selecting cts

8/14/2019 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


0.15 @E0.04, 0.15 @E0.20, 0.15 @BX.X RF 4.0

X CT TEST POINT


X X

ACTUAL

PROPOSED IEEE C57.13.6 ACCURACY

CURRENT (RF=4.0)MAXIMUM CONTINUOUS



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TEC POWERMETRIX


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


0.15S @E0.04, 0.15S @E0.20, 0.15S @BX.X RF 4.0

X CT 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




#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

selecting cts

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