Question

Explain the purpose of the following: (a) CT (Current transformer), (b) PT/VT (Potential Transformer/Voltage Transformer), (c) Relay, (d) Circuit Breaker.

Answer 1

A) Current transformer(CT): Current transformer is basically a step up transformer. It is used to measure very high current levels. It produces current in secondary proportional to its primary current. Due to being a step up transformer it's secondary current is less then primary current. By using this phenomenon the high level of current can be dropped down to a low level where it is measured using conventional measurement devices.

B) Potential Transformer (PT): Potential Transformer is just the reverse of CT. It is a step down transformer which decreases the high voltage to a low level where it can be measured by conventional measuring devices. In power system PT is used to measure very high level of voltages.

(C) Relay: In power system relay is the protection device. Relay takes input from CT of PT and compares it with the pre set values to find out fault. While senceing a fault it generates a signal for circuit breaker or other devices to clear the fault. Relay is used to protect a power system from over voltage, under voltage, over current, frequency change etc.

(D) Circuit breaker: Circuit breakers are basically switches. They are used for switching purpose in a power system. Circuit breakers are capable of breaking a circuit in case of fault of full load while high current is still flowing through it. When relay sences a fault it generates signal to following circuit breaker to clear the fault by opening the circuit.

#NOTE: Basically CT, PT are those devices which sences the power system parameters and provide those to Relays. Relay take input from CT, PT or other devices then compare them with reference values and generates signal accordingly. Circuit Breaker is the device which take commands from Relays of manual inputs to make or break a particular circuit in power system.