Definition :
Current transformer is a type of instrument transformer used to step down the heavy current to such a lower value which are suitable for the purpose of measurement, control and protection. In simple words, they provide a convenient way of measuring large heavy currents to the measuring instrument (ammeter) connected with their secondary winding.
Why Current Transformers are required?
Let assume that we want to measure the large current flowing in the transmission line. However, we cannot measure the high magnitude current due to the following reasons;
Standard current measuring device ammeter of such high rating is very costly and bulky. There will also be huge losses.
Ammeter needs to be connected in series with the circuit. This simply means that we need to break the line which is not possible.
Hence current transformers are used to reduce high level currents to much lower value and make it possible for current measuring devices to monitor the actual electric current flowing in high transmission lines. An ammeter is connected to the secondary terminal of current transformer and it is adjusted depending on the turns ratio to measure the primary current.
Construction of Current Transformer:
Current transformer consists of silicon steel ring core wound with many turns of copper wire. Conductor carrying the primary current passes through the ring as shown in the fig. Primary winding of the transformer consist of single or few turns and it is connected in series with the current carrying line to be measured. Since primary winding is connected in series with the current carrying conductor to be measured. Hence current transformer also termed as series transformer.
Secondary winding on the other side has a large number of coil turns of thin wire rated for standard 5amp or 1amp. Ammeter is connected directly across the secondary winding of transformer. Since ammeter is used at the secondary side to measure the current and its resistance is very low. Hence we can say that secondary side is short circuited.
How Current Transformers work ?
Current Transformer is basically a step up transformer (i.e. step up a voltage). It is used for the purpose of measurement and protection in the power system. It will reduce the large primary current to a standard secondary 1 amp or 5 amp. For example, let assume that we have a current transformer with a ratio of 1000:5. This means that current transformer is rated for 1000 primary amperes at full load and will produce 5 amps of secondary current when 1000 amperes flow through the primary. In simple words, it steps down the current from primary to secondary by a ratio 1000 :5.
Current transformer ratio like any other transformer follows the ampere turn equation. Turn ratio equation for current transformer is as follows;
Turns Ratio = 
Where
N1= Number of Primary turns.
N2= Number of Secondary turns.
I1= Primary Current
I2= Secondary Current
From the above turn ratio equation, we can say that by knowing the current ratio and ammeter reading, the actual line current flowing through the primary can be obtained. Let understand this by the following example.
A Current Transformer with a ratio of 1000:5 is used. If ammeter reading is 3.2 A, what will be the line current ?
Solution:
By given Condition,
Since ammeter is connected in series with secondary, I2= 3.2 A.
Putting the value of I2,we have
.
So the line Current is 640 Amp.
Burden of Current Transformer :
Value of the load connected across the secondary side is known as Burden. It is expressed in VA(i.e. product of secondary voltage and current). Burden (load) of current transformer includes protective relays, connecting wires and resistance of the secondary winding.
Rated Burden :
Specific VA loading at rated secondary current on which the current transformer performs with specified accuracy is known as rated burden. It is specified to ensure that errors in the measurements do not exceed the limits. The permissible load is called burden of current transformer. It is expresses in VA or ohms at rated secondary circuit.
Connections of Current Transformer :
It must be noted that secondary of the current transformer should never be kept open when the primary is energized. If current transformer is operated without a burden (load) on its secondary, current through the secondary becomes zero which means that the ampere turns of secondary becomes zero. Practically, secondary ampere turns opposes the primary ampere turns (magnetomotive force). If the secondary is left open, high flux is set up in the core since there is no unopposed secondary ampere turns to oppose the primary magnetomotive force. This will cause the huge emfs to be induced on both primary and secondary side. It will results in excessive core losses and also increases the heating of core beyond its limits . This will simply leads to damage the insulation and putting in danger the people (operator) near it.
Hence it is important that the secondary must be connected in series with low resistance coils of the ammeter (instruments) or simply short circuited. Moreover, most of the current transformers also have short circuit link which must must be closed if the secondary side of CT has to be opened for the maintenance of relays. Current Transformer connection diagram is given below.
One more thing to be noted during the connection of current transformer is that it must be installed around one phase conductor. Connecting a transformer with multiple phase and neutral conductors will result in the cancelling of flux due to which ammeter indicates zero reading.
Types of Current Transformer
Wound Type Current Transformer :
In wound type current transformer, primary winding of more than one full turn is wound on the core. The core material generally used is nickel iron alloy or oriented electric steel. Actually, steel is designed in such a way that it produces low hysteresis area which simply leads to low power losses↗ in the core.
For low voltage wound type current transformer, secondary winding is wound on Bakelite former. Bakelite is plastic made used for electrical non conductivity and heat resistant properties. While primary is wound on the top of secondary with suitable insulation between the two.
Bar Type Current Transformer :
In Bar type current transformer, primary winding is bar type of suitable size. Bar type primary is bakelized paper tube which has good electrical and mechanical properties. Moreover, the external diameter of bakelized paper tube is kept large in order to avoid the corona effect. Secondary winding and core is same in bar type transformer. Windings in bar type are placed closed to each other in order to minimize the leakage reactance.
For small line voltages, tape and varnish is used for insulation. In case of line voltage higher than 7 kV, oil immersed or compound filled current transformers are used.