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Transcript of 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
C20 METER 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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16 Slide Courtesy Kent Jones, GE
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
C20 METER 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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Errors with Current Transformers
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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Errors with Current Transformers
When you seea CT spec
sheet it will give
you the burden
at which the CTmeets a
specific
accuracy Class
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Errors with Current Transformers
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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Testing Current Transformers
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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Testing Current Transformers
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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Testing Current Transformers
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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Testing Current Transformers
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.
BurdenClass 0.1
CTs
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Testing Current Transformers
Ratio vs Applied Burden CTs installations can be fully verified in the field
CT Phase B highly harmonic load. CT Phase B harmonic load removed.
Same CT in BOTH cases.
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Testing Current Transformers
Ratio vs Applied Burden CTs installations can be fully verified in the field
These are the waveforms and harmonicswhen the CT failed.
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Testing Current Transformers
Ratio vs Applied Burden CTs installations can be fully verified in the field
Without the harmonic load this CT passed its test.
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Testing Current Transformers
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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Testing Current Transformers
Ratio Error In Whose Favor?
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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Testing Current Transformers
Ratio Error In Whose Favor?
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?
Same site as above: What went wrong???
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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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Testing Current TransformersWhats Wrong?
Phase A CT reversed.
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Testing Current TransformersWhats Wrong?
Phase A & B CTs swapped.