A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.

1. CURRENT TRANSFORMER

2. INTRODUCTION
• Current Transformers (CT’s) are instrument transformers
that are used to supply a reduced value of current to
meters, protective relays, and other instruments.
• CTs provide isolation from the high voltage primary,
permit grounding of the secondary for safety, and step-
down the magnitude of the measured current to a value
that can be safely handled by the instruments.
• CT ratios are expressed as a ratio of the rated primary
current to the rated secondary current.
• For example, a 300:5 CT will produce 5 amps of
secondary current when 300 amps flows through the
primary. As the primary current changes the secondary
current will vary accordingly.

3. APPLICATION

4. Classification of CT

5. DEAD TANK CT with CUT- SECTION VIEW LIVE TANK CT with CUT- SECTION VIEW

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8. NAMEPLATE DETAILS

9.  Current ratio error:- It is the ratio of difference of
transformation ratio to the actual ratio in CT
 Phase angle error:- The angular displacement
between Secondary current and primary current
 Composite error :- The current in the circuit contains
harmonic waves, for which the error due to different
wave forms becomes composite.
 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.
Meaning 0f 5P10:- indicates that the 5% composite error
in CT is limited up to 10 times the primary current flown
in the CT.
Terms and Definitions used in CT

10. ISF (Instrument Safety Factor) :- limit of allowable
primary current to flow in it up to which the instrument
can work safely.
Minimum Knee Point Voltage (Vk)
It is the point over and above which, increase of 10%
in excitation voltage causes increment of exciting
current of 50%
Maximum Excitation Current (Imag):- The current
required to identify the knee point voltage
Rated Burden: -The loads connected on the CT
secondary circuit are called as Burden.
Terms and Definitions used in CT

11. Maintenance of CT
• Clean the porcelain for dust etc. and check for any chipping.
• Check the Tightness of clamps for jumpers and tightness of nut bolts of supporting
structure and terminal bushings.
• Check the foundation for any crack etc.
• In case of hermetically sealed C.T./P.T. there is no necessity for extraction of sample
of oil for analysis or for re-conditioning of oil. However oil level has to be checked
periodically. Any abnormal change in the oil level indicates leak and should be
investigated and attended immediately check the diaphragm of pressure relief
vent for bursting.
• In case the transformer is not hermetically sealed then check the dielectric
strength of oil and record the result. It dielectric strength is poor then
recondition/replace the oil.
• Check the secondary terminal connections of the transformer and connection in
the junction box for tightness. When the current is flowing in the primary,
secondary circuit of C.T. should never be open as this may cause overheating of
core and breakdown of the insulation due to the high voltage developed across
the secondary terminals.
• Check equipment earthing and see that both the earthings are tight and intact.
• Measure IR value between primary and secondary primary and Earth
• Check earthing of earthed end of primary winding of transformers.

12. • Current Transformer is the sensor
for electrical system and play the most
vital role for protection and
measurement principle.
CONCLUSION

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