IEEE-Grounding-Transformers (1).ppt

8/18/2019 IEEE-Grounding-Transformers (1).ppt

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11February, 2007

Medford,Oregon

March 2011

Medford, Oregon

Grounding Transformers

John S. Levine, P.E.

Levine Lectronic and Lectric, !nc.


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• It is used to provide a ground path onIt is used to provide a ground path oneither an ungrounded Wye or a Deltaeither an ungrounded Wye or a Delta

connected systemconnected system• The relatively low impedance path to ground The relatively low impedance path to ground

maintains the system neutral at ground potentialmaintains the system neutral at ground potential


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• On Ungrounded systems you canOn Ungrounded systems you can

have overvoltages of 6 to 8 timeshave overvoltages of 6 to 8 times

normal with arcing faultsnormal with arcing faults


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•Plot of transient overvoltage for an arcingPlot of transient overvoltage for an arcing

ground faultground fault

!rcing "round #aults!rcing "round #aults

Intermittent or $estri%eIntermittent or $estri%e


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•Intermittent ground fault:Intermittent

ground fault: ! restri%ing ground fault can create a! restri%ing ground fault can create ahigh fre&uency oscillator '$() circuit*+ independent of ( and ) values+high fre&uency oscillator '$() circuit*+ independent of ( and ) values+

causing high transient overvoltages,causing high transient overvoltages,

– i,e, restri%ing due to ac voltage waveform or loose wire caused -yi,e, restri%ing due to ac voltage waveform or loose wire caused -y

vi-rationvi-ration

V V

480V Delta Source

3Ø Load

Cb bC

R fe

faS


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CONTROL OF TRANSIENT OVERVOLTAGE


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• It supports the voltage on aIt supports the voltage on a

faulted phasefaulted phase

– If a single linetoground fault occurs on anIf a single linetoground fault occurs on anungrounded or isolated system+ no return pathungrounded or isolated system+ no return pathe.ists and no current /owse.ists and no current /ows

– The system will continue to operate -ut the The system will continue to operate -ut the

other two unfaulted lines will rise in in voltageother two unfaulted lines will rise in in voltage-y the s&uare root of 0+ possi-ly overstressing-y the s&uare root of 0+ possi-ly overstressing

the transformer insulation+ and otherthe transformer insulation+ and othercomponents+ -y 102components+ -y 102


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• Provides a metering point to measureProvides a metering point to measure

faultsfaults


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! typical e.ample is a Wind #arm, Theyutili3e grounding transformers for faultprotection on ungrounded lines

When a ground fault occurs on a collector ca-le causes the

su-station circuit -rea%er to open+ the wind tur-ine string -ecomesisolated

Tur-ines do not always detect the fault and the generators continue toenergi3e the ca-le,

4oltages -etween the unfaulted ca-le and the ground rise -y the102

The transformer+ placed on the tur-ine string+ provides the groundpath


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A typia! e"amp!e is a #ind Farm$

#%en t%e feeder &rea'er

Opens( t%e o!!etor &us and

t%e step up transformer de!ta

onneted )V *indings

re!y on t%e Grounding

Transformer for t%eir ground

pat% and +o!tage support$


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Two di5erent constructions

ZIG ZAG (Zn)

WYE CONNECTED


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•What if no neutral e.ists 'i,e, delta systems*7What if no neutral e.ists 'i,e, delta systems*7 – ! grounding transformer is installed 'either a 3ig3ag or! grounding transformer is installed 'either a 3ig3ag or

a wyedelta* from all three phases to create an articiala wyedelta* from all three phases to create an articial

neutral for grounding purposesneutral for grounding purposes onlyonly,,

HRG

Broken Delta

Grounding

Transformers

Wye-Delta

Grounding

Transformers

HRGHRG

Zig-Zag

Grounding

Transformer

CØBØAØ AØ BØ CØAØ BØ CØ


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• The 3ig3ag transformer contains si. coilson three cores, The rst coil on each coreis connected contrariwise to the second

coil on the ne.t core,• The second coils are then all tied

together to form the neutral and thephases are connected to the primary

coils,• 9ach phase+ therefore+ couples with each

other phase and the voltages cancel out


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Typia! #ye onnetion *it%

Neutra! end of *indings onnetedToget%er

A , C

Symmetria! t%ree p%ase soure

A , C

Vetor diagram of

&a!aned system

A

,

C


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T%e -ig.-ag onnetion %as /

*indings on ea% !eg$

A

,

C

a

&


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C

,

A

&

aEa% !eg of t%e -ig.-ag

Connetion is onneted

to a *inding from anot%er *%i%is out of p%ase

,

C

A

&

a


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C0

A0

,0

T%e resu!ting -ig.-ag onnetion

Is p%ase s%ifted *it% respet to t%e

inoming t%ree p%ase soure


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• (imits circulation of triplen harmonics

' 0 rd+ 6th+ :th+ etc*

• )an -e used without a Deltaconnected or ;legged core

• 9limination of secondary windingresults in smaller footprint+ lowercost '


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• Includes a Delta or Wye connectedsecondary

• Utili3es > or ; legged core when Wyeconnected secondary is specied

• ?ultifunctional+ provides -enet ofau.iliary power


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• Primary 4oltage• Phase to Phase

continuous primarycurrent 'or $ated %4!*

• )ontinuous @eutralcurrent

• !vaila-le #ault )urrentand Duration

• Impedance as a 2 oras an ohmsAphasevalue

• Primary Windingconnection

• Becondary connection

• Casic overallconstruction

1no* t%e &asi parameters


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• This is the system voltage to whichthe grounded winding is to -econnected,

• Dont forget to specify the CI( also,

• In some cases the CI( will -e dictated-y e&uipment considerations+ such as

1;= %4 CI( on 0>,; %4 wind farms-ecause of the limitation of deadfront connectors


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• The Transformer must -e si3ed to carrythe rated continuous+ phasetophasecurrent without e.ceeding its temperature

limit• The higher the current+ the larger and

more costly the transformer

• Typical values can -e as low as ; amps to

as high as a few hundred

• Include any au.iliary loading re&uirements


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• Is dened as 0E the PhasetoPhasecurrent 'Fero Be&uence )urrent*

• It is the value that is e.pected to /owin the neutral circuit without trippingprotective circuits

• Used to design for thermal capacity

of the transformer


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• Used to determine the short timeheating resulting from a fault on thesystem which returns through the

transformer• Typical ranges run from a few

hundred to a few thousand amps

• Duration is e.pressed in seconds 'i,e,>== amps for 1= seconds*


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• )an -e e.pressed as either a percentage oras an ohmic value

• 9ither should -e chosen such that the un

faulted phase voltages are within thetemporary overvoltage capa-ility of• The Transformer

• !ssociated e&uipment 'i,e, arresters+ terminalconnectors+ etc,*

• Typical values can -e as low as 82 and asmuch as 1==2

• ?ust -e determined -y the system designer


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• Fig Fag or "rounded Wye


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• Bpecify the secondary voltageand connection for primary Wyeconnected transformers

• Bpecify si3e of au.iliary loadingto -e connected

• If two winding with no secondaryload+ advise if the delta windingcan -e G-uriedH 'not -roughtout* or if only one -ushing is to-e -rought out for grounding tothe tan% or testing


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• )ompartmentalpad mount or unitsu-station design

• Indoor or outdoor• #luid type '?ineralOil+ Bilicone+ or9nvirotemp #$0*

• Bite 9levation or9nvironmentalconditions

• )onnectivity• Dead #ront or (ive

#ront

• Bpade terminals

• )overmounted orsidewall

• 9.posed or enclosed

• Temperature $ise• 6; degrees )

• ;; degrees )• ;;A6; deg, )

• Bpecial )oatingre&uirements


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06= degree coolingducts

"ound #oi$"ound #oi$

Radia! fores are e2ua!i0ed during s%ort iruits 3 o+er!oads


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#rucifor% tac&ed core contruction#rucifor% tac&ed core contruction


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The following transformers are The following transformers are

for reference only to let you -efor reference only to let you -eaware of di5erent typeaware of di5erent type

transformers for di5erenttransformers for di5erent

applications,applications,


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• ! Bcott T Transformer is a! Bcott T Transformer is a

transformer that is designed fortransformer that is designed for

converting < phase current to 0converting < phase current to 0

phase current or viceversa,phase current or viceversa,


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• !n autotransformer is a transformer that only!n autotransformer is a transformer that only

have one winding with taps, @o isolation ishave one winding with taps, @o isolation isprovided -etween the primary and secondary,provided -etween the primary and secondary,


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• ! Cuc%Coost! Cuc%Coost

transformer is onetransformer is onethat is designedthat is designedto lower '-uc%* orto lower '-uc%* orraise '-oost* theraise '-oost* thevoltage in thevoltage in the

range of ;2 torange of ;2 to


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• ! drive isolation! drive isolationtransformer is onetransformer is onethat is typically usedthat is typically usedon !) or D) Driveon !) or D) Drivesystems, It typicallysystems, It typically

has the samehas the sameprimary andprimary andsecondary voltagessecondary voltagesand is used forand is used for

isolation, Inisolation, Inaddition youaddition youtypically have atypically have ashield to attenuateshield to attenuateline to ground noise,line to ground noise,


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• ! constant voltage transformer is one in! constant voltage transformer is one in

which the secondary stays constant withwhich the secondary stays constant with

large swings on the primary voltage,large swings on the primary voltage,

This is accomplished -y operating the This is accomplished -y operating theprimary in a saturated mode so evenprimary in a saturated mode so even

when the primary voltage dips you canwhen the primary voltage dips you can

still maintain a constant output, Typicalstill maintain a constant output, Typical

input may -e 8=2 to 11=2 with theinput may -e 8=2 to 11=2 with theoutput maintaining 12 voltageoutput maintaining 12 voltage

regulation,regulation,


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A copy of thisA copy of this

presentation presentation

can be foundcan be found

at:at:

www.L-3.comwww.L-3.com

or more information and


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www. PacificCrestTrans.com

or more information and!"e#i$#ation !%eet!
http://c/POWERPNT/SAMPLES/webpage.ppthttp://c/POWERPNT/SAMPLES/webpage.ppt

IEEE-Grounding-Transformers (1).ppt

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