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Insight - How Current Transformer Works

Written By: 

Abhimanyu Mathur

There are two primary winding of single turn and it overlaps the secondary. Insulators made from varnish or enamel is used to separate primary and secondary winding.Insulation in transformer prevents conductors shorting.

Wires in Current Transformer


After shredding some copper wires we saw the heart of the transformer “The Core”.This type of construction of core is known as Toroidal core in which the conductor tightly surrounds the core in a manner that there is no air gap. Ferrite Toroidal core is used for high frequency working to reduce losses as well as its physical size.

Core and Winding





Some more eleboration is required on a given topic.

Extra discription required.

@ Samir.dod


Will post clear content on Current Transformers

Instrument Transformers are classified into two types: Current Transformers(CT) and Voltage Transformers(PT). These Transformers are mainly used for Metering Purposes and for Protection Purpose in Electrical Power Systems. The Relays are used for protecting electrical  power system. These are made to operate at low voltages & currents. So Current & Voltages are stpped down by means of CT & PT and fed to protection system. In Current Transformers(CT) the primary is connected in series with power line. So current through its primary is nothing but the current flows through that power line. The primary current of the CT, hence does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden. Generally CT has very few turns in primary where as secondary turns is large in number. Say Np is number of turns in CT primary and Ip is the current through primary. Hence the primary AT is equal to NpIp AT. If number of turns in secondary and secondary current in that current transformer are Ns and Is respectively then Secondary AT is equal to NsIs AT. In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT.

Current Transformer Vector Diagram:


Is - Secondary Current

Es - Secondary induced emf

Ip - primary Current

Ep - primary induced emf

KT - turns ratio = numbers of secondary turns/number of primary turns

Io - Excitation Current

Im - magnetizing component of Io

Iw - core loss component of Io

?m - main flux.

Let us take flux as reference. EMF Es and Ep lags behind the flux by 90o. The magnitude of the passers Es and Ep are proportional to secondary and primary turns. The excitation current Io which is made up of two components Im and Iw.
The secondary current Io lags behind the secondary induced emf Es by an angle ? s. The secondary current is now transferred to the primary side by reversing Is and multiplied by the turns ratio KT. The total current flows through the primary Ip is then vector sum of KT Is and Io.

Current Error or Ratio Error in CT:

From above passer diagram it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turns ratio, i.e. KTIs. This difference is due to the primary current is contributed by the core excitation current. The error in current transformer introduced due to this difference is called current error of CT or some times Ratio Error in Current Transformer.


Current Error = (Ip-KTIs)/Ip



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