inst xformer

8/13/2019 Inst Xformer

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Instrument Transformer Theoryand Testing

Steve Hudson, PEEngineering Manager

10737 Lexington DriveKnoxville, TN 37932

Phone: (865) 966-5856

www.powermetrix.com


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What is a Transformer?

A TRANSFORMER is a

device used to changethe voltage levels ofelectricity to facilitate thetransfer of electricity from

generating stations tocustomers. A step-uptransformer increases thevoltage while a step-down

transformer decreases it.www.duquesnelight.com/understandingelectricityupdate/electricterms.html


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Basic Transformer Theory

Vp = primary voltage

Ip = primary current

Np = primary turns

Pp = primary power Vs = secondary voltage

Is = secondary current

Ns = secondary turns Ps = secondary power

VpNp

Ns

Vs=

Ip

Ns

NpIs =

IsVsPsIpVpPp ===

This is true for an IDEAL transformer!


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What is anInstrument Transformer?

Instrument Transformers

convert signal levels fromdangerous (high voltage) orinconvenient (high current,or current at high voltage) tolevels appropriate formetering.

There are two fundamentaltypes:

CTs (Current Transformers)

PTs (Potential Transformers)


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Potential Transformers (PTs) PTs step down high voltages to the

voltage needed by the meter (usually

120V occasionally 67V). They come in many shapes and sizesfor different applications

They work exactly as you would

expect them to: Vo=Vi(Ns/Np). They come in various power ratingsexpressed in VA.

They come in various accuracy

classes, however the 0.3% accuracyclass is generally used in NorthAmerica.


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Potential Transformers (PTs) 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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Potential Transformers (PTs) 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 direct connectionto 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 = Ip(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 1320200/1000105.3 5

==

Tests have shown valuesranging from 500 to 11,000volts.


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CT Accuracy Class/Burden

Most CTs used in North

America are 0.3 (0.3percent) Class devices.

When an accuracy class

is specified the maximumburden for which thedevice meets the classaccuracy is also

specified.


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CT Class 0.3 Metering error shall be less

than 0.3% when the CT is

used 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

MeetsAccuracy

Spec

Meets 0.6 Spec

No Spec


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14 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 GUARANTEED

IN 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

Accuracy 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

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


X X

ACTUAL

PROPOSED IEEE C57.13.6 ACCURACY

CURRENT (RF=4.0)MAXIMUM CONTINUOUS

Slide Courtesy Kent Jones, GE

Accuracy Class 0.15


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ACCURACY

CLASS

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


X X

ACTUAL

PROPOSED IEEE C57.13.6 ACCURACY

CURRENT (RF=4.0)MAXIMUM CONTINUOUS

X

Accuracy Class 0.15S


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Errors with Current Transformers


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CTs require a lot of care to insure

accurate metering. Burden Over burden reduces CT accuracy.

Wiring Faulty or improper wiring reduces

accuracy by increasing burden. Shunt Failure to remove the safety shunt

will not keep the CT from operating but it will

reduce the readings by 50-80%


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When you seea CT spec

sheet it will give

you the burden

at which the CTmeets 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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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 itmeans that at 30C it can be used upto 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 its

label current and maintainits accuracy Class. Operating temperature

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

only RF=3 at 55C


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CT Testing

Three Approaches in use today

Direct RATIO measurement with appliedburden

Most accurate approach tells us exactly what we

want to knowMeasures directly the quantity we care about CT

Ratio

Is more complicated to perform.


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CT Testing

Alternate Approaches Burden only

A compromise: tells us if circuit is stable underexcess burden

Cant give us the ratio which is what we really careabout.

Admittance TestingAllows us to look for changes from previous

measurements.

Doesnt directly give ratio

Accuracy typically 5%


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On Site CT Testing The meter measures ONLY the voltage and current reaching the

meter terminals.

To verify that the CT is working at the site we have to test the entirecircuit.


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CT TransformersField Verification Full Ratio Measurement

MeasureHere

Measure

Here


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CT Ratio with Burden Testing

Ratio Testing is thepreferred approach whenwe can gain access to the

CT primary. Various types of probes

can be used for primaryside. Flex

HV


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CT Ratio with Burden Testing

Secondaryconnection ismade throughthe test switch

Sameconnection thatis used for therest of the sitetesting.

Ratio Testing with applied burden is themost accurate and complete approach for

testing at CT in service.


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Testing Current Transformers

Ratio vs Applied Burden CT testing can be done with very high accuracy

Reference CTmeasured usingPowerMaster with752 clamp-on

probes.Essentially NOratio error, phaseshift, or change insecondary current

versus appliedburden.


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Ratio vs Applied Burden

Reference CTmeasured usingPowerMaster with752 clamp-on

probes.Essentially NOratio error, phaseshift, or change insecondary current

versus appliedburden.

CT testing can be done with very high accuracy


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Ratio vs Applied Burden

Reference CTmeasured usingPowerMaster with

752 clamp-onprobes.Essentially NOratio error, phaseshift, or change in

secondary currentversus appliedburden.

CT testing can be done with very high accuracy


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Ratio vs Applied Burden CTs installations can be fully verified in the field

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

Burden Class 0.1CTs variousissues: too smallgauge wire, bad

connections, highharmonic content.


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

BurdenClass 0.1

CTs


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CT Phase B highly harmonic load. CT Phase B harmonic load removed.

Same CT in BOTH cases.


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These are the waveforms and harmonicswhen the CT failed.


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Without the harmonic load this CT passed its test.


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Ratio Error In Whose Favor?

I did at CT ratio test. What does it mean when Imeasure a ratio of 99.68:5 on a 100:5 CT?


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In the phase B test we measured 35.86 amps on theprimary and 1.799 secondary amps.

Ratio = 5.0 * (Primary / Secondary) = 99.68

If the ratio had been 100:5 and the primary was 35.86

amps we should have measured 1.793 amps on the

secondary.We measured 1.799 MORE than we should have.

Therefore a tested ratio of 99.68 means that we are

measuring more current than we should. This error is inour favor.


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Suppose we add burden to the metering circuit.

Notice that the ratioincreases as we applyburden. With 0.3Ohms of burden theratio switches from less

than 100:5 to greaterthan 100:5. This is thepoint where thingsswing to being in favorof the customer.


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Testing Current TransformersBurden Test Only

If we cannot get to the primary side of the CT we cancheck that the output is constant as burden changes.

Burden Class 0.2

CTs can lookquite bad butactually be withinspecs.


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Testing Current TransformersBurden Test Only

If we cannot get to the primary side of the CT we cancheck that the output is constant as burden changes.

Same performancefrom Burden Class0.5 CTs is well outof specification.

A burden only testthat is GOODprobably means the

CT is GOOD. A testthat is bad maymean nothing.


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CT Accuracy Burden - Load

MeetsAccuracy

Spec

Meets 0.6 Spec

No Spec


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Testing Current TransformersWhats Wrong?

Same site as above: What went wrong???


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Testing Current TransformersWhats Wrong? Power Theft

A simple piece of wirewrapped around the CT.


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Phase A CT reversed.


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Phase A & B CTs swapped.

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