CT and PT for Protection purpose:
Current transformer (CT) and Potential transformer (PT) are instrument transformers, which are used in conjunction with protective relays for protection purpose. The CT has fewer number of turns (usually single turn or simply a conductor bar) on primary side and a large number of turns on the secondary side. Due to this difference in turns, a large current on primary side is transformed into current of low safe value on secondary side. The CT is connected with primary in series with power system component carrying large current and secondary winding feeds proportionately reduced current to protective relay. Potential transformer (PT) has large number of turns on primary side and fewer number of turns on the secondary side. Therefore, it steps down the primary voltage to lower safe secondary voltage. The PT is connected with primary connected to high voltage system bus and secondary winding feeds proportionately reduced voltage to protective relay. Protective relay is a device that detects abnormal condition in electrical system by continuously measuring / monitoring the electrical quantities such as current, voltage etc., which are different under normal and fault conditions. The current, voltage and phase angle are the basic quantities which change during fault conditions. On the occurrence of fault, the system voltage and current get affected. The CT and PT provides the information about the changed system condition to the relay. The relay determines the faulty condition and operates the trip contact as shown in the figure. The trip circuit of circuit breaker is energised. The current flows through trip coil of CB and the circuit breaker is tripped (opened) to disconnect the faulty part of system from the healthy part. In this way the CT and PT can be used for protection purpose.