Current & Voltage Transformer.pptx

7/28/2019 Current & Voltage Transformer.pptx

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Current &VoltageTransformer

M. M. Meraat

Spring 1392


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Instrument Transformer Standards

IEC 60044-1,6,8 CTs IEC 60044-2,5,7 VTs

BRITISH BS 3938:1973 CTs

BS 3941:1975 VTs

AMERICAN ANSI C51.13.1978 CTs and VTs


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Current transformer

The Current Transformers transform current fromsystem to other system

CT should produce Secondary current (Is) exactlyproportional to that flowing in the primary system

(Ip).CT should produce Secondary current (Is) exactly

in phase with the current flowing in the primarysystem (Ip).

A CT rated primary current value is indicated asIpn and is selected based on the current which ismaximum intended continuous current at which itsperformance is based


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Current transformer


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PrimaryConductor

Ring TypeCurrentTransformer

PrimaryInsulation

Core

SecondaryWinding


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RELAY

1A ?1000A ?

1000 turns sec. ?

Insulation covered wire,giving inter-turninsulation & secondaryto core insulation

Generator, orsystem voltagesource

Feeder or Bus-bar

forming 1 turn of primarycircuit

Insulation to stop flash-over

from HV primary to core &secondary circuit

Laminated strip wound steeltoroidal core


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Current transformer A CT rated secondary current (Isn), however, is fixed value

depending on the rated current of secondary device it feeds and isstandardized as 1A, 5A commonly to meet almost all applications

IEC 60044-1 defines the following standard rated currents:

IEEE C57-13 defines the following standard rated currents:


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Current transformer

rated continuous thermal current (Icth) the value of the current which can be permitted to

flow continuously in the primary winding, the

secondary winding being connected to the rated

burden, without the temperature rise exceeding

the values specified


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Current transformerAn Ideal CT can be defined as the CT which meets

the requirement of constant (non-changing) valueof k and zero phase displacement between the Ip& Is over the entire range of interest. It is possible toachieve this requirement to any imaginableaccuracy. But achieving higher and higher

accuracy requires more sophisticated designs andmost of the time is not feasible due to cost and sizeof such a CT.

A Practical CT is one which is designed to meet an

particular application maintaining a minimum levelof accuracy within the constraints of cost & size.Thus a Practical CT differs from an Ideal CT due toits accuracy in k (which we can also call as currentratio of a CT) and phase displacement from zero.


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Current transformer The error in the reproduction will appear both in

amplitude and phase. The error in amplitude iscalled current or ratio error and the error in phase iscalled phase error or phase displacement.

( = 1% = 1 centiradian = 34.4 minutes)


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Current Transformer Errors Phase Error Definition:

The displacement in phase between the primary and

secondary current vectors, the direction of the vectors

being chosen so the angle is zero for a perfect

transformer.

Phase Error is :

positive: When secondary current vector LEADS

the primary current vector. negative: When secondary current vector LAGS

the primary current vector.


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composite error under steady-state conditions, the r.m.s. value of

the difference between: a) the instantaneous values of the primary current,

and

b) the instantaneous values of the actual

secondary current multiplied by the rated transformation ratio

The composite errorc is generally expressed as apercentage of the r.m.s. values of the primarycurrent according to the formula

It includes amplitude (Ratio) and phase errors andalso the effects of any possible harmonics in theexciting current


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Current transformer

Two basic groups of C.T.

Measurement C.T.s

Limits well defined

Protection C.T.s

Operation over wide range of currents

Note : They have DIFFERENT characteristics


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DefinitionAccuracy Limit Factor ( A.L.F. ) or Saturation Factor

Ratio of Ip to Irated up to which the C.T. ratedaccuracy is maintained.

Marking: Accuracy limit factor is written after the

accuracy class.

E.g. 10 VA 5P10, 15 VA 10P10, 30 VA 5P20.

e.g. 200 / 1A C.T. with an A.L.F. = 5 will maintain itsaccuracy for Ip < 5 x 200 = 1000 Amps


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Current transformerMeasuring C.T.s

Require good accuracy upto approx 120% ratedcurrent.

Require low saturation levelto protect instruments, thususe nickel iron alloy core

with low exciting currentand knee point at low fluxdensity.

Protection C.T.s Accuracy not as important

as above.

Require accuracy up tomany times rated current,thus use grain orientatedsilicon steel with highsaturation flux density.

B

Protection C.T.

Measuring C.T.

H


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Measuring Class CTs: These CTs are intended for measurement which is

close to normal operating current values (load). InAC systems, thus are designed to provideaccuracy within a band close to normal ratedvalues of current at fundamental frequency of

primary system. This is also important when such a current is used

for revenue (billing) purposes. Thus specialapplication to meet load as low as 1% of rated at

specific accuracy becomes important. In IEC 60044-1 & IS standard, the following standard

accuracies for Metering class CTs are defined:


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Measuring Class CTs: IEC standard accuracy class of 0.1, 0.2, 0.5, 1 for Normal

Metering purpose (this value indicates percentage ratio errorat 100% & 120% of rated primary current).

IEC standard accuracy class of 0.2S & 0.5S for Tariff Metering

purpose (this value indicates percentage ratio error at 20%,

100% & 120% of rated primary current).


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Measuring Class CTs:

IEC standard accuracy class of 3 & 5 for roughDisplay reading meters (which is not required to bevery accurate) and this value indicatespercentage ratio error at 50% & 120% of rated

primary current.


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Protective class These CTs are intended for measurement which is usually

at rated values as well as at high currents encounteredduring system faults.

In AC systems, thus are designed to provide accuracy

within a band close to normal rated values of current at

fundamental frequency of primary system and accuracyat higher current expected at faults.

IEC Standard Protective CT Classes & IS Standard CT

classes:

IEC defines accuracy classes for Protective class CTs based on

steady state performance as well as on Transient performances.All CTs are indicated with letter P.

IEC standard classes for Protective CTs based on Steady Stateperformances: Classes defined: 5P, 10P, PX

IEC standard classes for Protective CTs based on Transient

performances: Classes defined: TPX, TPY, TPZ, TPS


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IEC Standard CTs for Steady State performances:

Protective class CTs in IEC standard for SteadyState performances are defined for meetingaccuracy based on fundamental component ofAC fault current.

Transient DC component performance is not

defined for these CTs.

These CTs are defined in three groups: without limiton remanence flux (5P, 10P), with 10% limit onremanence flux (5PR, 10PR) and a CT with 0.25%

limit turns error having low leakage reactancedesign (PX).


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Steady State performanceClass-5P & 10P CTs (No

limit defined for remanence flux & turns ratio error):

Steady State performance Class-PX CTs (turns error

limit defined & no limit defined for remanence flux): These CTs are also protective class CTs with letter P

but must be low leakage flux type. CT turns error shallnot exceed 0.25% and the knee-point of CT must bedeclared



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knee-point is the point on the secondary excitation

characteristics of CT at which 10% raise inrms secondary voltage results in 50% raisein the rms excitation current).

The Vk is also limited by practical design

and manufacturing consideration as:

Vk = Rated output in VA x ALFSecondary rated current


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Application of Class PX CTs: For sensitive application which compare phase or neutral (or

residual) currents from more than one CT such as highimpedance protection or sensitive differential protections, itis necessary to have CTs which are with low leakage flux andsecondary characteristics can be used to evaluateperformances of protection (e.g. to evaluate performancebased on differences in secondary current outputs,differences in time to saturate of CTs etc).



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IEC Standard CTs for Transient performances:

Four Protective class CTs are defined in IEC60044-6

standard based on the Transient Performances. LetterT is prefixed with letter P for these class CTs.

These are Class TPS, TPX, TPY & TPZ.

These CTs performances are defined for dc componentcurrent in addition to ac current. Flux in the core due to

dc current component is X/R times the flux due to accomponent.

Thus in applications which are fast and are requiringtransient performances like Distance Protection, theseCTs can be evaluated.

Steady state performance CTs can also be used for suchapplication as long as they are over dimensioned for dcflux based on evaluation of CT time to saturate, relayoperation time and relay design.


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Transient performance Class-TPS (No limit definedfor remanence flux, but turns ratio error shall be lessthan 0.25%):

These CTs are also protective class CTs with letter TP

but must be low leakage flux type. CT turns error shall

not exceed 0.25% and the knee-point of CT must bedeclared (knee-point is point on the secondary

excitation characteristics of CT at which 10% raise in

rms secondary voltage results in 100% raise in the peak

excitation current.

These CTs are with Iron core without air-gap.



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Transient performance Class-TPX (No limit definedfor remanence flux & turns ratio error):

These CTs shall be assigned duty cycle like auto-

reclose attempts. These CTs are with Iron core without

air-gap.



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Transient performance Class-TPY (10% defined forremanence flux & no limit on turns ratio error):

These CTs shall be assigned duty cycle like auto-

reclose attempts. These CTs may have small air-gaps.

Auto reclose duty may have to be considered.

Transient performance Class-TPZ (remanence fluxnegligibly small & no limit on turns ratio error):

These CTs shall be assigned duty cycle like auto-recloseattempts. These CTs may have large air-gaps.



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Accuracy limits defined for TPX, TPY & TPZ class CTs:



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AUTO-RECLOSE DUTY CYCLES: As applied to TPX &TPY class CTs:



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Optical CT Technology


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Voltage transformer

Similar to a CT an ideal VT is a VT which introducesno ratio errors and no phase displacement when ittransforms Voltage from Primary system to thesecondary system.

Ratio error or Voltage error is expressed as:


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Standard values for VTs as defined in IEC60044-2 for VTs are:


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Standard values for VTs as defined in IEC60044-2 for VTs are:


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Voltage Transformer

ACCURACY CLASSES: IEC 60044-2 defined accuracy classes for Measuring

class VTs are:


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Voltage Transformer

ACCURACY CLASSES: IEC 60044-2 defined accuracy classes for Protective

class VTs are:


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Capacitive voltage transformer


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Capacitive voltage transformer

O ti l VT T h l


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Optical VT Technology

F


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Ferroresonance:

Resonance:

In a resonant circuit, inductive and capacitive

reactances of the circuit are equal to eachother

The only opposition to current is the circuit

resistance,

This resonance effect presents one stable

operation state, and its effects are mitigated

by the system frequencies control or by the

introduction of pure resistances.


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Ferroresonance:

Ferroresonance is a resonance situation with

nonlinear inductance, so the inductivereactance not only depends on frequency, but

also on the magnetic flux density of an iron

core coil (e.g. transformer iron core).

Theoretically, this nonlinear inductance could

be represented by two inductive reactances,according to the situation on thesaturation

curve.

Linear zone fl XLlinear= Llinear

Saturation zone fl XLsat

= Lsat

Example:


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Ferroresonance: Operation point 1: It is a non-ferroresonant stable

operation point. This is an inductive situation ( XLlinear >XC fl E = VLVC).

Operation point 2: It is a ferroresonant stable operationpoint. This is a capacitive situation ( XLsat < XC fl E = VC VL).

Operation point 3: It is an unstable operating point.


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Ferroresonance: Damping of ferroresonance A ferroresonance

oscillation, which is not

damped out efficiently, isdangerous for thetransformer. Under suchcircumstances the core ofthe intermediate voltagetransformer works at fullsaturation and the exciting

current might be large, sothat there is a risk of afailure. A dampingarrangement that dampsany resonance oscillationseffectively is thus anecessity. The standards

specify certainrequirements on thedamping and these testsshould be performed inorder to verify that theseare fulfilled.


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Ferroresonance:

Damping of ferroresonance

Current & Voltage Transformer.pptx

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