Current Transformer

A current transformer (CT) is a transformer which measures the current of another circuit. It is coupled to an ammeter (A in the diagram) in its own circuit to perform this measurement. Measuring high-voltage current directly would require insertion of measuring instrumentation into the measured circuit--an unnecessary difficulty which would draw down the very current meant to be measured. Also, the heat generated in the measuring equipment from the high current could give false readings. Measuring current indirectly with a CT is much more practical.

Application

Current Transformers (CT’s) are instrument transformers that are used to supply a reduced value of current to meters, protective relays, and other instruments. CT’s provide isolation from the high voltage primary, permit grounding of the secondary for safety, and step-down the magnitude of the measured current to a value that can be safely handled by the instruments.

Common Uses 

Electric Meter

The one central function of a CT is to determine the current in a circuit. This is especially useful for monitoring high-voltage lines throughout the power grid. Another ubiquitous use of CTs is in domestic electric meters. A CT is coupled with a meter to measure what electrical usage to charge the customer. 

Current Transformer

Another function of CTs is protection of sensitive measuring equipment. By increasing the number of (secondary) windings, N2, the current in the CT can be made much smaller than the current in the primary circuit being measured. In other words, as N2 in the formula i1/(2πr) = i2 --- N2 goes up, i2 goes down.

This is relevant because high current produces heat which can damage sensitive measuring equipment, such as the resistor in an ammeter. Reducing i2 protects the ammeter. It also prevents heat from throwing off the accuracy of the measurement.

Protective Relay 

CT Mounted Over Bushing of Circuit Recloser

CTs also protect the main lines of the power grid. An overcurrent relay is a type of protective relay (switch) that trips a circuit breaker if a high-voltage current exceeds a certain preset value. Overcurrent relays use a CT to measure the current, since the current of a high-voltage line could not be measured directly.