A' !4 $.6-#1!1

5hartmann7ec5ithelectric0com55507ec5ithelectric0com



Introduction

• Ground faults in generator stator andfield/rotor circuits are serious events that

can:

– Lead to Damage – Cause Costly Repair

– Result in Etended !utage

– Cause Loss of Revenue

• "e #ill eamine traditional and advancedprotection

ADVANCED GENERATOR GROUND

FAULT PROTECTIONS

!



$ield/Rotor Ground $ault Damage

• Initial field/rotor circuit ground esta%lishes a groundreference – In the event of a second ground fault& part of the field/rotor

circuit is shorted out

• 'horted portion of the rotor causes une(ual flu in the air gap %et#een

the rotor and the stator

• )ne(ual flu in the air gap causes torsional stress and vi%ration& and

can lead to considera%le damage in the rotor and the %earings

• In etreme cases& rotor contact #ith the stator is possi%le

– * second rotor ground fault produces rotor iron heating from the

un%alanced currents& #hich results in un%alanced temperaturescausing rotor distortion and vi%ration

• $ield/rotor ground faults should %e detected and affected

generators alarmed at high resistance levels and tripped

at lo# resistance levels


FAULT PROTECTIONS

3



$ield/Rotor Ground $ault

• +raditional field/rotor circuit ground fault

protection schemes employ DC voltagedetection

– 'chemes %ased on DC principles are su%,ect to

security issues during field forcing& othersudden shifts in field current and system

transients


FAULT PROTECTIONS

.



-rushed and .-rushless Ecitation

-rushed

.-rushless


FAULT PROTECTIONS

$



$ield/Rotor Ground $ault 012$3

• +o mitigate the security issues of traditional

DC4%ased rotor ground fault protection

schemes& *C in,ection %ased protection

may %e used – *C in,ection4%ased protection ignores the

effects of sudden DC current changes in the

field/rotor circuits and attendant DC schemesecurity issues


FAULT PROTECTIONS

6



DC4-ased 12$


FAULT PROTECTIONS



*dvanced *C In,ection 5ethod


FAULT PROTECTIONS

8



*dvanced *C In,ection 5ethod: *dvantages

•'cheme is secure against the effects of DC transients inthe field/rotor circuit – DC systems are prone to false alarms and false trips& so they

sometimes are ignored or rendered inoperative& placing the

generator at ris6

– +he *C system offers greater security so this importantprotection is not ignored or rendered inoperative

• 'cheme can detect a rise in impedance #hich is

characteristic of grounding %rush lift4off – In %rushless systems& the measurement %rush may %e

periodically connected for short time intervals

– +he %rush lift4off function must %e %loc6ed during the time

interval the measurement %rush is disconnected


FAULT PROTECTIONS

(



'tator Ground $ault

• Ground faults in the stator #inding can cause severedamage as the level of fault current increases – Depending on the ground fault current availa%le& the damage

may %e repaira%le or non4repaira%le

• Generators are su%,ect to prolonged eposure to stator

ground fault damage due to the fact that even if the

system connection and ecitation are tripped& stored flu

remains and contri%utes to the arc as the generator

coasts do#n

• Due to the eposure to this damage& several types of

generator grounding are employed – +he stator circuit of a generator may %e ungrounded& lo#

impedance grounded& high impedance grounded or hy%rid

impedance grounded


FAULT PROTECTIONS

1#



'tator Ground $ault Damage0only 78* for 18 cycles3


FAULT PROTECTIONS

11



Generator GroundingUngrounded


FAULT PROTECTIONS

Resistance Grounded

High Impedance Grounded

Hybrid Impedance Grounded

1!



'tator Ground $ault

• +raditional stator ground fault protection

schemes include: – 9eutral overvoltage

– arious third harmonic voltage4dependent schemes

• +hese ehi%it sensitivity& security and

clearing speed issues that may su%,ect agenerator to prolonged lo# level ground

faults that may evolve into damaging faults


FAULT PROTECTIONS

13



9eutral !vervoltage 0;<G3

• ;<G provides <;= stator #inding coverage


FAULT PROTECTIONS

1.



;<G 'ystem Ground $ault Issue


FAULT PROTECTIONS

• 9%/ rovi,es caacitive couling for system groun,faults into generator :one

• /se t5o levels of $(9 5ith short an, long time,elays for selectivity

• Cannot ,etect groun, faults at;near the neutral very1$



5ultiple ;<G Element *pplication

• 59G-1 is %lind to

the capacitive

coupling %y the

G')> – 'hort time delay


FAULT PROTECTIONS

• 59G-2 is set to ;=& #hich may include the

effects of capacitive coupling %y the G')

– Long time delay

16



"hy Do "e Care *%out $aults 9ear 9eutral?

• * fault at or near the neutral shunts the high resistance that saves the stator

from large currents #ith an internal ground fault• * generator operating #ith an undetected ground fault near the neutral is a

accident #aiting to happen

• "e can use @rd Aarmonic or In,ection +echni(ues for complete 0788=3

coverage


FAULT PROTECTIONS

1



Generator Capacitance and @rd Aarmonics

• @rd harmonics are

produced %y some

generators

– *mount typically small• Lumped capacitance on

each stator end is C'/B>

– C+ is added at terminal

end due to surge caps

and isophase %us

– Effect is @rd harmonic

null point is shifted

to#ard terminal end and

not %alanced


FAULT PROTECTIONS

18



@rd Aarmonic )ndervoltage 0B+93

• * fault near the neutral shunts the @rd harmonic near theneutral to ground

• Result is a third harmonic undervoltage

• 'ecurity issues #ith generator operating mode and po#er

output 0real and reactive3


FAULT PROTECTIONS

1(



@rd Aarmonic Ratio or Difference 0;<R or ;<D3


FAULT PROTECTIONS

• * fault near the neutral or terminal shunts @rd harmonic

–+his upsets the difference or ratio %et#een the neutral or terminal ends of the stator

• Relia%ility may %e an issue #ith lo# levels of @ rd harmonic 0element %loc6s #ith lo#

values3

• 'ecurity issues #ith generator operating mode and po#er output 0real and

reactive3 as that can change ratios in unpredicta%le #ays !#

C G O G O



@rd Aarmonics at 9eutral ariations #ith Loading

Example Plot on Gas Turbine (Midsize, 180MVA)


FAULT PROTECTIONS

!1




)se of 'ymmetrical Component uantities

to 'upervise ;<G +ripping 'peed

• -oth B

and <B

implementation have %een applied

– * ground fault in the generator one produces primarily ero se(uence

voltage

– * fault in the + secondary or system 0G') coupled 3 generates

negative se(uence (uantities in addition to ero se(uence voltage


FAULT PROTECTIONS

!!




Intermittent *rcing Ground $ault +urned 5ultiphase


FAULT PROTECTIONS

!3




;<G/B+9 +iming Logic

Interval and Delay +imers used together to detect intermittent pic6ups

of arcing ground fault


FAULT PROTECTIONS

!.




'u%harmonic In,ection: 12'

• B8A in,ected into

grounding transformer

secondary circuit

• Rise in real component ofin,ected current suggests

resistive ground fault

• Ignores capacitive current

due to isophase %us and

surge caps – )ses it for self4diagnostic and

system integrity


FAULT PROTECTIONS

!$




'u%harmonic In,ection: 12'

• $unctions on4line and off4line

• Fo#er and fre(uency independent


FAULT PROTECTIONS

!6




'ummary and Conclusions

• $ield/Rotor Ground $ault – )se of *C in,ection offers greater security than traditional

DC systems& and also affords %rush lift4off protection

• <;= 'tator Ground $ault Frotection – )se of the ;<G element is a time4tested method of

protecting <;= of the stator for generator ground faults• +raditional approach to cope #ith G') capacitive coupling and

interference #ith the ;<G element is using t#o elements& one long #ith along time delay coordinated system ground protection& and the other #ith

a short time delay for in4one ground faults>

• *n advanced method of using se(uence component supervision allo#s

determination of eternal ground faults& and allo#s the ;<G element to

(uic6ly clear ground faults in the generator one>


FAULT PROTECTIONS

!





•788= 'tator Ground $ault Frotection –@rd harmonic protection implementations are availa%le to

complement the ;<9 element to provide 788= stator

ground fault protection>

• @rd harmonic protections may not #or6 #ith all generators& and

may not #or6 at all times on a given generator>

• @rd harmonic values availa%le for the protection vary #ith

operational mode and po#er 0real and reactive3 output> –-oth security and dependa%ility issues may develop>

–+ransient ground faults can %e detected #ith the use of an

interval timing scheme on the ;<G and B+9 protections>• +his enhancement affords the a%ility to detect transient ground

faults %efore a permanent ground fault develops>


FAULT PROTECTIONS

!8





• 788= 'tator Ground $ault Frotection – +he use of su%harmonic in,ection affords the a%ility to detect

ground faults any#here in the stator or in the unit4connected

one regardless of the generator operation and loading

– If the element uses the real component for fault declaration& it

is very sensitive

– *s long as eternal signals at or near the su%harmonic

in,ected fre(uency are %alanced& the element is highly secure>

• +he element only responds to ero se(uence current in the

generator neutral& not positive se(uence current from an eternal

%alanced system such as:

– *nother generator during %ac64to4%ac6 starting

–'tatic converter employed in starting com%ustion gas tur%ine generators


FAULT PROTECTIONS

!(



Advanced Generator

Ground Fault Protections

68th Annual Conference for Protective Relay EngineersMarch 31-Aril !" !#1$ College %tation" &'

Questions?

advanced generator ground fault protections

Documents

ground referencein

ground fault current

generator stator

fieldrotor circuits

rotor distortion

rotor contact

dc principles

important protection