Current Transformer:
The large alternating current can not be passed through the normal ammeter and current coils of wattmeters can easily be measured by the use of current transformers along with normal low range instruments.
The current transformer has primary and secondary windings. The primary windings consist of one or more turns with a larger cross-sectional area. The secondary windings consist of a larger number of turns of fine wire having a small cross-sectional area. The secondary windings of the current transformer are rated at 5A.
The current transformer primary winding is connected in series with the line carrying a high current. The current transformer is a step-up transformer i.e stepping up the voltage from primary to secondary and stepping down the current from primary to secondary. The secondary of current transformer is should not be open.
Potential Transformer:
The high alternating can be reduced in a fixed proportion for the measurement purpose with help of a potential transformer. In a potential transformer, primary windings consist larger number of turns and secondary windings consist less number of turns. The secondary windings are rated at 110V.
Comparison Table between the Current Transformer and Potential transformer:
| Current Transformer | Potential Transformer |
|---|---|
| The current transformer is treated as a series transformer under virtual short circuit conditions. | The potential transformer is treated as a parallel transformer under open-circuit secondary. |
| The secondary of the current transformer is should be short-circuited. | The secondary of the potential transformer is nearly open-circuited. |
| A small voltage exists across its terminals as connected in series with the transmission whose current can be measured. | The full line voltage across the potential transformer terminals. |
| The winding of the current transformer carries full line current. | The windings of the potential transformer carry full line voltage. |
| The primary current of the current transformer is independent of the secondary circuit conditions. | The primary current of the potential transformer is dependent on the secondary circuit conditions. |
| The primary current and excitation current varies over a wide range in the current transformer. | The primary Current is almost constant so excitation current and flux density vary over a limited range in the potential transformer. |
| In the current transformer, one bushing is needed as the two ends of primary windings are brought out through the same insulator. Hence there is a saving in the cost in the case of the current transformer. | Two bushings are needed when neither side of the line is at ground potential. Hence there is more cost in the case of a Potential transformer. |
| The current transformer converts higher-level current to lower level current. | The potential transformer transforms the higher voltage to lower voltage. |
| The core of the current transformer is built up with laminations of silicon steel. | The core of the potential transformer is built up with high-quality steel operating at low flux densities. |
| The primary windings of the current transformer carry a current which can be measured. | The secondary windings of the potential transformer carry voltage which can be measured. |
| The current transformer is connected in series with the line or instrument. | The potential transformer is connected in parallel with the line or equipment. |
| In the current transformer, primary windings consist of a small number of turns. | In a potential transformer, primary winding consists a larger number of turns. |
| In the current transformer, Secondary winding consists of a larger number of turns and can not be open-circuited. | In a potential transformer, Secondary winding consists of a smaller number of turns and one end of the secondary winding is earthed at ground potential. |
| The current transformer is a step-up transformer. | The potential transformer is step down transformer. |
| The secondary winding of the current transformer is rated at 5A or 1A. | The secondary winding of the potential transformer is rated at 110V. |
| The transformation ratio of the current transformer is high. | The transformation ratio of the potential transformer is low. |
| The current transformer does not depend on the secondary burden. | The potential transformer depends on the secondary burden. |
| The input of the current transformer is constant current. | The input of the potential transformer is constant voltage. |
| The primary winding of the current transformer consists of a full line current. | The primary winding of the potential transformer consists of full line voltage. |
| There are two types of current transformer i.e wound type and closed type. | There are two types of potential transformers i.e electromagnetic and capacitive voltage transformers. |
| The impedance of the current transformer is low. | The impedance of the potential transformer is high. |
| The current transformer is used for measurement of current, operating the relays. | The potential transformer is used for the measurement of voltage and operating the relays. |
Comparison of current transformer and potential transformer:
1. The current transformer is treated as a series transformer under virtual short circuit conditions. The potential transformer is treated as a parallel transformer under open-circuit secondary.
2. The secondary of the current transformer is should be short-circuited. The secondary of the potential transformer is nearly open-circuited.
3. A small voltage exists across its terminals as connected in series with the transmission whose current can be measured. The full line voltage across the potential transformer terminals.
4. The winding of the current transformer carries full line current. The windings of the potential transformer carry full line voltage.
5. The primary current of the current transformer is independent of the secondary circuit conditions. The primary current of the potential transformer is dependent on the secondary circuit conditions.
6. The primary current and excitation current varies over a wide range in the current transformer. The primary Current is almost constant so excitation current and flux density vary over a limited range in the potential transformer.
7. In the current transformer, one bushing is needed as the two ends of primary windings are brought out through the same insulator. Hence there is a saving in the cost in the case of the current transformer. Two bushings are needed when neither side of the line is at ground potential. Hence there is more cost in the case of a Potential transformer.
8. The current transformer converts higher-level current to lower level current. The potential transformer transforms the higher voltage to lower voltage.
9. The core of the current transformer is built up with laminations of silicon steel. The core of the potential transformer is built up with high-quality steel operating at low flux densities.
10. The primary windings of the current transformer carries a current which can be measured. The secondary windings of the potential transformer carry voltage which can be measured.
11. The current transformer is connected in series with the line or instrument. The potential transformer is connected in parallel with the line or equipment.
12. In the current transformer, primary windings consist of a small number of turns. In a potential transformer, primary winding consists of a larger number of turns.
13. In the current transformer, Secondary winding consists of a larger number of turns and can not be open-circuited. In a potential transformer, Secondary winding consists of a smaller number of turns and one end of the secondary winding is earthed at ground potential.
14. The current transformer is a step-up transformer. The potential transformer is step down transformer.
15. The secondary winding of the current transformer is rated at 5A or 1A. The secondary winding of the potential transformer is rated at 110V.
16. The transformation ratio of the current transformer is high. The transformation ratio of the potential transformer is low.
17. The current transformer does not depend on the secondary burden. The potential transformer depends on the secondary burden.
18. The input of the current transformer is constant current. The input of the potential transformer is constant voltage.
19. The primary winding of the current transformer consists of a full line current. The primary winding of the potential transformer consists of full line voltage.
20. There are two types of current transformer i.e wound type and closed type. There are two types of potential transformers i.e electromagnetic and capacitive voltage transformers.
21. The impedance of the current transformer is low. The impedance of the potential transformer is high.
22. The Current transformer is used for measurement of current, operating the relays. The potential transformer is used for the measurement of voltage and operating the relays.