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
1
TEC POWERMETRIX
Current Transformers (CT’s)
2
TEC POWERMETRIX
CT – Accuracy Class/Burden • Most CTs used in North America are 0.3 (0.3 percent) Class devices. • When an accuracy class is specified the maximum burden for which the device meets the class accuracy is also specified.
3
TEC POWERMETRIX
CT – Class 0.3 • Metering error shall be less than 0.3% when the CT is used at FULL RATED LOAD and with rated burden. • Metering error shall be less than 0.6% when the CT is used between 10% and 100% of full rated load. • Error is a combination of amplitude and phase error. 4
TEC POWERMETRIX
CT – Accuracy – Burden - Load
5
TEC POWERMETRIX IEEE C57.13 ACCURACY 0.3% @ BX.X RF 4.0 X CT TEST POINT C20 METER TEST POINT
0.6% ACCURACY REGION
0.60 0.3% ACCURACY
ACCURACY CLASS
ACTUAL
0.30 0.15 5% 10% 0.15
50%
100%
200
300
400
0.30
0.60
MAXIMUM CONTINUOUS AMPS (RF=4.0) NO ACCURACY GUARANTEED IN FRONT OF THIS LINE
RATING FACTOR
X
X
X
0.1
1.0
2.0
3.0
4.0
5.0
10
15
20
SECONDARY CURRENT
.25 1.0 0.5
2.5
Slide Courtesy Kent Jones, GE
6
TEC POWERMETRIX PROPOSED IEEE C57.13.6 ACCURACY 0.15 @ E0.04, 0.15 @ E0.20, 0.15 @ BX.X RF 4.0
0.60
X CT TEST POINT C20 METER TEST POINT
0.3% ACCURACY REGION
ACCURACY CLASS
0.15% ACCURACY ACTUAL
0.30 0.15 5% 10% 0.15
50%
100%
200
300
400
0.30 MAXIMUM CONTINUOUS CURRENT (RF=4.0)
0.60
RATING FACTOR
NO ACCURACY GUARANTEED IN FRONT OF THIS LINE
X
X
0.1
1.0
2.0
3.0
4.0
5.0
10
15
20
SECONDARY CURRENT
.25 1.0 0.5
2.5
Slide Courtesy Kent Jones, GE
7
TEC POWERMETRIX PROPOSED IEEE C57.13.6 ACCURACY 0.15S @ E0.04, 0.15S @ E0.20, 0.15S @ BX.X RF 4.0 X CT TEST POINT C20 METER TEST POINT
0.60
ACCURACY CLASS
0.15% ACCURACY REGION
0.30
ACTUAL
0.15 5% 0.15
50%
100%
200
300
400
10%
0.30 MAXIMUM CONTINUOUS CURRENT (RF=4.0)
0.60
RATING FACTOR
NO ACCURACY GUARANTEED IN FRONT OF THIS LINE
X
X
0.1
X
1.0
2.0
3.0
4.0
5.0
10
15
20
SECONDARY CURRENT
.25 1.0 0.5
2.5
Slide Courtesy Kent Jones, GE
8
TEC POWERMETRIX
Errors with Instrument Transformers CT - Burden
• Many CTs are only rated at B0.1 and B0.2 • • • • • •
#16 wire is 4.5 mΩ/ft #14 wire is 2.8 mΩ/ft #12 wire is 1.8 mΩ/ft #10 wire is 1.1 mΩ/ft #8 wire is 0.7 mΩ/ft 50 ft of #12 wire is nearly 100 mΩ 9
TEC POWERMETRIX
CT Transformers Field Verification – Full Ratio Measurement
Measure Here Measure Here 10
TEC POWERMETRIX
CT – Accuracy – Burden - Load
11
TEC POWERMETRIX
CT Rating Factor The MOST Misunderstood Spec
• Rating Factor has absolutely nothing to do with burden. • If a CT has a rating factor of 4 it means that at 30°C it can be used up to 4X its label current and maintain its accuracy Class. 12
TEC POWERMETRIX
CT Rating Factor • Rating Factor is a strong function of temperature. • If a CT has a rating factor of 4 it means that at 30°C it can be used up to 4X its label current and maintain its accuracy Class. • Operating temperature affects Rating Factor significantly. A CT with RF=4 at 30°C is only RF=3 at 55°C
13
Selecting Current Transformers
Presented to:
80th Southeastern Meter School & Conference By
William H. (Bill) Hardy, Ph.D. TEC Powermetrix
1
TEC POWERMETRIX
Current Transformers (CT’s)
2
TEC POWERMETRIX
CT – Accuracy Class/Burden • Most CTs used in North America are 0.3 (0.3 percent) Class devices. • When an accuracy class is specified the maximum burden for which the device meets the class accuracy is also specified.
3
TEC POWERMETRIX
CT – Class 0.3 • Metering error shall be less than 0.3% when the CT is used at FULL RATED LOAD and with rated burden. • Metering error shall be less than 0.6% when the CT is used between 10% and 100% of full rated load. • Error is a combination of amplitude and phase error. 4
TEC POWERMETRIX
CT – Accuracy – Burden - Load
5
TEC POWERMETRIX IEEE C57.13 ACCURACY 0.3% @ BX.X RF 4.0 X CT TEST POINT C20 METER TEST POINT
0.6% ACCURACY REGION
0.60 0.3% ACCURACY
ACCURACY CLASS
ACTUAL
0.30 0.15 5% 10% 0.15
50%
100%
200
300
400
0.30
0.60
MAXIMUM CONTINUOUS AMPS (RF=4.0) NO ACCURACY GUARANTEED IN FRONT OF THIS LINE
RATING FACTOR
X
X
X
0.1
1.0
2.0
3.0
4.0
5.0
10
15
20
SECONDARY CURRENT
.25 1.0 0.5
2.5
Slide Courtesy Kent Jones, GE
6
TEC POWERMETRIX PROPOSED IEEE C57.13.6 ACCURACY 0.15 @ E0.04, 0.15 @ E0.20, 0.15 @ BX.X RF 4.0
0.60
X CT TEST POINT C20 METER TEST POINT
0.3% ACCURACY REGION
ACCURACY CLASS
0.15% ACCURACY ACTUAL
0.30 0.15 5% 10% 0.15
50%
100%
200
300
400
0.30 MAXIMUM CONTINUOUS CURRENT (RF=4.0)
0.60
RATING FACTOR
NO ACCURACY GUARANTEED IN FRONT OF THIS LINE
X
X
0.1
1.0
2.0
3.0
4.0
5.0
10
15
20
SECONDARY CURRENT
.25 1.0 0.5
2.5
Slide Courtesy Kent Jones, GE
7
TEC POWERMETRIX PROPOSED IEEE C57.13.6 ACCURACY 0.15S @ E0.04, 0.15S @ E0.20, 0.15S @ BX.X RF 4.0 X CT TEST POINT C20 METER TEST POINT
0.60
ACCURACY CLASS
0.15% ACCURACY REGION
0.30
ACTUAL
0.15 5% 0.15
50%
100%
200
300
400
10%
0.30 MAXIMUM CONTINUOUS CURRENT (RF=4.0)
0.60
RATING FACTOR
NO ACCURACY GUARANTEED IN FRONT OF THIS LINE
X
X
0.1
X
1.0
2.0
3.0
4.0
5.0
10
15
20
SECONDARY CURRENT
.25 1.0 0.5
2.5
Slide Courtesy Kent Jones, GE
8
TEC POWERMETRIX
Errors with Instrument Transformers CT - Burden
• Many CTs are only rated at B0.1 and B0.2 • • • • • •
#16 wire is 4.5 mΩ/ft #14 wire is 2.8 mΩ/ft #12 wire is 1.8 mΩ/ft #10 wire is 1.1 mΩ/ft #8 wire is 0.7 mΩ/ft 50 ft of #12 wire is nearly 100 mΩ 9
TEC POWERMETRIX
CT Transformers Field Verification – Full Ratio Measurement
Measure Here Measure Here 10
TEC POWERMETRIX
CT – Accuracy – Burden - Load
11
TEC POWERMETRIX
CT Rating Factor The MOST Misunderstood Spec
• Rating Factor has absolutely nothing to do with burden. • If a CT has a rating factor of 4 it means that at 30°C it can be used up to 4X its label current and maintain its accuracy Class. 12
TEC POWERMETRIX
CT Rating Factor • Rating Factor is a strong function of temperature. • If a CT has a rating factor of 4 it means that at 30°C it can be used up to 4X its label current and maintain its accuracy Class. • Operating temperature affects Rating Factor significantly. A CT with RF=4 at 30°C is only RF=3 at 55°C
13
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