Introduction

A Current Transformer(CT) is a type of transformer that is used to reduce or multiply an alternating current (AC) (used along with measuring or protective device).It produces a current in its secondary which is proportional to the current in its primary.

Function of Current Transformer

The current transformers are basically a step up transformers.In case of high magnitude supplies we can not connected the measuring devices directly.we used current transformer to the supply of those devices with currents of magnitude proportional to these of power.Current transformer also isolates the measuring instruments from the high voltage circuits.current transformers supply the protective relays with current of magnitude proportional to those of power circuit but sufficiently reduce in magnitude.

Construction of Current Transformer

As we know that current transformer is a form of step up transformer,So it has primary winding of one or more turns with heavy cross sectional area.In some the bar carrying high current may act as primary.It is connected in series with the line carrying high current.The secondary of the current transformer is made up of large number of turns of small cross-sectional area.It is usually rated for 5A.It is connected to the coil of normal range ammeter.

There are three types of constructions used for the current transformers which are.

Wound Type CT. Bar Type CT. Toroidal (window) Type CT.

Wound Type Current Transformer

In this type of Current transformer the primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit.The magnitude of the secondary current is depend on the turns ratio of transformer.

Bar Type Current Transformer

In this type of current transformer we use the actual cable or bus -bar of the main circuit as the primary winding which is equivalent to a single turn.They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.

Toroidal Type Current Transformer

These type of current transformers do not contain primary winding,Instead the primary winding the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer.Some of the current transformers have Split Core which allows it to be opened installed and closed the advantages of such kind is we can installed with out disconnecting the circuit to which they are attached.

Ratio of Current Transformer

Current ratio of the current transformer is usually high.The current primary ratings vary from 1000A,3000A,5000A or more Where as the secondary current rating are of the values of 1A,3A,5A etc.

Working Principle of Current Transformer

As we know that current transformers are step up transformers but if we consider current point of view these are step down transformers.It means these are used to step down the current for measuring purpose and for the safety of the measuring device.

Let

N1=Number of primary turns

N2=Number of secondary turns

I1=Primary Current

I2=Secondary Current

As we know for a transformer

I1/I2=N1/N2

As we know in case of current transformer N2 is vary high compared to N1.

Thus the ratio of I1 to I2 is vary high it also show the range of current transformer.

For example a 5000:5 CT secondary winding will supply an output current of 5 Amperes when the primary winding current is 5000 Amperes.

Uses/advantages of Current Transformer

By using the current transformer we can measure the heavy current with the help of

conventional ammeters and CT electrically isolates the ammeters from high voltage primary circuits.Several measuring instruments can be operated from a single current transformer.Current transformer can be used to measure power or energy consumed by a circuit,for this a watt meter or energy meter is connected with secondary of the current transformer.By using the current transformer we can measure the large range by small range instruments.A 5A ammeter can read up to 5000A.Current Transformer can indicate power factor,frequency of a power system.Current transformer can also be used in power system to operate various relays.

Disadvantages of Current Transformer

The disadvantages of the current transformer is that it can be used only for AC circuit and not for Dc circuit.