CURRENT TRANSFORMER
What is Instrument Transformer?
A transformer intended to supply measuring instruments, Meters, Relays and other
similar apparatus.
What is Current Transformer?
It is a type of Instrument Transformer in which the secondary current is proportional to
Primary current with almost zero phase angle difference.
What is Primary winding?
Through which current to be transformed flows.
What is Secondary winding?
Through which transformed current flows to relays, meters, etc.
What is Secondary circuit?
The external circuit connected to CT secondary winding.
What is rated primary current?
Primary current on which CT performance based.
What is rated current?
Secondary current on which CT performance based.
What is actual transformation ratio?
The ratio of actual primary current to actual secondary current.
What is rated transformation ratio?
The ratio of rated primary current to rated secondary current.
What is ratio error?
Ratio of Actual transformation ratio to rated transformation ratio.
What is phase displacement error?
The phase angle difference between primary and secondary currents.
What is composite error?
The error caused by both ratio and phase displacement.
What is accuracy class?
Errors of which remain within specified limits under prescribed conditions of use.
What is burden?
The impedance of the secondary circuit in ohms and power factor.
What is exciting current or magnetizing current?
The r.m.s. value of current taken by the secondary when ac voltage applied ( by keeping
the other winding open)
What is secondary winding resistance?
Secondary winding DC resistance in ohms corrected to 75 o C.
Resistance at 75o C = Rt ( 1 + 0.00393 ( 75-t ) )
MEASURING CURRENT TRANSFORMER
What is measuring current transformer?
CT intended to supply indicating, integrating and recording apparatus.
What is rated instrument limit primary current ( IPL) ?
Minimum primary current at which the composite error is greater than 10%.
What is instrument security factor (FS)?
The ratio of rated IPL to rated Primary current.
What is secondary limiting e.m.f. (knee point)?
The product of FS, rated secondary current, vectorical sum of rated burden and R ct.
Note: knee point always less than this calculated value for measuring current transformer.
PROTECTIVE CURRENT TRANSFORMER
What is Protective current transformer?
CT intended to supply protective relays.
What is rated accuracy limit primary current ?
Primary current up to which the CT will comply composite error.
What is accuracy limit factor?
The ratio of rated accuracy limit primary current to rated Primary current.
What is secondary limiting e.m.f.(knee point)?
The product of accuracy limit factor, rated secondary current, vectorical sum of rated
burden and Rct.
Note: knee point always higher than this calculated value for protective current transformer.
What is Class PR protective current transformer?
CT with limited remanence for which Secondary loop time constant and Winding
resistance to be specified.
What is Class PX protective current transformer?
CT of low leakage reactance and performance shall be defined by Excitation
characteristic, Secondary winding resistance, Burden and ratio.
What is excitation characteristic ( magnetsing curve) ?
Graphical or tabular presentation of relationship between exciting current and voltage
applied, values in r.m.s.
What is rated knee point?
In the excitation characteristic, where 10% rise in voltage causes 50% rise in exciting
current at rated frequency.
What is dimensioning factor?
A factor to indicate multiples of rated secondary currents under fault conditions including
safety factors.
How the measuring current transformer class is defined?
CT accuracy followed by Instrument security factor.
For example, 0.2FS10, where 0.2 is the accuracy and 10 is the Instrument Security factor.
0.2sFS10 for special purpose measuring current transformer.
What is the difference between measuring current transformer and special purpose
measuring current transformer?
Normally accuracy is defined at rated current but in special purpose CTs accuracy is
defined from 20% of rated current.
What is ratio error limit for measuring current transformer?
Limits of Current error and phase displacement for measuring current transformer
(classes 0.1 to 1.0)
Accuracy ±% ratio error at diff ± phase displacement at diff rated current
Class rated current Minutes Centiradians
5 20 100 120 5 20 100 120 5 20 100 120
0.1 0.4 0.2 0.1 0.1 15 8 5 5 0.45 0.24 0.15 0.15
0.2 0.75 0.35 0.2 0.2 30 15 10 10 0.9 0.45 0.3 0.3
0.5 1.5 0.75 0.5 0.5 90 45 30 30 2.7 1.35 0.9 0.9
1.0 3.0 1.5 1.0 1.0 180 90 60 60 5.4 2.7 1.8 1.8
Limits of Current error and phase displacement for special purpose measuring current
transformer
Accuracy ±% ratio error at diff ± phase displacement at diff rated current
Class rated current Minutes Centiradians
5 20 100 120 5 20 100 120 5 20 100 120
0.2 0.35 0.2 0.2 0.2 15 10 10 10 0.45 0.3 0.3 0.3
0.5 0.75 0.5 0.5 0.5 45 30 30 30 1.35 0.9 0.9 0.9
How the protective current transformer class is defined?
Composite error followed by accuracy limit factor (ALF).
For example, 5P20 where 5 is composite error and 20 is ALF.
What is ratio error limit for protective current transformer?
Accuracy ±% current ratio ± phase displacement at rated % composite
Class error at rated current error at rated
current Minutes centiradians accuracy limit
primary current
5 1 60 1.8 5
10 3 --- --- 10
What is ratio error limit for PX class current transformer?
0.25%
How the knee point voltage is determined in PX class CT?
Ek = Kx . (Rct + Rb) . Isn
Where, Ek is rated knee point,
Kx is rated dimensioning factor,
Rct is secondary winding resistance at 75 deg.
Rb is rated resistive burden
Isn is rated secondary current.
Why the measuring CT knee point voltage is always less than Protective CT?
Under fault condition, measuring CT has to saturate to protect the measuring instruments
from damage. But, protective CT should not saturate because it has to operate the protective
relay to clear the fault.
Why class PX CT is always used for main protection instead of Protective CT?
In PX class CT, Excitation characteristic, winding resistance, resistive burden are well
defined. But, in Protective CT these are not exactly defined (only guidelines).
What is sequence of testing for current transformer?
1. Insulation resistance or IR value test.
2. Polarity test.
3. Winding resistance test.
4. Excitation characteristic or Magnetising current test.
5. Ratio test.
Why IR value test is always first?
Before proceeding to confirm the characteristics of CT, Insulation healthiness to be
ensured for transportation damage.
Why polarity test is always before winding resistance?
To measure the winding resistance, CT will be saturated by injecting dc current. Once CT
is saturated secondary will not reflect or follow primary. Hence, it is not possible to do the polarity
test after winding resistance test.
Why winding resistance test is always before excitation characteristic?
CT will be saturated during winding resistance test. The saturated CT will be normalized
or Desaturated during excitation characteristic test.
How will you Desaturate the CT?
By slowly applying r.m.s. voltage on the CT secondary terminals by keeping the primary
circuit open until the secondary current reaches the rated secondary current (1A).
Note: Excitation characteristic test shall be carried out only after desaturating the CT.
Why ratio test is always last?
Saturated CT during winding resistance test will only be Desaturated / normalized during
excitation characteristic test. After that only CT secondary will follow the primary to carryout the
ratio test.
What is the formula for converting resistance value from ambient temp to 75 deg.?
R75 = Rt (1+ 0.00393(75-t)), where t is ambient temperature.
What will you confirm during DC scheme check?
1. Non mixing of DC supplies between circuits.
2. Supervision relay at the end the circuit. In other words, all circuits are supervised
properly.
3. Polarity and non Grounding of circuits.
What will you confirm during TCS and TSS scheme check?
1. Trip Supply Supervision shall be at the beginning of the circuits and TCS shall be at
the end the circuit to differentiate whether it is supply failure or circuit failure.
2. Ensuring the TCS under both Pre close and Post close conditions.
3. Polarity and non Grounding of circuits.
What will you confirm during Primary Injection?
1. Ratio and Polarity.
2. Correctness of circuit as per drawing.
