General Notes on CTs

7/29/2019 General Notes on CTs

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Introduction to Instrument

Transformers

Presented to:

81th Southeastern Meter School &Conference

By

William H. (Bill) Hardy, Ph.D.

TEC Powermetrix


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What is a Transformer?

A TRANSFORMER is adevice used to changethe voltage levels of

electricity to facilitate thetransfer of electricity fromgenerating stations tocustomers. A step-uptransformer increases thevoltage while a step-downtransformer decreases it.www.duquesnelight.com/understandingelectricityupdate/electricterms.html


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OK, so whats anInstrument Transformer?

Instrument Transformersconvert signal levels from

dangerous (high voltage) or

inconvenient (high current,

or current at high voltage) tolevels appropriate for

metering.

There are two fundamental

types:

CTs (Current Transformers)

PTs (Potential Transformers)


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Potential Transformers (PTs)

PTs step down high voltages to thevoltage needed by the meter (usually120V occasionally 67V).

They come in many shapes and sizesfor different applications

They work exactly as you wouldexpect them to: Vo=Vi(Ns/Np).

They come in various power ratingsexpressed in VA.

They come in various accuracyclasses, however the 0.3% accuracyclass is generally used in NorthAmerica.


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PTs are available in Accuracy

Classes 1.2 Pecent

0.6 Percent

0.3 Percent

Burdens are expressed in VA W 12.5 VA

X 25.0 VA

M 35.0 VA

Y 75.0 VA

Z 200.0 VA ZZ 400.0 VA


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600V Type

240/416 2:1 288/500 2.4:1

300/520 2.5:1

480/480 4:1

600/600 5:1

5K Type

2400/4160 20:1

4200/7280 35:1

4800/8320 40:1

Medium Voltage 7200/12470 60:1

8400/14560 70:1

12000/20800 100:1

14400/24940 120:1

18000/18000 150:1

24000/24000 200:1

27600/27600 240:1

34500/34500 300:1

High Voltage Operate quite differently

May have no directconnection to transformer


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Current Transformers (CTs)

CTs allow the

measurement of highcurrents at potentially highvoltages.

They come in many shapes

and sizes for differentapplications

They are potentiallyextremely dangerous.

They can kill you!


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Current Transformers (CTs)


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Current Transformers (CTs)Basic Theory

Basic formula: Is = Ipy(Np/Ns) = Ip/Ns

Open Circuit Voltage:

Where: Zb = Burden Impedance

Ip = Primary Current

Ns = Number of Secondary Turns (Ratio to 1)

spb NIZV /5.3 =

VV 1320200/1000105.35

==

Tests have shown values

ranging from 500 to 11,000

volts.


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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 maximumburden for which the

device meets the class

accuracy is also

specified.


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CT Class 0.3

Metering error shall be lessthan 0.3% when the CT isused at FULL RATEDLOAD and with ratedburden.

Metering error shall be lessthan 0.6% when the CT isused between 10% and100% of full rated load.

Error is a combination ofamplitude and phase error.


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CT Accuracy Burden - Load


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13 Slide Courtesy Kent Jones, GE

0.3% ACCURACY

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

0.6% ACCURACY REGION

NO ACCURACY GUARANTEEDIN FRONT OF THIS LINE

0.3% @BX.X RF 4.0

X CT TEST POINT

C20 METER TEST POIN T

X X

ACTUAL

IEEE C57.13 ACCURACY

MAXIMUM CONTINUOUSAMPS (RF=4.0)

X

Burden Class 0.3


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0.15% ACCURACY

AC

CURACYC

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 0.3% ACCURACY REGION


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

Burden Class 0.15


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15 Slide Courtesy Kent Jones, GE

ACCURACYC

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

C20 METER TEST POINT

X X

ACTUAL

PROPOSED IEEE C57.13.6 ACCURACY

CURRENT (RF=4.0)MAXIMUM CONTINUOUS

X

Burden Class 0.15S


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Metering Use of ITs


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Metering Use of ITs 3 VTs

3 CTs Common

Return

When a meter isused with PTs andVTs :

Multiply thereading by CTR xVTR.


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Metering Use of ITs

2 CTs?

2? CTs

Common

Return

This installation doesnot conform toBlondels Theorem.


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Errors with Instrument TransformersPotential Transformers


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Primary source of errors is overloadingthe transformer.

Each PT has a burden rating, if we try

to draw too much power from the VT wewill cause an error in the reading.

The measured voltage will be low

thereby reducing the billing to thecustomer.


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Wiring is another source of errors. Loose or corroded wiring increases the

burden and reduces the voltage seen at

the meter.

The measured voltage will be low thereby

reducing the billing to the customer.


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Errors with Instrument TransformersCurrent Transformers


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Errors with Instrument TransformersCurrent Transformers

CTs require a lot of care to insureaccurate measurements.

Burden Over burden reduces CT accuracy.

Wiring Faulty or improper wiring reducesaccuracy by increasing burden.

Shunt Failure to remove the safety shunt

will not keep the CT from operating but it willreduce the readings by 50-80%


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Errors with Instrument TransformersCT - Burden

When you seea CT spec

sheet it will give

you the burden

at which the CT

meets a

specific

accuracy Class


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Many CTs areonly rated atB0.1 and B0.2

#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 isnearly 100 m


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CT TransformersField Verification Full Ratio Measurement

Measure

Here

Measure

Here


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CTs installations can be fully verified in the field

We can verify that the ratio is correct and constant as burden changes.


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If we cannot get to the primary side of the CT we can

check that the output is constant as burden changes.


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Errors with Instrument TransformersCT - Shunt

Leaving the shunt in the wrong position

produces wrong readings not no readings.

SHUNT CLOSED SHUNT OPEN


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Errors with Instrument TransformersCT - Polarity

Polarity of theconnection matters.

Wrong polarity

means totally wrongmetering.

When PF0,

reversed polaritiesmay not be obvious.


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CT Rating FactorThe MOST Misunderstood Spec

Rating Factor has absolutely nothingto do with burden.

If a CT has a rating factor of 4 it

means that at 30C it can be used up

to 4X its label current and maintain its

accuracy Class.


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CT Rating Factor

Rating Factor is a strong

function of temperature. If a CT has a rating factor

of 4 it means that at 30Cit can be used up to 4X itslabel current and maintainits accuracy Class.

Operating temperatureaffects Rating Factorsignificantly. A CT with RF=4 at 30C is

only RF=3 at 55C


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Introduction to InstrumentTransformers

Technology for Energy Corporation

10737 Lexington Drive

Powermetrix Division

Knoxville, TN 37932

Phone: 865-966-5856

FAX: 865-675-1241

www.powermetrix.com

William H. (Bill) Hardy, Ph.D.

General Notes on CTs

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