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INSTRUMENT TRANSFORMER.
By
Y. k. Pandharipande.
7/21/2019 Instrument Transformer Rev1
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INSTRUMENT TRANSFORMERS
CURRENT & VOLTAGE
TRANSFORMERS CT ! PT"
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INSTRUMENT TRANSFORMERS
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.
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INSTRUMENT TRANSFORMERS
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.
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INSTRUMENT TRANSFORMERS
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.
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INSTRUMENT TRANSFORMERS
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.
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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
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INSTRUMENT TRANSFORMER
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 -,.*.
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INSTRUMENT TRANSFORMER
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 .-*+.
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INSTRUMENT TRANSFORMER
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.
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INSTRUMENT TRANSFORMER
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.
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INSTRUMENT TRANSFORMER
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.
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INSTRUMENT TRANSFORMER
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.
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INSTRUMENT TRANSFORMER
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#)
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INSTRUMENT TRANSFORMER
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.
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INSTRUMENT TRANSFORMER
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
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INSTRUMENT TRANSFORMER
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)
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INSTRUMENT TRANSFORMER
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.