Instrument Transformer Rev1

7/21/2019 Instrument Transformer Rev1

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INSTRUMENT TRANSFORMER.

By

Y. k. Pandharipande.



INSTRUMENT TRANSFORMERS

CURRENT & VOLTAGE

TRANSFORMERS CT ! PT"




CURRENT ! VOLTAGE TRANSFORMER

Produce a scaled down replica of the Primary System Quantities(Current or voltage) to the accuracy expected for the particularmeasurement.

Performance depends upon ability to transform Primary Quantity

in the presence of transient D.C. ff!sets" non!sinusoidal waveforms etc" particularly for protection application.

C# Primary loading is independant of secondary loading" unli$e incase of Power % Potential #ransformers.




CURRNT TRANSFORMERS # $ BASIC TYPES % CT'

(. MEASURING CURRENT TRANSFORMER

&sed to supply indicating % integrating 'nstruments. perating ange *+ ! ,-+ of ated Current.

). PROTECTIVE CURRENT TRANSFORMER

&sed with Protective elays and similar /pparatus. peration specified over a wide range of Current ( /ccuracy

specified at rated to /01 times rated current).

$ *UAL PURPOSE CURRENT TRANSFORMER

'ntended for 2etering and Protection. Complies with the re3uirement of both 2easuring % Protective C#

in terms of ange % /ccuracy.




MEASURING CURRENT TRANSFORMER IS )+,- (//) # Par0 II"

Specified in terms of /ccuracy Class " 4/ ating and 'nstrumentSecurity 1actor ( e.g. Class ,." 4/ 5 ,*" 'S1 5 6.)

Standard /ccuracy Class5 .," .-" .*" ,." 6." *..

7rror specified at * 8 ,-+ of ated Current. 9igher errorspermitted at lower currents. 1or example" Class ,. C# has a ratioerror of

(:%!) , + at ,!,-+ of ated Current (:%!) ,.*+ at -+ of ated current (:%!) 6+ at * + of rated current.

7rror specified with -*!,+ of ated ;urden connected.




PROTECTIVE CURRENT TRANSFORMER IS)+,-1 (//)1 PART III"

Specified in terms of /ccuracy Class" 4/ ;urden < /ccuracy 0imit1actor (/01) e.g. *P," ,* 4/.

Standard /ccuracy Class *P" ,P" ,*P. Standard 4/ atings *" ," ,*" 6.

Standard /01 *" ," ,*" -" 6.

1or economical design 4/ x /01 should not exceed ,*.

7rror specified at ated Current < /01 times ated Currente.g. *P," ,* 4/ C# has a composite error of

(:%!) *+ at /01 times ated Current with rated burden connected (:%!) ,+ at ated Current with rated burden connected.

1or a given C#" burden < /01 are inversely related i.e. 'fconnected burden is less than rated" /01 is up.



INSTRUMENT TRANSFORMER

PROTECTIVE CURRENT TRANSFORMER

C!70/#'= ;7#>77= ;&D7= < /01

RATE* BUR*EN (-VA" CONNECTE*.

/01 #imes ated current 5 , x * 5 */

#otal ;urden 5 .? : .,* 5 .@* hms.

4oltage developed by C# at /01 (4) 5 * x .@* 5 6@.* 4




PROTECTIVE CURRENT TRANSFORMER

RE*UCE* BUR*EN -VA" CONNECTE*. #otal ;urden 5 .- : .,* 5 .6* hm.

2ax. Secondary current developed by C# at limiting voltage of6@.*4 5 6@.* % .6* 5 ,A/ (-,.* times ated Current)

#hus" at reduced burden of * 4/" /01 increases to -,.*.




PROTECTION CT FOR SPECIAL PURPOSE APPLICATIONSIS )+,-1 (//)1 PART IV"

Designated as Class BPS C#(Class B as per ;S). &sed forbalanced protections li$e Differential" 71 where re3uired

characteristic can not be conveniently expressed in terms of /ccuracy Class" 4/ and /01.

Specified in terms of

Enee Point 4oltage (4$) C# Secondary >inding esistance(ct)

2agnetising Current ('m) #ypical Specification atio5-%,/" 4$5 -4" ct 5 - hms" 'm

5 6 m/ at 4$%-.

0ow reactance type" #urns ratio error limited to .-*+.




CLASS PS CT # 2NEE POINT VOLTAGE

2NEE POINT # Point on the 2agnetisation Characteristic where,+ increase in secondary voltage causes F *+ increase in the

2agnetising current.




CURRENT TRANSFORMER

-A CT SECON*ARY

&sed where lead burden insignificant (e.g. C#s used in in!doormetal!clad switch!gear or with high Prim. ating say,"%*/).

1ine turns ratio adGustment not possible for ;ar Prim C# with lowprimary ratings (say -*%*/).

Comparatively low pea$ voltage on C# secondary open!circuit.

(A CT SECON*ARY

Preferred where Cts are ut!door and lead resistance high.

1ine turns ratio adGustment possible.

Comparatively high pea$ voltage when C# secondary is open!circuited.




CURRENT TRANSFORMER CBCT"

&sed for Sensitive 7arth 1ault % 7arth 0ea$age Protection.

7xcited by Primary residual current. 9ence no out put due tobalanced % unbalanced load.




CURRENT TRANSFORMER CBCT"

CBCT SPECIFICATIONS F%33%4in5 para6e0er' 0% 7e 'pe8iied.

2in. Primary 7arth 1ault Current to be detected (say ,/ or -/)

2in. Setting of the Sensitive 7%1 relay (e.g. , m/) hmic burden of relay at 2in. Setting. (say , ohm)

#o and fro lead burden between C;C# < elay (say , ohm)

uter diameter of cable on which C;C# to be mounted.

=ote 8 Source end earthing of /rmour to be routed through thewindow of C;C# and earthed on load side as shown" to preventearth return current through armour affecting sensitivity.




VOLTAGE TRANSFORMER VT"

TYPES

2etering 4# Protection 4# Dual Purpose 4# esidual 4#

SPECIFICATIONS

4oltage atio /ccuracy Class

ated 4/ ;urden ated 4oltage 1actor

ANOT9ER CLASSIFICATION

7lectro!magnetic 4oltage #ransformer

Capacitor 4oltage #ransformer (C4#)






PROTECTION VT

CLASS : RATIO ERROR P9ASE ANGLE ERROR Min"

6P (:%!) 6+ ,-

?P (:%!) ?+ -I

RESI*UAL VT

* P (:%!) *+ -

, P (:%!) ,+ !!!!!

REFERENCE CON*ITIONS 4oltage 8 *+ to 4oltage 1actor times rated voltage

;urden 8 -* 8 ,+

P.1. .A 0ag

1re3uency ! ated.




VOLTAGE TRANSFORMER VT"

RATE* VOLTAGE FACTOR IS $(-? PART I # (//)"

SYSTEM EART9ING RATE* V.F. RATE* TIME

7ffectively 7arthed ,.- Continuous

,.* 6 Seconds

=on!effectively ,.- Continuous 7arthed (>ith

/utomatic 7%1 trip) ,.H 6 Seconds

=on!effectively ,.- Continuous 7arthed (>% /utomatic 7%1 trip) ,.H A 9ours

'n non!effectively earthed system" healthy phase voltage rises to phase tophase value" in the event of an earth fault. #hus" considering ,+ " permissibleover voltage"

4 (9ealthy Phase) 5 ,., x ,.@6- 5 ,.H




VOLTAGE TRANSFORMER

STAR ! STAR CONNECTION

;oth Prim % Secondary neutrals to besolidly grounded.

e3uires 6 x ,Phase or 6 Phase" 6Core 4#.

V CONNECTION

e3uires - x ,Phase P#s (Phase 8Phase connected).

&sed where Phase 8 =eutral 4oltagenot re3uired (e.g. 6 Phase " 6 >ire

2eters)




VOLTAGE TRANSFORMER

RESI*UAL CONNECTION STAR ! OPN *ELTA"

Primary neutral solidly grounded" secondary connected in open delta.

&sed for polarising directional 7%1 relay or detection of earth fault in non!effectively grounded systems using =eutral Displacement elay.

P#s should be 6 Phase" * 0imb or 6 x Single Phase.

Date post:	04-Mar-2016
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Instrument Transformer Rev1

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