Current Transformer (CT) and Potential Transformer (PT) are instrument transformers basically used for measurement and protection purpose in power systems. A Current Transformer (CT) is used to obtain reduced current signals for purpose of measurement, control and protection. They reduce the higher current to lower values which are suitable for operation of Relays and other instruments connected to their secondary winding.
While a Potential Transformer (PT) used to reduce the voltage to lower values and to provide isolation between high voltage power network and the relays and the other instruments that are connected to their secondary.
Major differences between the Current Transformer (CT) and Potential Transformer (PT) are listed below.
Top Core Type CT is shown in figure below.
- The secondary of a C.T cannot be open circuited on any circumstance when it is under service.
- A CT may be considered as a series transformer.
- The primary current in a C.T is independent of the secondary circuit conditions (burden).
- The primary winding of the CT is connected in series with the line carrying the current to be measured. Hence it carries of the full line current.
- With the help of CT, a 5A ammeter can be used measure a high current like 2000A by using 2000/1 CT.
A Current Transformer (CT) is represented as shown below.
- The secondary of a P.T can be open circuited without any damage being caused either to the operator or the transformer.
- P.T may be considered as a parallel transformer.
- The primary current of a P.T depends upon the secondary circuit conditions (burden).
- The primary winding P.T is connected across the line of voltage to be measured. Hence the full line voltage is impressed across its terminal.