ct & vt

Instrument T fTransformers

Akhil Kumar GuptaAkhil Kumar GuptaSr. Faculty Member

CONTENTSCONTENTSCONTENTSCONTENTS

Introduction

Selection of Current Transformers

Theory of Current Transformer

Theory and Selection of Voltage Transformer

Selection of Current Transformers

Conclusion

IntroductionIntroduction

Protection System Analogy

BrainRelay

Eyes,Ears,Nose&Skin

CTs VTs

Hands&Legs

CTs,VTs

g

CircuitBreakers

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Protection System Analogy

Fault in the Power SystemFaultinthePowerSystem

SensedbyInstrumentTransformers&communicatedtoRelay

RelayIssuesTripCommandToBreaker

BreakerTrips&ClearsFault

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InstrumentTransformers

A Vital Part of the Protection and Metering Systemg y

Instrument Transformer transforms the high current orhigh voltage connected to their primary windings tothe standard low values in the secondary within therequired accuracy limits which feed the metering and

t ti tprotection apparatus

Provide insulation against High voltage (isolation)

Protect personnel and apparatus from high voltages

Provide possibilities of standardizing the relays andiinstruments

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Classification of Instrument Transformers Based on application Basedonapplication

Metering

Protection Protection

Basedonuse

I d Indoor

Outdoor

T f I T f TypesofInstrumentTransformer

CurrentTransformer(CT)

VoltageTransformer(VT)

ElectromagneticVoltageTransformer(EVT)

CapacitiveVoltageTransformer(CVT)3/4/20137:29:24PM 7

TheoryofTheoryofyyCurrentTransformersCurrentTransformers

What is Current Transformer Direct use of high current (in the tune of 100A or more)is not possible as protective relays and metering

devices are not designed to handle such huge amount

of current

Current Transformer is an instrument transformerwhich transforms current from one level to anotherwhich transforms current from one level to another

level, such as, 1000A/1A (CT ratio) i.e. transforms

t f th l l f 1000A i t t f 1A l lcurrent from the level of 1000A into current of 1A level

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S t h t b i i t

Why Current Transformer is required System has two basic requirementsmetering of energy sourced or consumedProtection of the electrical system fromfaults and disturbances

TypesofCurrentTransformer(CT) MeasuringCTs

ProtectionCTs

ProtectionCTsforspecialapplicationsp pp

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When alternating current flows in the primary

Current Transformer Theory When alternating current flows in the primarywinding, that current creates a MMF which results in aalternating flux in the core, which in turn induces analternating flux in the core, which in turn induces anEMF in the primary winding and in any other windingwound on, or linked with, the core

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It has a primary winding and one or more secondary windings

How Current Transformer is connected It has a primary winding and one or more secondary windings

wound on core of magnetic material

Metering and Protection devices are connected to thesecondaries of the CT

Primary winding connected in series and transforms the linecurrent to the standard 1A or 5A suitable for the meter or relaycurrent to the standard 1A or 5A suitable for the meter or relay

PR IM A RY W IN DI N GO R B US H IN GPR IM A RY W IN DI N GO R B US H IN G

M A GN E T ICC O R E

S E C O N D A R Y

M A GN E T ICC O R E

S E C O N D A R YS E C O N D A R YW IN D IN GS E C O N D A R YW IN D IN G

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I i i

How Current Transformer is connected

In current operation or seriesmode, the primary winding isconnected in series with thepower system whose relativelyhigh impedance determines themagnitude of primary windingmagnitude of primary windingcurrent which is independent ofthe secondary winding load

The current transformer hasassigned rated output termed asburden in VA which are invariablyburden in VA which are invariablysmall as against the high outputsin KVA or MVA of powertransformerstransformers

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F h t i it d t f th f ll i

Current Transformer Theory For a shortcircuited transformer the followingrelation holds good

Primary Ampere turns (I1N1) = Secondary Ampere Turns (I2N2)

21I N=

An ideal current transformer is a shortcircuited

12 NI=

An ideal current transformer is a shortcircuitedtransformer where the secondary terminal voltage iszero and the magnetizing current is negligible

The voltage across the secondary is very small, It isThe voltage across the secondary is very small, It isminimum when the secondary is short circuited andmaximum when open circuited

Current Transformer Theory

SimplifiedCTequivalentcircuit

Ip Is

Z Z

Ip

Zp ZsIe

V Es V ZB

Im Iw

Vp Es Vs ZB


Phasor Diagram

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Current Transformer Theory Primary current has two components, first is secondarycurrent which is transformed in inverse of the turnsratio and an exciting current which supplies the eddy &g pp yhysteresis losses and magnetize the core

Exciting current is not transformed and causes errors The exciting current determines the maximum The exciting current determines the maximumaccuracy that can be achieved with a currenttransformer

Ip = Ie + Is , orIs = Ip Ie

Current Transformer TheoryThe error in the reproduction will appear both inamplitude and phase. The error in amplitude is called

t ti d th i h i ll dcurrent or ratio error and the error in phase is calledphase error or phase displacement

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PrimaryIpp

IKn

Ip

: Phase error

SecondaryI

Kn.Is: Phase error

SecondaryIs

Kn =IpIs

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Since is a very small angle the current error and the phase

Current Transformer Theory Since is a very small angle, the current error and the phase

error could be directly read in percent on the axis

( = 1% = 1 centiradian = 0.572 dgree = 34.4 minutes)

the current error is positive if the secondary current is too high,and the phase error is positive if the secondary current isleading the primaryleading the primary

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h ll ?

Current Transformer AccuracyWhy at all CTs are inaccurate?The culprit is core loss and magnetizing current, whichintroduces ratio error as well as phase errorintroduces ratio error as well as phase error

What is inaccuracy?What is inaccuracy? The secondary current which we get is not truereflection of its primary current. for example, for a CTp y pwith CT ratio of 1000/1 amps, if we get 0.99 amps insecondary leading primary current by 15 minutes (0.25degree) for primary current of 1000 amps so the CTdegree) for primary current of 1000 amps, so the CThas ratio error of (0.991)/1 x 100= 1% and phaseerror of 15 minutes

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Th i i I i d i hi h i d fi d

Current Transformer Theory The exciting current Ie introduces ratio error, which is defined

as the difference in magnitude of the primary and secondarycurrent expressed as percentage of primary current

100.

)( = psnI

IIKErrorRatioCurrent

Kn= Rated transformation ratioI A t l i t

pI

Ip = Actual primary currentIs = Actual secondary current

The Phase angle error is the phase angle difference betweenthe primary current and the reversed secondary currentvectorsvectors

C itCurrent Transformer Theory

Composite error Under steadystate conditions, the r.m.s. value of the difference between

the instantaneous values of the actual primary current, and theinstantaneous values of the actual secondary current multiplied by the ratedtransformation ratio, integrated over one cycle including the effects of phasedisplacement and harmonics of excitation currentC it i ll d t f l f Composite error is generally expressed as a percentage of r.m.s. value ofprimary current according to the formula

( )T1100 ( ) = psnp

c dtiiKTI 021100

Kn istheratedtransformationratioIp isther.m.s.valueoftheprimarycurrentip istheinstantaneousvalueortheprimarycurrentis is theinstantaneousvalueofthesecondarycurrentTisthedurationofonecycle

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Knee Point Voltage The point on magnetizingKnee Point Voltage (KPV)

Knee Point Voltage The point on magnetizingcharacteristic (plot between secondary appliedvoltage and the corresponding magnetizing current)voltage and the corresponding magnetizing current)at which an increase of 10% in exciting e.m.f.produces an increase of 50% in the exciting current

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BurdenBurden of Current Transformer

BurdenThe external load (e.g. meters, transducers, relays etc)connected to the secondary of a CT is called the burdenThe burden can be expressed in voltamperes or in ohmsp p

VA = I2 x ZZ = Total CT secondary impedanceI = Secondary current (Generally 1A or 5A)

Total burden is the sum of1 D i ( d l ) B d F i h d b1. Device (transducer, meter, relay etc) Burden Furnished bythe manufacturer2. Burden of Interconnecting Leads can be calculated by usingthe above formula, use conductor resistance (total to the device, (and back) for Z3. Internal Burden of CT Windings This is so small that it cangenerally be ignored or specified by manufacturer

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Magnetization Curve

TheexcitationofCTdependon

a) Crosssectionalarea

b) Lengthofmagneticpathofcore

c) Numberofturnsinthewinding

d) Magneticcharacteristicsf hofthecore

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Basic Induced Voltage equation

Current Transformer TheoryBasicInducedVoltageequationEs =4.44*Bm *Aeff *f*Ns

where B flux density = /AwhereBm fluxdensity=m/AeffAeff Coreeffectiveareaf frequencyN secondary turnsNs secondaryturnsEs Inducedvoltageinthesecondary

A component of primary current excites the core to the flux densitynecessary to induce in the secondary winding an e m f sufficient to drive thenecessary to induce in the secondary winding an e.m.f. sufficient to drive thesecondary current through total impedance of secondary circuit

Hence core flux density is dependent on the magnitude of primary currentand the impedance of secondary circuit

Es isdecidedbythetotalburdenEs=Totalburden(VA+leadburden+sec.windingburden)*IsN i d id d b th ti i N I I

and the impedance of secondary circuit

Ns isdecidedbytheratioi.e.Np,Ip,IsNs=NpIp/Is3/4/20137:29:24PM 27

Current Transformer TheoryA b d fl d i i di l hAt constant burden, core flux density varies directly as thesecondary current, hence, as the primary and consequentlysecondary current increases, a point is reached when core

B is decided by the required error Lower B for better

material start saturating and exciting current becomesexcessive, thus resulting in excessive current error

Bm isdecidedbytherequirederror.LowerBm forbetteraccuracy.LowertheBm loweristheexcitationcurrentyieldingbetter

b t laccuracybutlargercorearea

RewritingtheequationAeff=Es/(4.44*Bm*f*Ns)eff s/( m s)

Highercoreareaisrequiredfor betteraccuracy(lowerBm ,lowerIe Excitationcurrent), lower ampturns (lower N ) lowerampturns(lowerNs), andhigherburden(higherEs)3/4/20137:29:24PM 28

Effect of Secondary Open Circuiting E.M.F. induced in secondary winding is that required to drive

secondary current through total impedance of secondarycircuit, and core flux inducing this e.m.f. is provided by a smallcircuit, and core flux inducing this e.m.f. is provided by a smalldifference between primary and secondary m.m.f. (ampereturns)

With secondary open circuited, there are no secondary ampereturns to oppose those due to primary current and whole ofprimary m.m.f. act on the core as an excessive exciting force,p y g ,which drive core into saturation on each half wave of thecurrent

Thi hi h t f h f fl i th i f i t This high rate of change of flux in the region of primary currentzero induces an e.m.f., Es of high peak value in the secondarywinding

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Effect of Secondary Open Circuiting

With rated primary current, peak value may be as low as fewhundred volts in small measuring CT with 5A secondarywinding, but it might reach many kilovolts, in the case of, say2000/1A protective CT with a large core section/ p g

With system fault currents flowing in primary, even highervoltages would be induced and not only constitute hazard toi l ti f CT it lf d t d i t t l dinsulation of CT itself and connected instruments, relays andassociated wiring, but also to life

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SelectionofSelectionofCurrentTransformersCurrentTransformers

Current Transformer Secondary Rating

Choice of CT secondary rating 5A Secondary

Preferred where lead burden is insignificant (e g indoor Preferred where lead burden is insignificant (e.g. indoorswitchgear cubicles with closely located relays)

Preferred where primary current ratings are very high Comparatively low peak voltage when secondary getsopen

Fine turns ratio adjustment is not possible when primaryj p p yrating is low

1A SecondaryP f d h CT td d l d b d hi h Preferred when CTs are outdoor and lead burden are high

Comparatively high peak voltage when secondary is open Fine turns ratio adjustment possible

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Current Transformer Accuracy Measuring CTs are required to be accurate overnormal working range of current, while protective CTsrequired to maintain the accuracy up to several timesrequired to maintain the accuracy up to several timesof the rated current

Metering if we want to measure current for meteringpurpose, we desire thatwhatever current we measure, that should be veryaccurate as the metered data may be used for tariffaccurate as the metered data may be used for tariffpurpose

Accuracy ClassA designation assigned to a current transformer, theerrors of which remains within specified limits underprescribed condition of useprescribed condition of use

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Classification of Current Transformer MeteringClassCTs0.1class:Highprecisiontesting

0.2class:Laboratoryclass

0 5 class : industrial metering0.5class:industrialmetering

1.0class:Firstgradeindicatingwattmeter

l l /3.0&5.0class:Forgeneraluse/WTI

ProtectionClassCTs5P,10P,15PPS classPSclass

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D i ti f M t i CT

Measuring Current Transformer Designation of Metering CTsMetering CTs are specified in terms of R i A l B d (VA i ) ISFRatio, Accuracy class, Burden (VA rating), ISF(Instrument Security Factor)Example: 2000/1 Class 0 2 20VA ISF 5Example: 2000/1, Class 0.2, 20VA, ISF 5

Standard Error Class 0.1, 0.2, 0.5, 1.0, 3.0, 5.0Standard Error Class 0.1, 0.2, 0.5, 1.0, 3.0, 5.0 The errors are specified between 5120% of ratedcurrent and 25100% of rated burden connected

Higher errors are permitted at lower currents

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Current Transformer Accuracy Limits

IEC 600441 Limits of error for accuracy Class of metering cores

Metering Cores

Class 5% of t d I

20% of t d I

100% of t d I

120% of t d I

IEC 60044 1 Limits of error for accuracy Class of metering cores

rated I rated I rated I rated I

0 2 0 75 0 35 0 2 0 20.2 0.75 0.35 0.2 0.2

0.5 1.5 0.75 0.5 0.5

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IEC600441 has laid down standards on this

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Th i d h d f CT h ld b

Instrument Security factor (ISF) The instruments connected to the secondary of a CT should be

protected from getting damaged during primary fault condition,when primary current is many times higher than the rated value,the core should get saturated

For this purposes, Instrument Security Factor (ISF) for MeteringCTs has been definedCTs has been defined

The CT cores should be such that it saturates at its instrumentsecurity factor (ISF) for safeguarding the instrument from gettingd d d f lt t ditidamaged under fault current condition

ISF is defined as the ratio of rated instrument securityprimary current to rated primary currentprimary current to rated primary current

ISF is expressed as 3,5,7 or 10 (it shall be chosen assmall as possible)

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Protection Current Transformer Protection Class During fault condition, value of primary current maybe 10 to 20 times the rated primary current

Here, main requirement is ability of CT to faithfullyf h i d i f l di itransform the primary current during fault condition

At such high level of primary current, if CT is notl d i d it t t d l illproperly designed, it may saturate and relay will

receive very less current and, therefore, would notmake right decisionmake right decision

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D i ti f P t ti CT

Protection Current Transformer Designation of Protection CTsProtection CT are specified in terms of R i A l B d (VA i ) ALF (ARatio, Accuracy class, Burden (VA rating), ALF (AccuracyLimit Factor)Example: 200/1 5P20 10VAExample: 200/1, 5P20, 10VA

Standard Error Class/ALF/VA ratingStandard Error Class/ALF/VA rating Error Class 5P, 10P, 15P ALF 5, 10, 15, 20, 25, 30 VA rating 5, 10, 15, 30

The errors are specified at rated current and ALF timesrated current with rated burden connectedrated current with rated burden connected

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Protection Current Transformer

.


Protection Cores BS 3938:1973 Limits of error for accuracy Class 5P and 10P

Accuracy Class

Current Error at rated

Phase displacement error at rated

Composite Error at rated accuracy

BS 3938:1973 Limits of error for accuracy Class 5P and 10P

Class at rated Primary Current

error at rated Primary Current

at rated accuracy limit (ALF)

Primary CurrentP 1% 60 i 1 8 5%5P 1% 60 min 1.8

centiradians5%

10P 3% 10%10P 3% - - 10%

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U lik i CT hi h i d b

Accuracy Limiting Factor (ALF) Unlike measuring CTs, which are required to be accurate over

the normal working range of currents, protective CTs are usuallyrequired to maintain their ratio up to several times the ratedprimary current

At some value of primary current above the rated value, corecommence to saturate resulting in increase in secondarycommence to saturate, resulting in increase in secondarycurrent error

Protection Class CTs cores should not get saturated below itsA Li iti F t (ALF) t hi h th i tAccuracy Limiting Factor (ALF) up to which the primary currentshould be faithfully transformed to the secondary side,maintaining the specified accuracy

ALF is defined as the ratio of the rated accuracy limitprimary current to the rated primary current

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For a given CT, VA and ALF are inversely related, for example, ifProtection Current Transformer

For a given CT, VA and ALF are inversely related, for example, ifconnected burden is less than rated then ALF would increase

Applications of this CT are Over current relay, Inverse relay,earth fault protection, Phase fault protection etc.

While selecting 5P10 class CT for IDMT O/C or Earth fault relays

CT should have optimum ALF/VA rating, so that they do notsaturate up to at least 20 times current rating (either byselecting low burden relays or by selecting a ratio ofselecting low burden relays or by selecting a ratio ofappropriate high value)

Over rated CTs having high VA rating and ALF may produceOver rated CTs having high VA rating and ALF may producehigh secondary currents during severe faults (in excess of 20times setting) that may cause thermal stressing of relaycurrent coils and eventual failurescurrent coils and eventual failures

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f f l l

Protection Current Transformer Designation of Protection CTs for special applicationsFor protection like circulating current differential, restrictedearth fault etc where balanced of current/turns is requiredearth fault etc. where balanced of current/turns is requiredbetween associated CTs with close toleranceSpecial class Protection CT of are specified in terms of 1) R ti1) Ratio2) Accuracy class3) Knee Point Voltage (Vk)) d i di i ( ) d O4) CT Secondary winding resistance (RCT) corrected to75OC

5) Excitation current (Ie) usually at Knee Point Voltage or a statedpercentage thereof

Example 200/1, PS Class, Vk > 200V, RCT < 2.0 ohms, Ie < 30mA at Vk/4 The turn ratio error are limited to +0.25% which helps in

maintaining balance between the protection system duringg p y gmaximum through fault condition

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Which Current Transformer is connected

Substation to be protected

External/Through Fault

Internal Fault


Equipment to be protectedI1 I1

Equipment to be protectedI2

i1 i1i1 i1 i2

S bili i i1 + i2Stabilising Resistance

Operating relay

UNIT PROTECTIONUNIT PROTECTION


ForUnitTypeProtection Here, requirements are rather stringent as wecompare current of two or more CTs and rely on thecompare current of two or more CTs and rely on thetheir mutual faithfulness, moreover, our aim is thatthe protection must be stable for even worst throughfault condition and fast acting for internal faultconditionF d ti f l it i ll d i bl t For speedy operation of relay, it is usually desirable tomake the knee point voltage of the CT magnetizingcurve not less than twice the relay operating voltagey p g g

F hi h i d i l ti t diff ti l


Forhighimpedancecirculatingcurrentdifferentialscheme

voltsRRIV leadCTfK )2(2 +RCT= CT secondary winding resistance

R = lead resistance of the farthest CT in parallelRlead = lead resistance of the farthest CT in parallelgroup

If = Maximum through fault current up to which relayIf Maximum through fault current up to which relayshould remain stable (referred to CT secondary)

F T f I M i th h f lt t


ForTransformers, If =Maximumthroughfaultcurrentlimitedbyleakageimpedanceoftransformer

For Busbar If = Maximum through fault current limitedForBusbar,If =Maximumthroughfaultcurrentlimitedtoswitchgearbreakingcapacity

ForGenerator, If =Maximumthroughfaultcurrentf glimitedbysubtransientreactance(Xd)ofthegeneratorF M t I M i t ti t ( b t 6 ForMotor, If =Maximumstartingcurrent(about6xloadcurrentforDOLMotors)

For Shunt reactors I = Maximum charging current ofForShuntreactors,If =Maximumchargingcurrentofreactor

ForShortfeeders,If =Maximumthroughfaultcurrent, f gforfaultatbusbar

F bi d diff ti l l


Forbiaseddifferentialrelay

voltsRRIKV )]2(2[ +I Relay rated current

voltsRRIKV leadCTRK )]2(2[ +IR= Relay rated current

K = Constant specified by the manufacturer usuallybased on conjunction test (the constant is usuallybased on conjunction test (the constant is usuallychosen to ensure positive operation of highestdifferential unit on severe internal fault with extremeCT saturation)

F Di t P t ti


ForDistanceProtection

voltsnRRZIXV )]()[1( +++X/R Primary system reactance/resistance ratio (to

voltsnRRZIR

V leadCTrelayfK )]()[1( +++X/R = Primary system reactance/resistance ratio (toaccount for the DC component of the fault current)

I = Maximum CT secondary current for fault at zone1If= Maximum CT secondary current for fault at zone1reach point

Z = Relay ohmic burdenZrelay = Relay ohmic burden

RCT= CT secondary winding resistance

nR = Lead resistancenRlead= Lead resistance

Outdoor Current Transformer

OutdoorCTsarebasicallyof3typesofConstruction

DeadTank withU(HairPin)shapedprimaryprimary

DeadTankwithEyeBoltprimary Live Tank or Inverted primary CTLiveTankorInvertedprimaryCT

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PPBus Feeder

Dead Tank Current TransformerPP

CB

Bus Feeder

Insulator

Primary winding

Secondary winding

Core

Terminal BoxSS

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Dead Tank Current Transformer

1) Eye Bolt Type 2) Hair Pin Type

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P2P1

CORE 5CORE 1

CORE 2CORE 4

CORE 3

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Outdoor Current Transformer

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Manufacturing of Current TransformerManufacturing of Current Transformer

CT Secondary in Progress


Shell Preparation & Assembly


CT Tank Assembly & Welding


This type construction, cores situated in atank close to the ground, the primaryconductor is Ushaped (hair pin) or coilconductor is Ushaped (hair pin) or coilshaped (eye bolt)

low centre of gravity & high earth quakeg y g qwithstand

using heavy cores without stressing theporcelain insulatorporcelain insulator

Oil circulation in the primary conductor (tube)gives even temperature and not hot spotsgives even temperature and not hot spots

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420kVdeadTankCT(HairPinDesign)( g )

TheheatinCTismainlyproducedintheprimarywinding.Thethickinsulation P2P1

encirclinghairpinprimarypreventsheattransferfrommostofthistube

P l i i h f Porcelaincasingpreventsheattransfertoairasitisabadconductorofheat.Theoilcarries heat from inside the primary coppercarriesheatfrominsidetheprimarycoppertubetotopwherecoolingtakesplace

Therefore, the cooling area is severely Therefore,thecoolingareaisseverelyrestrained.Duetolimitedcoolingareaatthetop,raisingtheCTcurrentratingis

CORE 5

CORE 4

CORE 1

CORE 2

CORE 3

difficult3/4/20137:29:24PM 63

Live Tank Current Transformer

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Live Tank Current Transformer

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420 kV Live Tank CT420kVLiveTankCT

The heat in CT is produced only in the TheheatinCTisproducedonlyintheshortconductorlengthofprimarywinding.TheheattransferiseasilydoneatthetopchamberlevelascomparedtopresentHairpindesign

Therefore the cooling area is Therefore,thecoolingareaisadequateforsmallerlosses.RaisingtheCTcurrentratingismucheasier

AllovertheworldCTs>3000AmpsareproducedwithInverteddesignonlyonly

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k d d k420kVLiveTankCT

420kVdeadTankCT(HairPinDesign)3/4/20137:29:24PM 67

TheoryofTheoryofyyVoltageTransformersVoltageTransformersgg

What is Voltage Transformer

Voltage Transformer is an instrumenttransformer which transforms voltagef l l t th l l hfrom one level to another level such as400KV/3:110V/3 (VT ratio) i.e.transforms voltage from the level oftransforms voltage from the level of400KV/3 into voltage of 110V/3 level

Direct measurement of high voltage (inDirect measurement of high voltage (inthe tune of 3.3kV or more) is notpossible as devices used formeasurement of voltage are notdesigned to handle such high level ofltvoltage

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h b

Why Voltage Transformer is Required

Systemhastwobasicrequirements

t i fmetering ofenergysourcedorconsumed

protection of theprotection oftheelectricalsystemfromfaults andfaultsanddisturbances

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Why Voltage Transformer is Required

Faults can be of many kinds, some faults such as O/Ccan be detected solely on current measurement, but

t d t id di ti b t t dcurrent does not provide discretion about nature andlocation of the fault

Therefore when voltage is also measured along withTherefore, when voltage is also measured along withcurrent during faults, we can in a way compute poweror impedance of system along with its directiong

Moreover O/V, U/V, O/F, U/F and over fluxingprotections are also configured from VTs

Voltage signal also used for synchronizing, Disturbancerecorders and event logs

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h d d

How Voltage Transformer is connected

VT has a primary and one or more secondarywindings

M t i d P t ti d i t d t Metering and Protection devices are connected tothe secondaries of the VT

In voltage operation or shunt mode the primary In voltage operation or shunt mode, the primarywinding is connected in parallel with the powersystem to transform the phase voltage to usually 63.5system to transform the phase voltage to usually 63.5volts suitable for the meter or relay

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f l d h f ll l d

Voltage Transformer Theory For a transformer in no load the following is valid

Voltage transformation is proportional to the ratio of primaryand secondary turnsand secondary turns

11

EE

NN=

An ideal voltage transformer is a transformer under noload

22E N

conditions where the load current is zero and the voltage drop isonly caused by the magnetizing current and is thus negligible

Voltage Transformer Theory

SimplifiedVTequivalentcircuit

Ip Is

Z Z

Ip

Zp ZsIe

V Es V ZB

Im Iw

Vp Es Vs ZB

Voltage Transformer Theory

IpRpVp

VIsRs Es

Vp

IpI

IeRpVs

Is

Ie

Phasor diagram with referance to voltage error

0

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h h d f d h d ff

Voltage Transformer Theory Ratio error, which is defined as the difference in

magnitude of the primary and secondary voltageexpressed as percentage of primary voltageexpressed as percentage of primary voltage

100.

)( = pns VKVErrorRatioVoltageKn= Rated transformation ratio

)(pV

g

Vp = Actual primary voltageVs = Actual secondary voltage

Phase Angle error is the difference between thereversed secondary and the primary voltage vectorsreversed secondary and the primary voltage vectors

V lt f t d t i th i ti lt f lt

Voltage Factor Voltage factor determines the maximum operating voltage for voltage

transformers expressed in per unit of rated voltage, which in turndependent on the system and voltage transformer earthing conditions

VT d i ff ti l th d t h hi h lt f t i i VTs used in noneffectively earthed system have high voltage factor since inthe event of an earthed fault in one of the phases, the healthy phasevoltage may rise to phase to phase value

VoltageFactorVF

Duration Earthing conditions

V.T.primarywinding

System

1.2 Continuous Nonearthed Effectivelyornoneffectivelyearthed

1.5 30s Earthed Effectivelyearthed

1.9 30s Earthed NoneffectivelyearthedwithautomaticE/Ftripping

1.9 8h Earthed IsolatedneutralorresonantearthedwithoutautomaticE/Ftripping

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Protection of EVT from accidental overloads and short circuit

Protection of Voltage Transformer Protection of EVT from accidental overloads and short circuit

across its secondary terminal is achieved by incorporating fusesor MCB in secondary circuit located near to transformer aspossible

Normal secondary current is not more than 5A and short circuitcurrent in the range of 100A simple fuses can be employedcurrent in the range of 100A, simple fuses can be employed

Short circuit on secondary winding gives only a few amperes inprimary winding and is not sufficient to rupture a high voltagefuse at primary side (HRC fuses on primary side up to 66kV)

Hence high voltage fuse on primary side do not protecttransformer they protect only network in case of any shorttransformer, they protect only network in case of any shortcircuit on the primary side

CVT invariably solidly connected to the system so that there isno primary protection

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Voltage Transformer Accuracy

As stated for CT, we need it forMetering voltage measurement, energy, powermeasurementProtection for distance protection, O/V, U/V, O/Fand U/F protections field failure overfluxing etcand U/F protections, field failure, overfluxing etc

For metering VTs we need high accuracy in the voltagemeasurement during stable conditions i.e. 80% tog120% of nominal system voltage with burdens from25% to 100% of rated burden at power factor of 0.8lagginglagging

Combination of magnitude and phase error dependson the power factor of the burdenon the power factor of the burden

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IEC600442and600445definesthisas

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For Protection VTs we need faithfulness ofvoltage measurement in the higher range ofvoltage measurement in the higher range ofvoltage such as from value as low as 2% ofnominal voltage to the rated voltage multipliednominal voltage to the rated voltage multipliedby rated voltage factors such as 1.2, 1.5, 1.9with burden of 25% to 100% of rated burdenwith burden of 25% to 100% of rated burdenat 0.8 pf lagging

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d d f h

Voltage Transformer Accuracy IEC600442and600445definesthisas

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Voltage Transformer Connections

There are three types of connections VV connection Star/Star connection Star/Open delta connectionV V ti VV connection Used for measurement and for those protections which donot require phase to neutral voltage input (2 VTs are used)not require phase to neutral voltage input (2 VTs are used)

Primary of VTs is connected in V (one VT primary across RYphase and other across YB phase) with identical Vconnection for the secondary

In this connection zero sequence voltage can not beproducedproduced

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Voltage Transformer Connections

Star/Star connection Either 3 separate single phaseVT i l 3 h 3 li b VTVTs or a single 3 phase, 3 limb VTis used

Both primary and secondariesp yare connected in star with bothstar neutrals solidly grounded

E h i h li b i th Each primary phase limb is thusconnected between phase toearth of the supply circuit andreplicate similar phase to earthvoltage on the secondary

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S /O D l i

Voltage Transformer Connections Star/Open Delta connection

Primary windings are connected instar with star neutral solidlyygrounded and the secondaries areconnected in series to form anopen delta connectionp

This type of connection is calledresidual connection and requireeither 3 single phase VTs or aeither 3 single phase VTs or asingle 3 phase 5 limb VT

This residual connection is used forl i i di i l h f lpolarising directional earth fault

relays or for earth fault detectionin noneffectively grounded orisolated neutral system

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f l f ( )

Types of Voltage Transformer TypesofVoltageTransformer(VT)

ElectromagneticVoltageTransformer(EVT)

C iti V lt T f (CVT) CapacitiveVoltageTransformer(CVT)

MM

P

P

M

PPP

INDUCTIVE VOLTAGE TRANSFORMER

CAPACITIVE VOLTAGE TRANSFORMER

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El t ti V lt T f i il t ll

Types of Voltage Transformer Electromagnetic Voltage Transformers similar to a small power

transformer and differs only in details of design that controlratio accuracy over the specified range of output, cooling( t t t th 200 300 VA) i l ti (d i d f(output not more than 200300 VA), insulation (designed forsystem impulse voltage level) and mechanical aspects

At high system voltages the cost of conventional potential At high system voltages the cost of conventional potentialtransformer is high, due to prohibitive cost of insulation,hence, at 132 kV and higher voltages, CVT may be moreeconomical than EVT particularly when the high voltageeconomical than EVT particularly when the high voltagecapacitors can serve also for carrier current coupling (PLCC),but may be inferior in transient performanceC i ll h i j i f hi h f i l Capacitors allow the injection of high frequency signals ontothe power line conductor to provide endtoendcommunications between substations for distance relays,l / i d i i itelemetry/supervisory and voice communication

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Capacitive Voltage TransformerD fi i iDefinition

A CVT is a voltage transformer comprising of capacitor dividerunit and an electromagnetic unit so designed andunit and an electromagnetic unit so designed andinterconnected that the secondary voltage of theelectromagnetic unit is substantially proportional to and inphase with the primary voltage applied to the capacitor dividerphase with the primary voltage applied to the capacitor dividerunit (IEC 186)

What does a CVT do?

Inputs to measuring and protection devices Galvanic isolation

Main Parts of a CVT

Capacitor Part Capacitor Stack, Insulator Electromagnetic Unit PT, HV Choke, FR circuit

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Capacitive Voltage Transformer

Primary Terminal

Capacitor Part

Electromagnetic Unit

HF Terminal

Sec. Terminal Box

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CVT Internal Components

Tank

PT

Resistor

Capacitor

FR

C

FR Choke

Ckt.

HV Choke

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CVT Internal Components

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Why intermediate PT is required

Assumingtheintermediatepotentialtransformerisabsent

C1 ExpressionforUs

U2ZU p C2 Burden

C1Up

Us

21

2

KRUZZ

U

p

ps += R

121

1

ZandRZR

U

Cj

Cj

ps

==+=

1

21

21

CK

CCCand Cj+=

21

1

CCCK +=

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Why intermediate PT is required

Theperuniterroris

KU

UKU

p

sp =

ConsideringR

UP s2

=Onsimplifying

Thisleadstotheconclusionthatforgivenerrorthepower

KCUP s /2 12=

g poutputisproportionalto SecondaryoutputvoltageUs Upper stack capacitance C UpperstackcapacitanceC1

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Why intermediate PT is required As the output voltage Us is usually constant, very large

capacitance (C1) is required to get sufficient power

output

This is economically unacceptable

Two modifications required to improve the situation Introduction of an intermediate stepped potential

transformer to boost Us , it can be 20 kV primary, the burden

is connected at its secondary at Volts

li i i f h i f h l d3

110

Elimination of the main source of phase angle error due to

the capacitance C(=C1+C2 ) by a series inductance tuned to

resonate with C at the system power frequencyresonate with C at the system power frequency

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L i i bl

Why HV Choke is required Lisvariableinductivechokeusedforphaseangle C1 Lerrorcorrection

Itistunedtoresonate with C C2

Up

R

L

resonatewithC(=C1+C2)atnominalpowerfrequency

UsC2 R

Wound PT

WoundPTisusedtoincreasetheavailableoutputpower,for

Wound PT

agivenmaximumerrorlimitandC1

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Equivalent Circuit Diagram of CVT

Leqisthesumofchokeinductanceandleakagei d t f th CU

I1

inductanceofthewoundPT

Magnetizinginductance

C1Up

LeqU1

I

+ UL -g gofthePTisneglected

Itcanbeseenthattheh i f i bl

UsC2 RU2 I2

UL

choiceofasuitablevalueofLtendstoreducethephaseangle

Wound PT

error

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As the value of L increases I1CVT Under Steady State

AsthevalueofLincreasesUp decreasesuntilitisinphasewithUs andthen

1

Iincreases

AtthefrequencywhenLandCareresonantand

UL

Us

U2I2

I

canceleachother,thecircuitwillbehave,understeady state condition as a Up

U2steadystatecondition,asaconventionaltransformer

p

U1 If the burden is short circuited a considerable overvoltageIftheburdenisshortcircuitedaconsiderableover voltage

appearsacrossC2,duetoresonanceofLandC ThisvalueofU2ishowever,usuallylimitedbysparkgap

connected across C or by arranging the inductance L to saturateconnectedacrossC2 orbyarrangingtheinductanceLtosaturateatcurrentsabovetheratedvalue

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A practical CVT consists of capacitance, tuning inductance andFerro-resonance

A practical CVT consists of capacitance, tuning inductance andwound PT which is having exciting impedance of nonlinearcharacteristicsWh it d li i d t t d i Whenever a capacitor and nonlinear inductor are connected inseries, there is a danger of nonlinear energy interchanges atsubharmonic frequencies and causes sustained oscillation andconsequently large overvoltage in the circuit

Such oscillations are less likely to occur when the losses in thecircuit are high, hence resistive load is increased in CVT (it alsocircuit are high, hence resistive load is increased in CVT (it alsoimpair the transient response)

To avoid Ferroresonance the operating flux of iron parts is keptat 1/2 to 1/3rd of the sat ration fl densit hich pre entsat 1/2 to 1/3rd of the saturation flux density, which preventshigh exciting currents during circuit transients

Alternately a special provision for damping the oscillations isprovided

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Capacitive Voltage Transformer

Coupling Capacitor

In Power Line Carrier Communication (PLCC), CouplingCapacitor (CC) is used as coupling device between

li d i i t ll hi hpower line and carrier accessories to allow highfrequency (40500KHz.) carrier signals into/out ofcarrier accessories (Line Matching Unit (LMU) etc )carrier accessories (Line Matching Unit (LMU) etc.)

Some times, the capacitor part in CVT is used as CC inPLCC

When CVT is used as CC the terminal HF will beconnected to carrier accessories (carrier coupling unit)

d f dinstead of grounding it

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PowerLineCarrier(PLC)equipmentC1

Wave Trap>500KHZ NOISE PICKUP

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ct & vt

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