1. CHAPTER 6
SPECIAL TRANSFORMERS
Electrical Machines

2. Current Transformer (CT)
The primary winding of a CT is connected in series with the line whose current needs to be measured. The primary winding has less number of turns and so the voltage drop across it is also less. The secondary winding, on the other hand, has large number of turns.
Electrical Machines

3. Phasor Diagram of CT
Electrical Machines

4. Transformation Ratio of CT
Electrical Machines

5. Phase angle of CT
Electrical Machines

6. Potential Transformer (PT)
In a potential transformer (PT), the primary is connected across the line whose voltage needs to be measured and the voltage circuit is connected across the secondary winding. Potential transformer uses large core and conductor sizes in comparison to conventional power transformer. Shell-type configuration is used for low-voltage and core-type configuration is used for high-voltage transformers.
Electrical Machines

7. Phasor Diagram of PT
Electrical Machines

8. Transformation Ratio of PT
Electrical Machines

9. Phase angle of PT
Electrical Machines

10. Capacitive Potential Transformer
Capacitor voltage transformers are used for metering and protection in high-voltage network systems.
A capacitive voltage transformer works on the principle of voltage division, by using two capacitances, one many times larger than the other.
Electrical Machines

11. Earthing Transformer
The main purpose of an earthing transformer is to pass the faulty current during earth fault condition. It provides a neutral point for grounding purpose and carries no useful load. Low-impedance ground path can be obtained with the help of three pairs of concentric coils. There coils are connected in the manner so that they oppose the mmf or ampere turns.
Electrical Machines

12. Pulse Transformer
A pulse transformer is a special transformer that is designed for producing rectangular electrical pulses. Pulses are mainly used in electronic circuits. Special high-voltage pulse transformers are also used to generate high-power pulses for radar, particle accelerators, or other high-energy pulsed power applications. A pulse transformer generally has low values of leakage inductance and distributed capacitance, and a high open-circuit inductance.
Electrical Machines

13. Furnace Transformer
Furnace transformers are used to step down from voltages between 11 kV and 33 kV to levels of several hundred volts only. This results in massive secondary currents.
Due to the high secondary currents and resistive losses, the furnace layout is such as to limit the bus-bar length.
Electrical Machines

14. Rectifier Transformer
Rectifier transformers are used to supply rectifier bridges for DC applications such as DC arc furnaces. These transformers step down the distribution voltages to the level used by between 500 V and 3300 V depending on the configuration of the power electronics.
Electrical Machines