CT (current transformer) current setting is an important aspect of electrical installations and power distribution systems. CTs are used to measure the current flowing in electric power systems, which is then converted into a signal that can be used for measurement and control purposes. However, the accuracy of the CT current setting is crucial for reliable performance and safety
(a). The rated primary current should be selected from standard values. The value should be so chosen that it is suitable for all normal currents and permissible overload currents in the primary circuit.
Current transformer operating a high-set instantaneous over-current relay may be selected for higher rated primary current; thereby the required accuracy limit may be reduced.
Referring values of rated primary currents: 0.5 – 1 – 2.5 – 5 – 10 - 12.5 – 15 – 20 – 25 -30 – 40 – 50 – 60 – 75 – 100 – 125 – 150 – 200 – 250 – 300 - 400 – 500 – 600 – 750 –800 – 1000 – 1250 – 3000 – 4000 – 5000 – 6000 – 7500 - 10000 Amperes.
Reference values of rated secondary currents: 1-2-5 Amperes. Reference values of rated output: 2.5-5-7.5-10-15-30 VA and up.
(b) Rated short time current and its duration example:
750 A for 0.5 sec., 525 for 1 sec., 300 for A for 3 sec.
Current Transformer Classification of Tests: -
Type Tests:
The following shall constitute the type tests:
a) Short-time current tests
b) Temperature-rise test.
c) Lightning impulse test for current transformers for service in electrically
exposed installation
d) Switching impulse voltage tests for current transformers of 420 kV and above
e) High voltage power-frequency wet withstands voltage tests on outdoor current
transformers up to and including 245 Kv.
f) Determination of errors or other characteristics according to the requirements of
the appropriate designation or accuracy class.
Routine Tests:
The following shall constitute the routine tests:
a) Verification of terminal marking and polarity
b) Power frequency dry withstand tests on primary windings.
c) Power frequency dry withstand tests on secondary windings
d) Over-voltage inter-turn test
e) Partial discharge tests in accordance with
f) Determination of errors or other characteristics according to the requirements of the appropriate designation or accuracy class.
Optional Tests:
The following tests where applicable, shall be carried out by mutual agreement between the purchaser and the manufacturer:
a) Chopped lightning impulse test as a type test
b) Measurement of dielectric dissipation factor for oil-immersed current
transformers of 72.5 kV and above
c) Commissioning tests on new current transformers up to and including 36kV.