1. Instrument Transformer
Magnetic circuit (core)
Primary
Windings
Secondary
Windings U1 U2
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2. Instrument Transformer
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3. Definition of an Instrument Transformer
Voltage Transformer :
Primary U1
K : Transformation ratio
Secondary U2
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4. Definition of an Instrument Transformer
Current Transformer : I1
Primary
K : Transformation ratio
Ampere-Turns
Conservation
Law
Secondary I2
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5. Instrument Transformer Error
Secondary
Primary Up Us K
Voltage Transformer :
Up (theoretically = k.US)
e:Ratio error
f: Phase error
K.Us
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6. Instrument Transformer Error
Secondary
Primary Ip Is K
Current transformer : Ip
e:Ratio error
K.Is
f: Phase error
TC : K= Ip Is
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7. Internal details Eye bolt design 1. Oil filling plug 2. Dome
3. Nitrogen filling valve
4. Collar
5. Primary terminal
6. Porcelain insulator
7. Insulated primary
8. Cover plate for tank
9. Tank
10. Secondary cores
Eye bolt design
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8. Manufacturing Process
Primary steel pipe
Paper insulation
Seconday cores
IT range
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9. Active Part Manufacturing
IT range
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10. Hair Pin Design Hair-Pin design 1. Dome 2. Nitrogen filling valve
3. Primary terminal
4. Collar
5. Porcelain insulator
6. Primary conductor with insulation
7. Adaptor cylinder
8. Secondary cores
9. Base
10. Oil drain plug
IT 400 Cross section
Hair-Pin design
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11. Current Transformers Protection accuracy classes
CEI
Accuracy classes (Protection)
Maximum current error in % of IP
Accuracy limit primary current = fL.IN
Burden :SN
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12. Factors for Protection
Over Current and Earth Fault Protection
Parameters
1. ALF ( accuracy limiting factor)
2. Composite error
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13. Factors for Protection
1. Accuracy Limiting Factor
What is Accuracy Limiting factor ?
It is the factor of over current above the rated current which
determines the capability of CT to maintain the error at such
a condition.
2. Composite error
It is the error of the CT when this over current is applied.
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14. Current Transformers Protection accuracy classes
CEI
Composite error :
Under steady-state conditions, the r.m.s. value of the difference between:
a) the instantaneous values of the primary current, and
b) the instantaneous values of the actual secondary current multiplied by the rated transformation ratio
Kn is the rated transformation ratio;
Ip is the r.m.s. value of the primary current;
ip is the instantaneous value or the primary current;
is is the instantaneous value of the secondary current;
T is the duration of one cycle.
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15. Factors for Protection
1. Accuracy Limiting Factor/composite error
For e.g if the class designation is 5P20
20 is the Accuracy limiting factor which signifies that when
20 times the rated primary current is applied the composite
error of 5P( +/- 5%) is maintained.
Typical Class designations are
5P10, 5P20, 10P10, 10P20 etc.,
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16. Factors for Protection
Transformer and Busbar Differential Protection
Parameters
1. Knee Point Voltage requirement(Vk)
2. Exciting current ( Io)
3. Resistance of CT (Rct)
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17. Current Transformers Saturation curve
Induction
B [T]
Protection CT
18500 gauss
Metering CT
8000 gauss
Primary current
Ip/In 10 20 30 40
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18. Factors for Protection
1. Knee Point Voltage(Vk):
Knee point voltage is point beyond which an application of 10% of voltage increases the exciting current by 50%.
The typical equations for Vk ( based on relay used)
Vk > 24 In (Rc t+2RL)- for Transformer Differential
Where In: Relay rated current
RL = Total lead Resistance
Rc t = CT secondary resistance
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19. Current Transformers Type Tests
a) short-time current tests
b) temperature rise test
c) lightning impulse test
d) switching impulse test
e) wet test for outdoor type transformers
f) determination of errors
IEC
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20. Current Transformer Routine tests
CEI
CEI 60-1
Routine tests
The following tests apply to each individual transformers:
a) verification of terminal markings
b) power-frequency withstand test on primary winding
c) partial discharge measurement
d) power-frequency withstand test on secondary windings
e) power-frequency withstand tests, between sections
f) determination of errors
The order of the tests is not standardized, but determination of errors shall be performed after the other tests.
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21. CT Failure and remedial action
Remedial actions in CT at site to avoid failures
Tangent delta and Capacitance measurement from the C terminal at periodic intervals once in three years or during shutdown.
Dissolved gas analysis of oil taken out from CT alteast once in five years.
Thermo vision scanning of CTs of rating 400kV ( or above).
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22. Partial discharge test
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23. Multiple Chopped Impulse test
As per IEC ( 2002)
Application of 100 chopped impulses of negative polarity
on CTs of ratings above 300kV. These impulses will be
applied at the rate of one impulse per minute. The test
Voltage shall be 60% of the rated lightning impulse voltage
Before the test and three days after the test the dissolved
gas analysis of oil taken from CT will be carried out.Analysis
procedure and fault diagnosis shall be as per IEC
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24. Special test A. Thermal Stability test :
This involves simultaneous application of rated voltage
(1.1Um/Sqrt3) and rated simultaneous current (1.2,1.5 etc)
by using a synthetic test circuit.Capacitance, tangent delta,
secondary resistance and temperature of primary terminal
are recorded until stable values are acquired.
This test demonstrates the insulation capacity ( healthiness)
under energised conditions.
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25. Special test B. Temperature coefficient test:
The CT is heated in a oven to approximately 90Deg C. The tan delta is measured at ambient, 80 and 90 deg C at voltages of 0.3,0.7,1.0 and 1.1Um/Sqrt3.
This test demonstrates the healthiness at high extreme temperature conditions.
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26. Accurate Measurement of HV
Capacitor Voltage Transformers Coupling Capacitors CCV / CC 72.5 to 765 kV
Line protection
HF transmission
Compliance with IEC, ANSI or equivalent standards
Reduce the slope of the RRRV (Rate of Rise of Recovery Voltage (Ik >40 kA)
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27. Capacitor Voltage Transformer Definitions
Element Pack
(or pack)
Pile of elements : ± 10 to 25 kV
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28. Capacitor Voltage Transformer Definitions
HV Power line 
Ground
Capacitor Unit
(or unit)
Assembly of elements in an insulating container: ± 245 kV.
Can be connected to a HV line
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29. Capacitor Voltage Transformer Definitions
HV Power line 
Ground
Capacitor STACK
(or stack)
Assembly of elements to reach higher voltage levels : ± 800 kV
In general, the term
CAPACITOR
stands for a capacitor element
as well as a capacitor stack.
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30. Capacitor Voltage Divider
HV Power line
Ground 
Capacitor Voltage Divider
( or CVT )
Connecting an MV inductive voltage transformer to a tap.
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31. P1 P2 C1 C2
Intermediate
Voltage :
10 to 20 kV/3 S1 S2
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32. Lµ = Inductance equivalent to magnetic losses of the magnetic circuit.
Equivalent Diagram Ce La
Lfs Ra Rs Lμ Rw Zc Us
U’P =
Vp . C1
k . (C1+C2)
Lµ = Inductance equivalent to magnetic losses of the magnetic circuit.
Rw = Resistance equivalent to the watt losses of the magnetic circuit.
Lfs = Secondary leakage inductance of the magnetic VT.
Rs = Resistance of the secondary winding
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33. Capacitor stack
Inductive VT
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34. Secondary terminal box
Oil expansion device
CCV 72.5 to 765 kV
Capacitor elements
Capacitor column
Insulating oil
Insulator flange
Secondary terminal box
Inductance
MV Transformer
Damping circuit
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35. Capacitor Voltage Transformer Capacitor Elements
All Paper Dielectric design
Mixed dielectric design
Paper
PPR film + paper
Copper tabs for
connection
Aluminum foil
folded for contact
with next element
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36. Capacitor Voltage Transformer Typical Error Curve
Typical graph of variation of errors with VA & frequency
Nominal burden OVER-ESTIMATED Real load < 25% VAN
Phase error
Ratio error f
25% VA
f+Df
f -Df
100% VA
+20’
- 20’
+0.5
- 0.5
Excessive Frequency Variations
Nominal burden UNDER-ESTIMATED Real load > 100% VAN
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37. Voltage transformers Protection accuracy classes
IEC
Accuracy classes (Protection)
Maximum error in % of VP
- Voltage between 5 % and fT x VNP
- Burden between 25% and 100% of SN
- Maximum error doubled for VNP=2%
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