selecting cts
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TEC POWERMETRIX
Selecting Current Transformers
Presented to:
80th Southeastern Meter School &Conference
By
William H. (Bill) Hardy, Ph.D.TEC Powermetrix
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TEC POWERMETRIX
Current Transformers (CTs)
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TEC POWERMETRIX
CT Accuracy Class/Burden
Most CTs used in NorthAmerica are 0.3 (0.3
percent) Class devices.
When an accuracy classis specified the maximum
burden for which the
device meets the class
accuracy is also
specified.
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TEC POWERMETRIX
CT Class 0.3
Metering error shall be lessthan 0.3% when the CT isused at FULL RATED LOADand with rated burden.
Metering error shall be lessthan 0.6% when the CT isused between 10% and 100%
of full rated load. Error is a combination ofamplitude and phase error.
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TEC POWERMETRIX
CT Accuracy Burden - Load
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TEC POWERMETRIX
Slide Courtesy Kent Jones, GE
0.3% ACCURACY
ACCURACYC
LAS
S
RATING
SECONDARY
FACTOR
CURRENT
20
4.0
15
3.02.0
10
1.0
5.02.51.00.5
.25
0.1
5%10%
50% 100% 200 300 400
0.60
0.30
0.15
0.15
0.30
0.60
0.6% ACCURACY REGION
NO ACCURACY GUARANTEEDIN FRONT OF THIS LINE
0.3% @BX.X RF 4.0
X CT TEST POINT
C20 METER TEST POINT
X X
ACTUAL
IEEE C57.13 ACCURACY
MAXIMUM CONTINUOUSAMPS (RF=4.0)
X
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TEC POWERMETRIX
0.15% ACCURACY
ACCURACYC
LAS
S
RATING
SECONDARY
FACTOR
CURRENT
20
4.0
15
3.02.0
10
1.0
5.02.51.00.5
.25
0.1
5%10%
50% 100% 200 300 400
0.60
0.30
0.15
0.15
0.30
0.60 0.3% ACCURACY REGION
NO ACCURACY GUARANTEEDIN FRONT OF THIS LINE
0.15 @E0.04, 0.15 @E0.20, 0.15 @BX.X RF 4.0
X CT TEST POINT
C20 METER TEST POINT
X X
ACTUAL
PROPOSED IEEE C57.13.6 ACCURACY
CURRENT (RF=4.0)MAXIMUM CONTINUOUS
Slide Courtesy Kent Jones, GE
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TEC POWERMETRIX
Slide Courtesy Kent Jones, GE
ACC
URACYC
LASS
RATING
SECONDARY
FACTOR
CURRENT
20
4.0
15
3.02.0
10
1.0
5.02.51.00.5
.25
0.1
5%
10%
50% 100% 200 300 400
0.60
0.30
0.15
0.15
0.30
0.60
0.15% ACCURACY REGION
NO ACCURACY GUARANTEEDIN FRONT OF THIS LINE
0.15S @E0.04, 0.15S @E0.20, 0.15S @BX.X RF 4.0
X CT TEST POINT
C20 METER TEST POINT
X X
ACTUAL
PROPOSED IEEE C57.13.6 ACCURACY
CURRENT (RF=4.0)MAXIMUM CONTINUOUS
X
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TEC POWERMETRIX
Errors with Instrument TransformersCT - Burden
Many CTs are onlyrated at B0.1 andB0.2
#16 wire is 4.5 m/ft
#14 wire is 2.8 m/ft
#12 wire is 1.8 m/ft
#10 wire is 1.1 m/ft
#8 wire is 0.7 m/ft 50 ft of #12 wire is nearly
100 m
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TEC POWERMETRIX
CT TransformersField Verification Full Ratio Measurement
Measure
Here
Measure
Here
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TEC POWERMETRIX
CT Accuracy Burden - Load
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TEC POWERMETRIX
CT Rating FactorThe MOST Misunderstood Spec
Rating Factor has absolutely nothing to dowith burden.
If a CT has a rating factor of 4 it meansthat at 30C it can be used up to 4X its
label current and maintain its accuracyClass.
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TEC POWERMETRIX
CT Rating Factor Rating Factor is a strong
function of temperature. If a CT has a rating factorof 4 it means that at 30Cit can be used up to 4X its
label current and maintainits accuracy Class.
Operating temperatureaffects Rating Factor
significantly. A CT with RF=4 at 30C is
only RF=3 at 55C
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