2. Unit Transformer
Unit transformer are step up transformer which is connected to generating house & step up voltage from 15kV voltage to 132 voltage level as requirement or line design parameter.
It is just like transformer but connected to unit of the generating house that's why we called it unit transformers.
Transformers used in three-phase systems may consist of a bank of three single-phase transformers or a single three-phase transformer which is wound on a common magnetic core.
In Tongi power plant they use three single phase transformer which make a bank and produce three phase supply.
Fig: Unit Transformer used in Tongi plant

3. Cont. Main transformer specifications: Application standard IEC 60076
Rated parameters
Type DFZ-55000/132W2 TH (single-phase composite type) transformer)
Cooling type ONAN/ONAF (75%/100%)
Frequency Hz
Impedance voltage %
Rated power ×550000kVA
Rated voltage /√3×(1±8×1.25%)/15kV
Wiring Ynd1
Insulation level LI650AC275-LI650AC275/LI105AC45
Rated current / A

4. Instrument Transformer

5. Current Transformer
Current transformers are used for reducing/stepping down a.c. current from higher value to lower value for measurement /protection/control.Typical secondary current of CTs are 5A or 1A r.m.s
There are two classes of current transformer:
Measuring current transformer
Protective current transformer
Fig: Current Transformer used in Tongi plant

6. Cont..
Here is difference between the protection CT and metering CT in operating conditions…
The CT using for protection will have to carry the fault currents which are 10 times the normal full load current.
The CT used for metering will have to carry only full load current

9. Voltage transformer
Voltage transformers are use for measurement and protection. It stepping down the voltage for the purpose of protection, measurement and control.
The primary of voltage transformer is connected directly to power circuit between phase and ground depending upon rated voltage and application.
Typical secondary voltage of VTs are 110V or 100V.
Fig: Voltage transformer used in Tongi plant

13. The trip circuit When a fault occurs in the
protected circuit, the relay
(2) connected to the CT an PT
actuates and closes its contacts
(6). Current flows from the
battery (5) in the trip circuit (4).
As the trip coil of the circuit
breaker(3) is energized, the circuit
breaker operating mechanism is
actuated and it operates for the
opening operation.