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SUBSTATIONCOMMISSIONING COURSE
MODULE THREE
INSTRUMENT TRANSFORMER
TESTING PROCEDURES
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INTRODUCTION
The electrical commissioning andacceptance testing of electric
power systems is essential for theenergization of any electrical
system for the first time.
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INTRODUCTION
Module Three
Discuss theory and ratings
Safety issues Test equipment operation &
hands on testing
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INTRODUCTION
Skills learned in MODULE THREE:
Acceptance testing for CTs and VTs
Operate CT tester Apply safe work practices Analyze test values Complete test forms
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INSTRUMENT TRANSFORMERS
A general classification
Current transformers (CT) Voltage transformers (VT)
Isolate primary quantitiesand convert into useable
secondary values via
transformation
Standard secondary rating:
5A for CT 120V for VT < 24 kV 115V for VT > 24 kV
Measurement values from
CTs and VTs:
Voltage, current,
frequency, real / reactivepower, energy, power
factor, impedance
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INSTRUMENT TRANSFORMERS
Polarity markings
A designation of the relative instantaneous direction ofcurrents
Current flowing into the polarity mark in the primary isrelatively in phase with current flowing out the polarity
mark in the secondary
As if the primary and secondary are connected straightthrough
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INSTRUMENT TRANSFORMERS
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INSTRUMENT TRANSFORMERS
Manufacturers name ortrademark
Manufacturers type Serial number Rated Voltage, maximum
voltage or nominalsystem voltage
Rated frequency
Basic impulse insulationlevel (N/A for bushing
CTs) Continuous rating factorfor CT (1.0 or more)
Thermal burden rating forVT (VA)
Accuracy rating
Nameplate Data
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INSTRUMENT TRANSFORMERS
Ratio expression betweenwinding groups
Example: VT with oneprimary and onesecondary winding
14,400 : 120 VRatio 120 : 1
Ratio between primary
and secondary amperes
Example: CT with one
primary and one
secondary winding
100 : 5 A
Ratio 20 : 1
Nameplate Symbols (: colon)
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INSTRUMENT TRANSFORMERS
Voltage rating or ratios
between winding groups
Example: VT with two
windings for series or
parallel connection
2,400 x 4800 V
Ratio 20 x 40 : 1
Current rating with two or
more coils for series of
parallel connection
Example: CT with two
primary windings for
series or parallel conn.
100 x 200 : 5 A
Nameplate Symbols (x multiplication sign)
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INSTRUMENT TRANSFORMERS
(Not used for VT) Ampere rating ofseparate secondary
winding with independentcore
Example: CT with twosecondary windings and
two cores.
100 : 5// 5A
Nameplate Symbols (// double slant lines)
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INSTRUMENT TRANSFORMERS
Voltage rating or ratios ofseparate winding on one
core.
Example: VT with oneprimary and twosecondary winding
14,400 V: 120 & 72 V
Ratio 120 & 200:1
Ampere rating ofseparate primary winding
on one core.
Example 1: CT with twoprimary windingsdesigned to be used
individually.
100 & 600 : 5A
Nameplate Symbols (& ampersand)
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INSTRUMENT TRANSFORMERS
Example 2: Totalizing CT
with four primary
windings designed to beused simultaneously.
5 & 5 & 5 & 5 : 5A
Nameplate Symbols (& ampersand)
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INSTRUMENT TRANSFORMERS
Example 3: CT for three-
wire single phase.
100 & 100 : 5A
Nameplate Symbols (& ampersand)
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INSTRUMENT TRANSFORMERS
Two or more primary orsecondary ratingsobtained by taps in the
secondary winding
Example 1: VT withsecondary tap for moreprimary voltage rating
12,000/14,400 :120 V
Ratio 100/120 : 1
Different primary currentratings obtained by tapsin the secondary winding
Example: CT with taps inthe secondary foradditional ratios
100 : 5// 5A
Nameplate Symbols (/ single slant line)
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INSTRUMENT TRANSFORMERS
Example 2: VT with
secondary tap for
additional secondaryvoltage ratings
14,400 : 120/72 V
Ratio 120/200:1
Nameplate Symbols (/ single slant line)
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INSTRUMENT TRANSFORMERS
CSA Standards CAN/CSA-C60044-1 to -8 (part 1 to part 8), 2007
Part 1: Current Transformers Part 2: Inductive Voltage transformers
Part 3: Combined Transformers Part 4: N/A Part 5: Capacitive Voltage transformers Part 6: Requirements for Protective Current
Transformers for Transient Performance Part 7: Electronic Voltage Transformers Part 8: Electronic Current Transformers
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INSTRUMENT TRANSFORMERS
What is a CT ?
A device that isolates and transforms the primary currentquantity into a proportional secondary current quantity
The CT has: Primary winding or primary conductor Magnetic core
Secondary winding
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INSTRUMENT TRANSFORMERS
Distributed uniformwinding with negligible
Leakage Flux
Non-uniform distributedwinding with some
Leakage Flux
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INSTRUMENT TRANSFORMERS
How does a CT work ?
Primary current flows generates magnetic field aroundthe conductor
Magnetic field flows in the magnetic core Magnetic field induces a voltage in the secondary
winding
Burden connected to secondary winding completes the
circuit and allows current to flow
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INSTRUMENT TRANSFORMERS
Ipri x Npri = Isec x Nsec
Ipri = Current flowing in primary winding
Isec = Current generated in secondary windingNpri = Number of turns in primary winding
Nsec = Number of turns in secondary winding
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INSTRUMENT TRANSFORMERS
What is the secondary current ?
CT ratio: 5000:5A; 1000:1
Iprimary = 1000A
Isec = 1000A / (ratio) = 1000A /1000
= 1A
In reality, there will slightly less than 1A flowing.
The difference is the known as the Ratio Error in %
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INSTRUMENT TRANSFORMERS
Never open the secondary of a CT when
pr imary cu rrent is f low ing because
dangerous high voltages will appear.
IEEE C57.13 allow open c ircu it crest
vo l tages up to 3500 V peak w ithou t
vo l tage lim it ing devices.
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INSTRUMENT TRANSFORMERS
CT Types
Torroidal Wound Primary
Bar Type Split Core
Tororoidal
Low flux leakage Typical of bushing and
window CTs Primary conductor passthrough center
600V insulation can beused in MV application
with insulated primaryconductors
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INSTRUMENT TRANSFORMERS
CT Types
Torroidal (donut) Wound Primary
Bar Type Split Core
Wound Primary
Has an internal primarywinding
Wound more than 1
full turn on the core Core can be torroidal Must be inserted in series
with the conductors
Bar provides primaryconnection points, oftenmistaken for bar type
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INSTRUMENT TRANSFORMERS
CT Types
Torroidal (donut) Wound Primary
Bar Type Split Core
Bar Type
Has a continuous bar forthe primary winding
Only 1 turn primary Core can be torroidal Must be inserted in series
with the conductors
Primary insulation part ofCT construction
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INSTRUMENT TRANSFORMERS
CT Types
Torroidal (donut) Wound Primary
Bar Type Split Core
Split Core
Core is separated intotwo parts
For use where primaryconductor cannot be
easily removed
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INSTRUMENT TRANSFORMERS
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INSTRUMENT TRANSFORMERS
CT Ratio
Ratio or primary current to secondarycurrent
Expressed as current quantities 1000 : 5 A
Expressed as ratio numbers 200 : 1
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INSTRUMENT TRANSFORMERS
CT Standard Burden @ 5A, @ 60 Hz
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INSTRUMENT TRANSFORMERS
Basis for Accuracy class for Metering
CTs and VTs correction factor is within specified limitsfor specified load conditions
.6 lagging to unity PF
CT operating between 10% to 100% rated current
VT operating between 90% to 100% rated voltage
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INSTRUMENT TRANSFORMERS
IT Standard Accuracy Class for Metering @ 60 Hz
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INSTRUMENT TRANSFORMERS
CT Accuracy Rating for Metering
Accuracy for a specified burdenExample: 0.3 B 1.8
.3% accuracy for all other burden up to 1.8
Example: 0.3 B @ 0.2
.3% accuracy for burden of .2 and 3
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INSTRUMENT TRANSFORMERS
CT Accuracy Rating for Relaying @ 60 Hz
Accuracy classification apply to full
winding unless specified
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INSTRUMENT TRANSFORMERS
Relaying CT Class
C (Calculated) T
X
Class C
Low leakage flux CTs Torroidal type
3% error @ 5A 10% error @ 100A
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INSTRUMENT TRANSFORMERS
Relaying CT Class
C T (Tested)
X
Class T
High leakage flux CTs Primary Wound type
% error obtained fromtesting
Tested to 100A or > until10% error is establish at
specified burdens Test curve plotted for IprVs Isec, the overcurrent
curve
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INSTRUMENT TRANSFORMERS
Relaying CT Class
C T
X (Defined)
Class X
User defined along withexpected accuracy rating
for protection application Minimum knee point
voltage; Vk
Maximum excitation
current Ik at Vk Maximum secondarywinding resistance at
75C, Rs
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INSTRUMENT TRANSFORMERS
Secondary Terminal Voltage
Saturation voltage level at20X rated current
100 A
Vsat = 100 A x Burden
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INSTRUMENT TRANSFORMERS
Saturation Curve
B-H CurveExcitation Current Vs
Excitation voltage
Knee Point voltage, Vk
10% Ve 50% Ie
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INSTRUMENT TRANSFORMERS
Transformer Formula
Erms = 2 f N a Bpeak / 2 4.44 f N a B
Where:
f = the supply frequency
N = number of turns
a = cross sectional area of the coreB = peak magnetic flux density
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INSTRUMENT TRANSFORMERS
Class C Saturation Curve
Plotted on log-log scale Induced voltage Y-axis
Excitation current X-axis Voltage begins at 1% Voltage ends at 5X Irated Vk identified where curve
is at 45 tangent axis
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INSTRUMENT TRANSFORMERS
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INSTRUMENT TRANSFORMERS
Class T O/C Ratio Curve
Plotted of linear scale Secondary current Y-axis
Primary current X-axis 1 to 22 x rated I-primary
for all standard burdens
Curve identifies the
burden which causes50% ratio correction
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INSTRUMENT TRANSFORMERS
5 Groups of VT
All capable of operating at 110% for emergencycondition
Emergency for 1 minute Some have special emergency rating conditions All must operate within thermal limits and or specified
temperature rise
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INSTRUMENT TRANSFORMERS
Thermal Burden Rating
The maximum volt-ampere (VA) it can carry withintemperature design limits
Each winding can have its own rating One specified rating is for combined distribution
between the 2ndaries, including taps
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
Safety Consideration
Many testing involves using high voltages Should be done by trained qualified personnel
Safe work practice and procedures required
Review Module 2 IEEE Standard 510 1983,
Recommended Practice for Safety in HighVoltage & High Power Testing
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Testing
Primary InsulationResistance
Dielectric Withstand Secondary Insulation
Resistance
Secondary Burden
Saturation Ratio and Polarity
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Testing
Primary InsulationResistance
Dielectric Withstand Secondary Insulation
Resistance
Secondary Burden
Saturation Ratio and Polarity
Primary Ins. Resistance Test
For Bar and Primary woundCTs
DC voltage test at the ratedcrest voltage on the primary
winding
Secondary grounded
1 minute test
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERSCT Insulation Resistance: Bar Type and Primary Wound
DC Voltage test
Isolate Primary
Isolate & Gnd Secondary Connect HV lead to H1/H2 Connect LV to Gnd /
2ndary
Test for 1 minute Record test value
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Testing
Primary InsulationResistance
Dielectric Withstand Secondary Insulation
Resistance
Secondary Burden
Saturation Ratio and Polarity
Dielectric Withstand Test
For Bar and Primary woundCTs
AC voltage test at the 75%factory test voltage
Secondary grounded 1 minute test
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Dielectric Withstand: Bar Type and Primary Wound
AC Voltage test
Isolate & Gnd Secondary
Connect HV lead to H1/H2 Connect LV to Gnd /
2ndary
Test for 1 minute
Record test value
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TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Secondary Circuit Insulation Test
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Testing
Primary InsulationResistance
Dielectric Withstand Secondary InsulationResistance
Secondary Burden
Saturation Ratio and Polarity
2ndary Burden Test
Conducted in 2 parts Part 1, 1A current injection
Measure circuit burden Part 2, four-wire resistancemeasurement
Measure secondary
winding resistance Burden = part 1 + part 2
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
Part 1: CT Secondary Circuit Burden Measurement
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TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
Total burden calculation
ZT = RCT + (RL + ZB)
WhereZT = Total burden ()
RCT = DC Winding resistance ()RL = Lead resistance ()ZB = Device impedance ()
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Testing
Primary InsulationResistance
Dielectric Withstand Secondary InsulationResistance
Secondary Burden Saturation Ratio and Polarity
Saturation Test
Secondary winding excitationtest
Primary open circuited Step voltage increase,current proportional
Test for knee-point voltage Rapid rise is I-excitation 10% V = 50% I
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Saturation test
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Testing
Primary InsulationResistance
Dielectric Withstand Secondary InsulationResistance
Secondary Burden Saturation Ratio and Polarity
Ratio and Polarity test
Secondary winding excited at1V per turn
Primary open circuited H1-H2 compared to X1-X2 Phase measured, ratio
calculated
Phase difference 0 meanspolarity correct
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Ratio and Polarity Test
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Testing
Primary InsulationResistance
Dielectric Withstand Secondary InsulationResistance
Secondary Burden Saturation Ratio and Polarity
Ratio and Polarity test (ALT)
Use individual meters, phaseangle and voltmeter
Secondary winding excited at1V per turn
Primary open circuited H1-H2 compared to X1-X2 Phase measured, ratio
calculated
Phase difference 0 meanspolarity correct
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
CT Ratio and Polarity Test (alternate connections)
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TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
VT Testing
Insulation Resistance Dielectric Withstand
Ratio and Polarity
Insulation Resistance Test
DC voltage test at the ratedcrest voltage
3 tests HV to Gnd HV to LV LV to Gnd
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TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
VT Testing
Insulation Resistance Dielectric Withstand
Ratio and Polarity
Dielectric Withstand Test
AC voltage test at the 75%factory test voltage
Secondary grounded 1 minute test
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
VT Dielectric Withstand Tests
TESTING INSTRUMENT
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TESTING INSTRUMENT
TRANSFORMERS
VT Testing
Insulation Resistance Dielectric Withstand
Ratio and Polarity
Ratio and Polarity test
Primary winding excited at1V per turns ratio
Secondary open circuited X1-X2 compared to H1-H2 Phase measured, ratio
calculated
Phase difference 0 meanspolarity correct
Compare ratio withnameplate
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NETA STANDARD
5. NETA Acceptance Testing Procedure
5.1 Visual and Mechanical Inspection
5.2 Electrical Test 5.2.1 Current Transformer
5.2.2 Voltage Transformer
5.3 Test Value Analysis
5.3.1 Visual and Mechanical
5.3.2 CT Electrical Test
5.3.3 VT Electrical Test
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TEST SET MANUAL
Module 3: CT Tester
Module 2:
15 KV Insulation Resistance Test set AC High Potential Test set
DC High Potential Test set
Digital Low Resistance Test set
S O S
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TEST FORMS
Current Transformer Test Form
Voltage Transformer Test Form
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