Current Transformer and Potential Transformer

One of the major difference between them is that the current transformer converts the high value of current into low value whereas the potential or voltage transformer converts the high value of voltages into low voltage.

1. INSTRUMENT
TRANSFORMERS

2. Uses and Definitions
•Usedinacsystemformeasurementofcurrent,
voltage,powerandenergy
•Usedinmeasuringpf,frequencyandindicationof
synchronism
•Transformerusedformeasurementpurposeiscalled
“InstrumentTransformers”
•Usedformeasurementofcurrent“Current
Transformer”“C.T.”
•Usedformeasurementofvoltage“Potential
Transformer”“P.T.”

3. Use of Instrument Transformer
•MeasurementofHighvoltagesandCurrents
•C.T. Primarywindingisconnectedwiththe
currentbeingmeasuredandsecondarywindingto
theammeter.CTstepsdownthecurrent.
•P.T. Primarywindingisconnectedwiththe
voltagebeingmeasuredandsecondarywindingto
thevoltmeter.PTstepsdowntheVoltage.
•Extensionofrangecanbedonebytheuseofshunt
forammeterandmultipliersforvoltage
measurementind.cmeasurement.Thismethodis
suitableonlyforsmallvaluesofcurrentandvoltage

4. Disadvantages of Shunt
•It is difficult to achieve accuracy using shunt in ac
•The method of using shunts is limited to capacities of
few hundred amperes since power consumption by
shunts would be large
•The problem of insulation of instrument and shunt is
quite difficult if measurements are done at high
voltages
•The measuring circuit is not isolated electrically from
the power circuit

5. Disadvantages of Multipliers
•Thepowerconsumptionislargeasthe
voltageincreases
•Constructionofmultiplierisverycostlyand
complicatedforhighvoltagetoprotect
leakagecurrent
•Themeasuringcircuitisnotelectrically
isolatedfromthepowercircuit

6. Adv of Instrument Transformers
•WheninstrumentsareusedinconjunctionwithInstrument
Transformers(IT)theirreadingdonotdependuponconstants
(R,L,C)asinthecaseofshuntandmultipliers.ITproduce
samereadingregardlessofconstants
•Verycheapmoderateratingmaybeusedtomeasurelarge
currentsathighvoltages(1000A/5AOR66kv/100to120v)
•WiththestandardizationofCTandPTsecondarywinding
ratingsitispossibletostandardizeinstrumentsaroundthese
ratings.ThereforegreatreductionofthecostofITand
Instruments
•Themeasurementcircuitisisolatedfromthemaincircuit
•ReplacementofITiseasyonaccountofthestandardizationof
theratings

7. ADVANTAGES OF IT
•Instrumentsofmoderatesizeareusedformetering
•Instrumentsandmeterscanbestandardizedsothat
thereissavinginoverallcosts.Replacementofdamaged
instrumentiseasy
•Singlerangeinstrumentscanbeusedtocoverlarge
currentorvoltagerangeswhenusedwithsuitablemulti-
rangeITorwithseveralsinglerangeIT
•Themeteringcircuitisisolatedfromthehighvoltage
powercircuit.Isolationandsafetyisassured
•Thereislowpowerconsumptioninmeteringcircuit
•SeveralinstrumentcanbeoperatedfromthesingleIT

8. RATIOS OF IT phasor
Ratio R
sec phasor
winding current
; a C.T.
secondary winding current
winding voltage
; a P.T.
secondary winding voltage

min Ratio K
n
primary
Transformation
ondary
primary
For
primary
For
rated pri
No al



winding current
; C.T.
rated secondary winding current
winding voltage
; P.T.
rated secondary winding voltage
mary
For
rated primary
For

9. No of turns of secondary winding
Ratio=n ; C.T.
No of turns of primary winding
No of turns of primary winding
; P.T.
No of turns of secondary winding
Ration Correcton Fac
Turns For
For


tor (RCF). RCF of a transfromer is the
transformation ratio divided by nominal ratio
Transformation ratio=Ratio correction factor x Nominal ratio
R=RCF x K ; the ratio marked on tfr is the Nominal Rat
n
 io RATIOS OF IT

10. Burden of IT
•Itisconvenienttoexpressloadacrossthesecondarywending
terminalsastheoutputinvolt-ampattheratedsecondary
windingvoltage.Theratedburdenisthevolt-amploading
whichispermissiblewithouterrorsexceedingthelimitsforthe
particularclassofaccuracy.
•Thetotalsecondarywindingburden2
2
(secondary winding induced voltage)
( of secondary winding ckt and secondary winding)
(secondary winding current) ( of secondary-
winding ckt and secondary winding)
win
impedance
x impedance
Secondary

2
2
ding burden due to load
(secondary winding terminal voltage)
=
( of load on secondary winding)
(secondary winding current) ( of load-
on secondary winding)
impedance
x impedance

11. CURRENT TRANSFORMER
•Primarywindingconsistsoffewturnsandis
connectedinserieswiththelinecarrying
current
•Secondarywindinghaslargernoofturns
andconnectedwiththeinstruments
•CToperatesitssecondarywindingnearly
undershortcircuitconditions

12. Relationship in a CT of secondary winding turns
Ratio=
of Primary winding turns
Re tan of the secondary winding
Re tan of the secondary winding
Re tan of the external burden (resitance of meter
s
s
e
No
n Turn
No
r sis ce
x ac ce
r sis ce



 , Curent coil, leads etc)
Re tan of the external burden (reactiance of meter, Curent coil, leads etc)
E Pr winding induced voltage;E winding in duced voltage
of Primary windin
e
ps
P
x ac ce
imary Secondary
N No


 g turns; of secondary winding turns
of secondary winding terminals,
s
s
N No
V Voltage



13. I secondary winding current; I Primary winding current
=phase angel of transformer; Working frux of transformer
between secondary winding induced voltage and secondary winding current
sp
Angle
pha





1
1
0
angle of total burden incl inpedance of secondary winding=tan
=phase angle of scondary winding load ckt/External burden=tan
current; I magnetizing component of exiting
se
se
e
e
m
xx
se
rr
x
r
I exiting





 current
I loss component of exiting current; =angle between exing current and working flux
e
 Relationship in a CT

14. Transformation Ratio0
0
0
00
0
00
2 2 2
2 2 2
00
2 2 2 2
0 0 0
90 ; ; ; and oc=
bc= sin(90 ) cos( );
cos(90 ) sin( )
, (oc) (oa+ab) (bc)
( ) [ sin( )] [ cos( )]
( ) sin ( ) 2 sin( ) c
sp
ps
ss
bac ac I oa nI I
II
ab I I
Now
I nI I I
nI I nI I I

   
   
   
   
     
    
    

     
     
2
22
00
2 2 1/ 2
00
os ( )
( ) 2 sin( )
[( ) 2 sin( ) ]
ss
p s s
nI nI I I
I nI nI I I




   
    

15. 2 2 1/ 2
00
00
2 2 2 1/ 2
00
0
[( ) 2 sin( ) ]
Ratio=R=
a well designed CT ; Usually is less than 1 % of
[( ) 2 sin( ) sin ( )]
R
sin( )
p ss
ss
sp
ps
p ss
ss
s
s
I nI nI I I
Transformation
II
In I nI I I
I nI
I nI nI I I
II
nI I
n
I

   

  


   
  

  
=
0
0
00
sin( )
sin cos
(sin cos cos sin )
, cos and sin
s
me
ss
me
I
I
I I I
R n n
II
Where I I I I


   



     
 Transformation Ratio

16. Phase Angle
•Theanglebywhichthesecondarycurrentphasor
whenreversed,differsinphasefromtheprimary
current,isknownasthephaseangleofthe
transformer
•Thisangleistobetaken+veifthesecondarycurrent
reversedleadstheprimarycurrent.Theangleistaken
–veifsecondarycurrentlagsbehindtheprimary
current

17. Phase Angle0
0
0
0
0
angle between I reversed and I is . therfore the phse angle
I cos( )
tan
I I sin( )
I cos( )
is very small we can write,
I I sin( )
I is very small compa
sp
s
s
The
bc bc
ob oa ab n
As rad
n








   
  



0
0 0 0
red to I , we can neglect the term I sin( )
I cos( ) I cos cos I sin sin )
II
I cos I sin ) I cos I sin )180
deg
II
s
ss
m e m e
ss
n
rad
nn
rad x ree
nn

     

   



  



18. Error in CT
•Thevalueoftransformationrationisnotequaltotheturns
ratio.Thevalueisnotconstant;dependsuponthemagnetizing
andthelosscomponentoftheexcitingcurrent,thesecondary
windingloadcurrentanditspf.
•Thesecondarywindingcurrentisnotaconstantfractionofthe
primarywindingcurrentbutdependsupontheabovefactors
•Itisnecessarythatthephaseofthesecondarywindingcurrent
shallbedisplacedbyexactly180
0
fromtheprimarywinding
current.Thephasedifferenceisdifferentthanthatof180
0
by
theta
•Hencetwotypesoferror;oneduetoactualtransformationratio
beingdifferentfromtheturnsratioandtheotherdueto
secondarywindingcurrentnotbeing180outofphasewiththe
primarywindingcurrent

19. Ratio Error & Phase Angle Errormin ratio-Actual ratio
Percentage ratio errror= 100
Actual ratio
100
I cos I sin )180
angle error. Phase Angle = deg
I
Formula for Error
n
me
s
No al
x
KR
x
R
Phase x ree
n
Approximate






: The usual instrument burden is largely
resistive with some inductive and is positive and small
Hence sin 0 and cos 1
I180
= deg
I
I180
I I =n(1 ) And = deg
I
em
ss
e e m
ps
p p p
I
R n x ree
In
nI I
As n R n x
II







  
     ree

20. Characteristic of CT
Effect of PF of Secondary Winding Burden on Error
RatioError: Forinductiveburdensthesecondary
windingcurrentI
slagsthesecondaryinducedvoltageE
s
so,δispositive.Underthisconditiontheactual
transformationratioisalwaysgreaterthantheturnsratio
ForsufficientlycapacitiveI
sleadsE
sandδisnegative;the
actualTransformationratiodecreasesbecominglessthan
theturnratioforvaluesofδapproaches-90
0

21. •PhaseAngle:Forinductiveburden,the
phaseangleθispositiveforsmallvaluesofδ
(highpf)butbecomesnegativeasthesecondary
burdenbecomesmoreinductiveandδapproaches
90
0
•Fornegativevaluesofδ,sufficientlycapacitive
burdenθisalwayspositive
•Itisassumedthatthemagnitudeofsecondary
impedanceisconstant
Characteristic of CT

22. Effect of Change of Primary
Winding Current
•Iftheprimarywindingcurrentchangesthe
secondarywindingcurrentchanges
proportionately.AtlowvaluesofcurrentI
pI
s;
thecurrentI
mandlosscurrentI
eareagreater
proportionofandtheerrorsaregreater.
•AsthecurrentI
pincreases,thereisanincrease
inI
Sandthereisadecreaseinratioerrorand
phaseangel

23. •Anincreaseinsecondarywindingcircuits
burdenimpedancemeansanincreaseinvolt
amprating.Thisnecessitatesandincreasein
thesecondarywindinginducedvoltagewhich
canbegeneratedbyanincreasedfluxandflux
density.Thereforeboththemagnetization
currentincreased
Effect of Change of Secondary
Winding Circuit Burden

24. •Theeffectofincreaseinfrequencywillresult
inproportionatedecreesinfluxdensity.Thus
theeffectofincreaseinfrequencyissimilarto
thatproducedbydecreaseinimpedanceof
secondarywindingburden
•Acurrenttransformerisseldomusedata
frequencywhichisverydifferentfromtheone
orwhichisdesignedandconsiderationofthis
effectisnotveryimportant
Effect of Change of Frequency

25. Causes of Error in CT
•Thereissomeexitingmmfrequiredbytheprimary
windingtoproducefluxandthetransformerdrawsa
magnetizingcurrent
•Thetransformerinputmusthaveacomponentwhich
suppliesthecorelosses(eddycurrentandhysterisis)and
I
2
Rlosses
•Thefluxdensityinthecoreisnotalinearfunctionofthe
magnetizingforce
•Thereisalwaysamagneticleakageandconsequentlythe
primaryfluxlinkagesarenotequaltothesecondaryflux
linkages
Effect of Change of Primary
Winding Current

26. Means to Reduce Error in CT
•There are some design featurewhich helps to
minimize the errors
–Core
–Primary winding current ratings
–Leakage reactance
–Turns compensation
–Use of shunts
–Wilson compensation method
–Two stage design

27. Construction of CT
•WoundType ACThavingaprimary
windingofmorethanonefullturnwounded
oncore
•BarType ACTinwhichtheprimary
windingconsistsofabarofsuitablesizeand
materialforminganintegralpartof
transformer

28. CLAMP ON AMMETERS
•ACTwithasingleconductorisusedin
combinationwithabridgerectifierandadc
milli-ammetertoproduceaveryusefulservice
meter
•Bychangingtheshuntresistanceofthemilli-
ammetercircuitrangesfrom0-5Ato0-600A
•Thesamemilli-ammeterandrectifierareused
withtwoexternalbindingpostsandarange
selectingswitchforamulti-rangeacvoltmeter

29. Effect of Secondary Open Ckt
•CTalwaysusethesecondarywindingclosed
throughammeterswattmetercurrentcoilsor
relay.
PRECAUTION
•NEVER OPEN THE SECONDARY
WINDING CKTOFACTWHILEITS
PRIMARYWINDINGISENERGISED
•ifthesecondaryisopen,theprimarymmf
remainsthesamebuttheopposingsecondary
mmfreducestozero.Thelargemmfproduce
HIGHVOLTAGE atthesecondary;destroy
insulation

30. POTENTIAL TRANSFORMER

31. Connection of CT and PT

32. Potential Transformer
•PTisusedtooperatevoltmeters,thepotential
coilsofwattcmetersetc
•Theprimarywindingisconnectedacrossthe
linevoltagetobemeasuredandthevoltage
circuitisconnectedacrossthesecondary
winding
•ThedesignofPTissimilartothepower
transformerbuttheloadingofaPTisalways
small
•Secondaryvoltageisnormally100-120V

33. Difference Between CT & PT
•PTmaybeconsideredas‘parallel’transformerwithits
secondaryoperatingnearlyunderopencircuit;whereasCT
maybea‘series’undervirtualshortcircuit.Thesecondaryof
PTcanbeopencircuitwithoutanydamage
•TheprimarywindingcurrentinaCTisindependentofthe
secondarywindingcircuitconditionswhiletheprimarywinding
currentinaPTcertainlydependsuponthesecondarycircuit
burden
•InaPT,fulllinevoltageisimpresseduponitsterminal
whereasaCTisconnectedinserieswithonelineandasmall
voltageexistsacrossitsterminals.TheCTcariesthefullline
currents
•undernormaloperationthelinevoltageisnearlyconstantand
thefluxdensityandtheexitingcurrentofaPTvariesonlyover
arestrictedrangewhereastheprimarywindingcurrentand
excitationofaCTvaryoverwidelimitsinnormaloperation

34. RELATIONSHIPS IN A PT
•ThepowerloadingofPTisverysmalland
consequentlytheexitingcurrentisofthesame
orderasthesecondarywindingcurrentwhile
inaPTtheexitingcurrentisverysmall
fractionofsecondarywindingloadcurrent
•Theequivalentcircuitandthevectordiagram

35. Let,0
flux; I = magnetizing component of no load current
I = iron loss component of no load (exiting) current
I = no load (exiting) current
E = Secondary winding induced voltage; V = Secondary windin
m
e
ss
working
g terminal voltage
= Primary winding turns; = Secondary winding turns
= Secondary winding currents
= Resistance of secondary winding; x = Reactance of secondary winding
= Resistance of seconda
Ps
s
ss
e
NN
I
r
r ry load ckt; x = Reactance of secondary load ckt
angle of secondary load ckt
E =Primary winding induced voltage; I = primary winding current;
= Resistance of primary winding; x = Reactance of
e
pp
pp
phase
r

primary winding
Turns ratio n=N /N ; n=E /E
p s p s


41. Ratio (voltage) Error

42. ERRORS IN PT
Ratio (voltage) Error
•Theactualratiooftransformationvaries
withoperatingconditionsandtheerrorin
secondaryvoltagemaybedefined;
•percentagerationerror=[(K
n-R)/R]x100

43. •In an ideal PT there should not be any phase
difference between primary winding voltage and
the secondary winding voltage reversed. In actual
transformer there exists phase angle error between
V
Pand V
s reversed.
ERRORS IN PT
Phase Angle Error

44. Reduction of Error in PT
•Considerableimprovement inthe
performancecanbemadebyreducingthe
valuesofmagnetisingcurrentsandrequires
shortmagneticpath,goodqualitycore
material,lowfluxdensityincoreandsuitable
precautionsinassemblingandinterleaving
thecore
Reduction of Magnetizing and Loss Components

45. Reduction of Resistance and
Leakage Reactance's
•Windingresistancecanbeminimizedbyusingthick
conductorsanbyadoptingthesmallestlengthof
meanturn
•Theleakagereactancedependsuponthemagnitudeof
primaryandsecondarywindingleakagefluxesand
shouldkeepthetwowindingsascloseaspossible.
•Thefluxdensityinthecoreshouldbekeptashighas
possible
•Highfluxdensitymeansahighfluxinthecoreandso
thewindingshavelessernumberofturns.Asmaller
noofturnsnaturallyresultsinsmallerleakage
reactanceofthewindings

46. Turns Compensations
•Atnoloadtheactualratioexceedstheturnsratiobyan
amount(I
er
p+I
mx
p)/V
s.Withandinductiveorresistive
loadthereisfurtherincreaseinratiobecauseofvoltage
dropsinresistanceandleakagereactanceofthe
windings
•Iftheturnsrationisequaltothenominalratio,the
actualratiodiffersfromthenominalratioandthus
thereareerrors
•Thesolutionliesintheturnsratiomustbelessthan
nominalratio.Thiscanbedonebyeitherreducingthe
numberofprimarywindingturnsofincreasingthe
mumberofsecondarywindingturns

47. Construction of PT
ThedesignandconstructionofPTisbasicallythe
sameasthoseofpowertransformerwithafew
majordifferences:
PThaslargercoreandconductorsizes.Economic
designmayleadtolargeratioandphaseangle
errorswhichareundesirable
TheoutputofPTisalwayssmallandthesizeis
quitelarge,nothermalproblem.
LoadingofPTislimitedbyaccuracy
considerationswhileinapowertransformerthe
loadlimitationisonheatingbasis.

48. •Core.Thecoremaybecoreorshelltype.Shelltype
isforlowvoltage.Laminationsshouldbesuchthat
theeffectoflargeairgapsatthejointmaybe
minimized
•Windings. Theprimaryandthesecondary
windingsareco-axialtoreduceleakagereactanceto
minimum.Theprimarywindingmaybeasinglecoil
inlowvoltagetransformersbutmustbesib-divided
intoanumberofshortcoilsinhighvoltage
transformerinordertoreducetheinsulationneeded
betweencoillayers
Construction of PT

49. •Insulationscottontapeandvarnishedcambric
areusedasinsulationforcoilconstruction.Atlow
voltagesthetransformersarefilledwithout
compoundbutPTvoltageabove7KVareoil
immersed
•Bushingsoilfilledbushingsareusedforoil
filedPTasthisminimizetheoverallsizeofthe
transformer.Twobushingsareusedwhenneither
sideofthelineisatgroundpotential
Construction of PT

50. High Voltage PT
•ConventionaltypePTusedinhighvoltagesof
100kvandabove,areverylargeinsizeand
costlytobuildbecauseofinsulationrequirement
•Recentdevelopmentindesignandconstruction
hasreducedthesizeconsiderably.The
eliminationsofbushingreducesthesizeandcost
oftransformer.Designsareintendedtomeasure
linetogroundvoltagesin3phasesystem:
•Thedesignemploy:

51. •InsulatedCasing Thetransformerisbuiltentirely
inanoiledfiledhighvoltageinsulator
•MouldedRubberPTMoulded rubberinsulated
transformerhasreplacetheinsulatingoiland
porcelainbushingsandislessexpensive
•CascadedTransformerThevoltagesisamonga
numberoftransformers.Inthiswayinsulationreduces
tothelowervoltagesandsavescostsandspaces
Construction of PT

52. Characteristics of PT
Increaseinsecondaryburden,thesecondarycurrent
increasesi.e.primarycurrentincrease;foragiven
valueofV
PthevalueofV
sdecreasesandtheactual
rationincreasesastheburdenincreases
Theratioerrorincreasesbecomingmorenegative
withincreaseinburden.Thisvariationofratioerror
isalmostlinearwithchangeinburden
V
pismoreadvancedinphasebecauseofincreased
voltagedropswithincreaseinsecondaryburden.The
phasorV
sisretardedinphaseowingtincreasein
secondaryvoltagedrop;thusthephaseanglebetween
V
PandV
sreversedincreases,becomingmorenegative
Effect of Secondary Current or VA

53. •Ifthepfofsecondarycircuitburdenis
reduced,ΔincreasedandI
PshiftedtowardsI
0
•ThevoltagesV
PandV
Snearlycomesinphase
withE
PandE
S;sincethevoltagedropis
almostconstant.
•Thetransformationratioincreasesasthepf
ofsecondaryburdenreduces
Characteristics of PT
Effect PF of Secondary Burden

54. Effect of Frequency
•Thefluxisinverselyproportionaltofrequencywith
constantvoltage.Increaseinfrequencyreducesthe
fluxandmagnetizationcurrentdecreasesandthe
voltageratiodecrease
•Asregardstophaseangleerror,botheffectsdueto
increaseinfrequencyadvanceV
Pandtheincreasein
secondaryreactanceretardsV
sandphaseangleis
increasedasthefrequencyincreases

55. Effect of Primary Voltage
•Thereisnowidevariationofsupply
voltagetowhichtheprimarywindingof
thePTisconnected.Thereforethestudy
ofvariationofratioandphaseangle
errorswithsupplyvoltageareofno
importance

56. Polarity of CT & PT
•Anunderstandingofpolarityisessentialtocorrectlyconstructthree-
phasetransformerbanksandtoproperlyparallelsingleorthree-phase
transformerswithexistingelectricalsystems.Aknowledgeofpolarity
isalsorequiredtoconnectpotentialandcurrenttransformerstopower
meteringdevicesandprotectiverelays.
•Thebasictheoryofadditiveandsubtractivepolarityistheunderlying
principleusedinstepvoltageregulatorswheretheserieswindingof
anautotransformerisconnectedtoeitherbuckorboosttheapplied
linevoltage.
•TransformerPolarityreferstotherelativedirectionoftheinduced
voltagesbetweenthehighvoltageterminalsandthelowvoltage
terminals.DuringtheAChalf-cyclewhentheappliedvoltage(or
currentinthecaseofacurrenttransformer)isfromH1toH2the
secondaryinducedvoltagedirectionwillbefromX1toX2.In
practice,Polarityreferstothewaytheleadsarebroughtoutofthe
transformer.
The Importance of Polarity

57. •BushingArrangement ThepositionoftheHighVoltage
Bushingsisstandardizedonallpowerandinstrument
transformers.Theruleisthis:whenfacingthelowvoltage
bushings,thePrimaryBushingH1isalwaysontheleft-handside
andthePrimaryBushingH2isontheright-handside(ifthe
transformerisathree-phaseunit,H3willbetotherightofH2).
•DistributionTransformersareAdditivePolarityandtheH1and
X1bushingsarephysicallyplaceddiagonallyoppositeeach
other.SinceH1isalwaysontheleft,X1willbeontheright-hand
sideofadistributiontransformer.Thisstandardwasdeveloped
veryearlyinthedevelopmentofelectricaldistributionsystems
andhasbeenadheredtoinordertopreventconfusioninthe
fieldwhentransformersneedtobereplacedorparalleledwith
existingequipment.
•InstrumentTransformers(PT’sandCT’s)andlargesubstation
transformersareSubtractivePolarity,sotheH1andX1
Bushingswillbeonthesamesideofthetransformer.This
standardwaslateradoptedtomakeiteasiertoreadelectrical
schematicsandconstructphasordiagrams.

58. Polarity of CT & PT

59. Polarity of CT & PT

60. POLARITY TEST
•Insituationswherethesecondarybushingidentificationisnotavailableor
whenatransformerhasbeenrewound,itmaybenecessarytodeterminethe
transformerpolaritybytest.Thefollowingprocedurecanbeused.
•TheH1(left-hand)primarybushingandtheleft-handsecondarybushing
aretemporarilyjumperedtogetherandatestvoltageisappliedtothe
transformerprimary.Theresultantvoltageismeasuredbetweentheright-
handbushings.
•Ifthemeasuredvoltageisgreaterthantheappliedvoltage,thetransformer
isAdditivePolaritybecausethepolarityissuchthatthesecondaryvoltageis
beingaddedtotheappliedprimaryvoltage.
•If,however,themeasuredvoltageacrosstheright-handbushingsislessthan
theappliedprimaryvoltage,thetransformerisSubtractivePolarity.
•Note:Forsafetyandtoavoidthepossibilityofdamagingthesecondary
insulation,thetestvoltageappliedtotheprimaryshouldbeatareduced
voltageandshouldnotexceedtheratedsecondaryvoltage.

61. POLARITY TEST

62. •PolarityMarksToinsurecorrectwiring,polaritymarksare
shownonInstrumentConnectionDiagrams,ControlSchematics,
andThree-LinePowerDiagrams.Thepolaritymarkisusually
shownasarounddot,onoradjacentto,theH1andX1terminals
ofPT’sandCT’s.Sometimesalternatemarking,intheformofa
squaredot,slashmark(/),orplus/minussign(+)willbeused
toidentifythepolarityterminalsonelectricaldrawings.
•Instrumenttransformersmayalsohavetheterminalsidentified
withpolaritymarksasshownintheillustrationofthe300:5CT
onSheet3.Ifinstrumenttransformersdonothavepolarity
marksonthem,itisunderstoodthattheH1(primary)andthe
X1(secondary)terminalsarepolarity.
•Meters,relays,andotherequipmentwhichrequireproper
polarityconnectionsmayalsohavepolaritymarks,butusually
thisinformationmustbeobtainedfromtheInstrument
ConnectionDiagram.
POLARITY MARKING

63. POLARITY MARKING

64. •ThePT,CT,andinstrumentpolaritymarksare
shownbythereddotsontheabovedrawing.(Red
dotswereusedinthisexampleonlyforclarity.)
•Currentelementsoftheinstrumentsareconnectedin
series,voltageinputsareconnectedinparallel.
•Polarityisnotaconsiderationonsingle-element
devicessuchasanammeterorvoltmeter,butis
essentialforproperoperationofpowermeasuring
devices,andfordirectionalordifferentialprotective
relays.
POLARITY MARKING

65. Current Flow Analysis
•InanalyzingthecurrentflowinasystemutilizingCT’sthe
followingobservationcanbemade:
•WhencurrentflowsintheCTprimaryfromtheH1lead
(polarity±)tothenon-polarityH2lead,currentwillbeforced
outthesecondaryX1(polarity±)lead,throughtheburden
(load),andreturntothesecondaryX2non-polaritylead.The
nexthalf-cyclethecurrentwillreverse,butforthepurposeof
analysisandforconstructingphasordiagrams,onlytheabove
indicatedone-halfcycleisanalyzed.
ElectricalDrawingConventions
•Thepolaritymarkingonelectricaldrawingsmaybemadein
severaldifferentways.Thethreemostcommonschematic
conventionsareshownbelow.Thedrawingsymbolformeters
andrelaysinstalledinadraw-outcasethatautomaticallyshort
theCTsecondaryisshowninthedrawingatthelowerright.