A current transformer (CT) is a type of transformer that is used to measure alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.
Current transformers, along with voltage or potential transformers, are instrument transformers. Instrument transformers scale the large values of voltage or current to small, standardised values that are easy to handle for instruments and protective relays.
The instrument transformers isolate measurement or protection circuits from the high voltage of the primary system. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. The current transformer presents a negligible load to the primary circuit.
Current transformers are the current-sensing units of the power system and are used at generating stations, electrical substations, and in industrial and commercial electric power distribution.
Basic Operation :
The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary. The primary circuit is largely unaffected by the insertion of the CT. Accurate current transformers need close coupling between the primary and secondary to ensure that the secondary current is proportional to the primary current over a wide current range. The current in the secondary is the current in the primary (assuming a single turn primary) divided by the number of turns of the secondary. In the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is an AC ammeter.
Current transformers typically consist of a silicon steel ring core wound with many turns of copper wire as shown in the illustration to the right. The conductor carrying the primary current is passed through the ring. The CT's primary, therefore, consists of a single 'turn'. The primary 'winding' may be a permanent part of the current transformer, i.e. a heavy copper bar to carry current through the core. Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. To assist accuracy, the primary conductor should be centered in the aperture.
Thumb Rules & Nomenclature of CT:
Ratio: input / output current ratio
Burden (VA): total burden including pilot wires. (2.5, 5, 10, 15 and 30VA.)
Class: Accuracy required for operation (Metering: 0.2, 0.5, 1 or 3, Protection: 5, 10, 15, 20, 30).
Accuracy Limit Factor:
Nomenclature of CT: Ratio, VA Burden, Accuracy Class, Accuracy Limit Factor.Example: 1600/5, 15VA 5P10 (Ratio: 1600/5, Burden: 15VA, Accuracy Class: 5P, ALF: 10)
As per IEEE Metering CT: 0.3B0.1 rated Metering CT is accurate to 0.3 percent if the connected secondary burden if impedance does not exceed 0.1 ohms.
As per IEEE Relaying (Protection) CT: 2.5C100 Relaying CT is accurate within 2.5 percent if the secondary burden is less than 1.0 ohm (100 volts/100A).