application of instrument transformers - ct.ppt
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APPLICATION OFINSTRUMENT TRANSFORMERS
Current TransformersVoltage Transformers - Electro-magnetic Type
- Capacitor Type
for
Metering &Protection
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ESSENTIAL PRE-REQUISITES FORINSRTUMENT TRANSFORMERS
1.HIGH RELIABILITY 2.CAPABILITY TO WITHSTAND HIGHEST SYSTEM
LOAD AND FAULT CURRENTS AND TRANSIENTVOLTAGES
3.LOW MAINTENANCE 4.LONG AND TROUBLE-FREE SERVICE
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Current Transformers
Service Conditions:Pollution Levels-- Normal, Heavy Rated Primary Current
Rated Short Time Current B.I.L Number of Ratios
Number of Cores Output ,Accuracy,Accuracy limit factor etc of Individual Cor
Specification Requirements
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Requirement of Metering And ProtectiveCurrent Transformers
MeteringShould maintain accuracywithin
limits over-Entire Current range of 20 to120 % of rated current-Burden Range of 25 to 100 %&Preferably saturate beyond theRated current
ProtectionShould maintain accuracy from
beyond the rated current to the
highest possible fault current inany location
Consequently should notsaturate up to highest fault
current
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Current Transformers
Definition of Accuracy
Ratio Error or Current Error:Kn Is - Ip * 100 percent
Ip
Phase Error or Phase displacement:Difference between the Primary Current and
Secondary Current Vectors
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Vector Diagram of C.T
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Variation of Ratio Error Permitted With 0.5 & 1.0 Accuracies
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VARIATION OF PHASE ERROR WITH BURDEN
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VARIATION OF ERRORS WITH PRIMARY CUAccuracy % Ratio Error at Phase Displaceme Class % of Primary Current % of Primary Curr 5 20 100 120 5 20 100 120
0.2 0.75 0.35 0.2 0.2 30 15 10 10
0.5 1.5 0.75 0.5 0.5 60 45 30 30
1.0 3.0 1.5 1.0 1.0 180 90 60 60
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P1
S1 S2 S3 S4 S1 S2
P1 C2 C4 C6
C3 C5 P2
SECONDARY RECONNECTION PRIMARY RECONNECTION
PRIMARY SECONDARY RATIOTERMINALS TERMINALS P1 P2 S1 S2 100/1 P1 P2 S1 S3 200/1 P1 P2 S1 S4 400/1
PRIMARY CONNECT SECONDARY RATIOTERMINALS TERMINALS P1 P2 C1-C2,C3-C4,C5-C6 S1 S2 100/1 P1 P2 C1-C2,C3-C4,C5-C6,C4-P2 S1 S2 200/1 P1 P2 C1-C3-C5-P1,C2-C4-C6-P2 S1 S2 400/1
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SECONDARY RECONNECTON V/S PRIMARY RECONNECTION
SECONDARYRECONNECTION ADVANTAGES
INDEPENDENT SELECTION OFRATIO ON INDIVIDUAL CORES RATIO CHANGE POSSIBLE
WITHOUT INTERRUPTINGPOWERE SUPPLY
DISADVANTAGE . OUTPUT , ACCURACY ON
LOWER RATIOS LOWER THANWITH HIGHER RATIOS
PRIMARY RECONNECTION ADVANTAGE
OUTPUT , ACCURACY UNIFORMLYSAME ON ALL RATIOSDISADVANTAGES
RATIOS ON INDIVIDUAL CORESCAN NOT BE INDEPENDANTLYCHOSEN
NECESSARY TO INTERRUPTPOWER SUPPLY FOR MAKINGCHANGE IN RATIOS
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Force Of Repulsion =
K I * I d
Force Of repulsion = K N I* NI d
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Normal VA Burden Of Instruments and
Meters Instrument/Meter VA BurdenAmmetersRecording Ammeters
3
Current Coils of -Wattmeters, Power Factor Meters, Kwh, Kvar meters, Recording Wattmeters
5
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Accuracy Classes for Protective Current Transformers
AccuracyClass
Ratio Error atRated Current
Composite Error at RatedA.L.F times Rated Current
5P + or- 1 % + or 5%
10P + or 3 % + or 10%
15P + or 5 % + or 15 %
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INTERDEPENDANCE OF VA , ACCURACY, A. L. F
VA= OUTPUT OF CT IN VOLTAMPERES A= CROSS SECTIONAL AREA OF CORE OF CT
B normal=FLUX DENSITY IN CORE AT NORMAL OUTPUTB sat =FLUX DENSITY IN CORE AT A.L.FA.L.F- ACCURACY LIMIT FACTOR
TAKING A SECONDARY CURRENT OF 1 AMP , VOLAGE ACROSS SECONDARYWINDING
VAV=--------=VA
1THEREFORE, VA= 4.44*FREQUENCY(f)*B normal* A* No OF TURNS IN SECONDARY(N)
AS FLUX DENSITY AT A.L.F I S RESTRICTED BY SATURATION IN THE COREHIGHER A.L.F MEANS LOWER VA AND VICE-VERSA
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Requirements for Special Purpose
Current Transformers The performance is specified in terms of
_ Minimum Knee Point Voltage
_ Maximum Exciting Current at the Rated KneePoint Voltage(K.P.V )
_Maximum Resistance of Secondary Winding atK.P.V
_ No Turns Correction
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Dead Tank Type Current Transformer
N 2
Oil
Dead Tank Type Current Transformer(Hair Pin Type) (Eyebolt Type)
N 2
Oil
1opics for Discussion
1.Pry Current2.Cost3.Dynamic withstand
.Porcelain cost
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N 2
Live Tank Current Transformer
Topics for Discussion1.Primary Taping2. E.Q withstand3.Secondary Terminals4. Placement of Insula-ted Secondary
5.Susceptibility forfailure in case of oilleakage
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CAUSES OF FAILURE
SEAL FAILURE----Causing CT to breathe atmospheric air.Moresevere in the case of LIVE TANK CTs(NTPC 400kV CTs )
LOOSE H.V CONNECTION-----Causing severe local heating andmelting of soldered joints
FORMATION OF CONDUCTING DRY BAND----due to pre-monsoon showers and un-seasonal rains causing external flashover
LOOSE P.F TERMINAL CONNECTION----Causing local arcing andconsequent weakening of insulation
MECHANICAL FAILURE OF PORCELAIN DUE TO GROUNDVIBRATIONS----More severe in LIVE TANK CTs due to inherentconcentration of weight on top of porcelain
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Schematic Diagram of Zero-flux C.T
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