what is Current transformer ?
The design and use of this transformer is quite different from that of well-known power transformers. In current transformers, primary current is not controlled by condition of secondary circuit. Protective relays in AC power system are connected in the secondary circuit of the transformers.
it is a step up transformer which work on principle of mutule induction . it gives output 5A current .
Current transformer is must be classified in two groups.
1. Protective current transformer.
It is used in association with relays, trip coils, pilot wires etc.
2. Measuring current transformers
This is used in conjunction with watt meter, ammeter etc.
The major criterion of selection of the current transformer is the ratio at maximum load current through primary and secondary. In other words, the current transformer secondary current at maximum load should not exceed the continuous current rating of the applied relay. This is particularly applicable to phase type ralys where load current flows through the relays.
Terms and definition to related CT
1. Instrument transformer
The transformer which are used in conjunction with measuring instruments, protective relays and control circuit. It is include measuring and protective current transformer and voltage transformers.
2. Current transformer
Instrument transformer used in conjunction with ammeters, over current relays,etc. current transformer step down the current from high to low current. It is also increase the voltage of the system which means current transformer is a step up transformer and it is step voltage from low voltage to high voltage. So simply we can say this step up transformer.
3. Rated primary current
The value of primary current on which the performance of the current transformer is specified by the manufacturing
4. Rated short time current (primary).
It is defined as R.M.S value of A.C component of current which the CT can carry for rated time without damage due to thermal or electrodynamics stresses.
The value of secondary current market on the rating plate
6. Rated transformation ratio
The ratio of the rated primary current to rated secondary current
7. Actual transformation ratio
The ratio of the actual primary current to the actual secondary current
Other related words of current transformer
8. Existing current
The RMS value of current taken by the secondary winding of a CT when sinusoidal voltage of rated frequency is applied to secondary, with primary winding open circuited
9. Rated saturation factor
The ratio of rated primary saturation, current to rated primary current
10. Overcurrent factor
The ratio of rated short-time current to rated primary current.
11. Rated burden
The burden assigned by manufacture ar which the CT performs with specified accuracy.
12. Rated primary saturation current
The maximum value of primary current at which the required accuracy current
13. Phase angle error
The phase angle between primary current vector and the reversed secondary current vector.
Testing of current transformer
Type and name different testing in CT
• Verification of terminal markings and polarities
• Short time current test
• Temperature rise test
• Power frequency voltage with stand test on primary
• Error management test
• Turns ratio test
Application of current transformer
- It is used for protection for over current in different circui
- It is used in HT panel with relays circuits
- CT is also used for calibration of ammeter
- It is also use for metering purpose
- It reduce range of current so ammeter can easily measure of current for high range
- This is also used in control circuit
- Basically it gives 5A current which can be used as power supply in different panels