What is a Current Transformer : Working & Its Applications

The transformer is an electrical device used to transfer electric power from one circuit to another without changing its frequency and it achieves by electromagnetic induction. Basically, transformers are available in two types namely shell type and core type. The main function is to step up and step down the voltage. For measurement purposes, instrument transformers are used because these transformers measure current, voltage, energy and power. These are used in different instruments with a conjunction like a voltmeter, ammeter, wattmeter & energy meter. These transformers are classified into two types namely the current transformer and potential transformer.

What is the Current Transformer?

Definition: An instrument transformer that is used to generate an AC within the secondary winding of the transformer is known as a current transformer. This is also known as a series transformer as it is allied in series with the circuit for measuring different parameters of electric power. Here the current in the secondary winding is proportional to the current in the primary winding. These are used to reduce high voltage currents to low voltage currents.

Current Transformer Device
Current Transformer Device

Working Principle

The working principle of the current transformer is somewhat different when we compare it with a normal voltage transformer. Similar to the voltage transformer, it includes two windings. Whenever AC supplies throughout the primary winding, then alternating magnetic flux can be generated, then AC will be induced within the secondary winding. In this type, the load impedance is very small. Thus, this transformer works under short circuit conditions. So the current within the secondary winding depends on the current in the primary winding but doesn’t depend on the load impedance.

Current Transformer Construction

The construction of this transformer includes different features based on the design like primary ampere-turns, core, windings, and insulation.

Current Transformer Construction
Current Transformer Construction

Primary Ampere Turns

The no. of primary ampere-turns in the transformer ranges from 5000 to 10000 so these are decided through the primary current.


To achieve the low magnetizing ampere twists, the core material must include low iron losses and low reluctance. Core materials like nickel and an alloy of iron include different properties like low loss, high permeability.


The leakage reactance in the transformer can be reduced by placing the windings close to each other. The wires used in the primary winding are copper strips and for secondary, SWG wires are used. The designing of these windings can be done for appropriate strength & fixed bracing without any harm.


The windings of the transformer are insulated using varnish & tape. The applications of high voltage need insulation arrangements which are absorbed by the oil used for the windings.

The designing of the core in the transformer can be done using silicon steel lamination. The transformer’s primary winding carries the current & it is connected to the major circuit. The current in the secondary winding is proportional to the current in the primary winding & it is connected to the meters or instruments.

The primary and the secondary windings are insulated from the cores. The primary winding includes a single turn that carries the full load current whereas the secondary winding includes a number of turns.
The ratio of the current in the primary and secondary is called a current transformer ratio. Usually, the transformer’s current ratio is high. The current ratings in the secondary are 0.1A, 1A & 5A whereas the current ratings in the primary range from 10A – 3000A.

Types of Current Transformers

These are classified into four types which include the following.

Indoor Current Transformer

Indoor type transformers are applicable in low voltage circuits. These are classified into different types like wound, window, and bar. Similar to the basic type, wound type includes two windings like primary and secondary. These are used in summing applications due to high accuracy & high values of primary ampere twists.

The bar-type transformer includes bar primary with secondary cores. In this type, bar primary is an essential part. The accuracy of this transformer can be reduced because of the magnetization in the core. Window type can be installed in the region of the primary conductor because the designing of these transformers can be done without primary winding.

These types of transformers are accessible in solid & split-core designs. Before connecting this kind of transformer, the primary conductor should be detached whereas, in split-core, it can install directly in the region of the conductor without separating it.

Outdoor Current Transformers

Outdoor type transformers are used in high voltage circuits like substations & switchyards. These are available in two types namely oil-filled & SF6 gas insulation. SF6 insulated type transformers are lightweight when we compare with oil-filled type transformers.

The pinnacle tank can be connected toward the primary conductor which is known as live tank construction current transformer. In this construction, small bushings are utilized because both the tank and the primary conductor are at the same potential. For multi-ratio CTs, the split type primary winding is used.

Thus taps are arranged on the tank intended for the primary winding so variable current ratio can be obtained by using these transformers. Once the taps are given to the secondary winding, then the operating ampere-turns can be changed while provided to the primary winding, so unused copper space can be left excluding in the lowest range.

Bushing Current Transformer

This kind of transformer is similar to the bar type, where core & secondary are placed in the region of the primary conductor. The secondary winding in the transformer can be turned into a circular otherwise annular-shaped core. It is connected to the high voltage bushing within the circuit breakers, power transformers, switchgear otherwise generators.

Once the conductor flows throughout bushing then it acts as primary winding & the arrangement of the core can be done by enclosing an insulating bush. These types of transformers are used in the high voltage circuits for relaying purposes because these are not expensive.

Portable Current Transformers

These types of transformers are high precession type mainly used for power analyzers and high accuracy ammeters. These transformers are available in different types like flexible, clamp ON portable and split core. The measurement of the current range for the portable CTs ranges from 1000A-1500 A. These transformers are mainly used to provide isolation for the measuring instruments from the circuits with high voltage.

Errors in Current Transformer

The errors that occurred in this transformer include the following.

  • The primary winding of this transformer requires MMF (magnetomotive force) to generate flux which draws magnetizing current.
  • The transformer’s no-load current includes an element of core loss component and occurs hysteresis and eddy current losses.
  • Once the transformer’s core is saturated, then the magnetizing force’s flux density can be stopped & other losses can occur.

Applications of Current Transformers

These transformers are used to measure electric power in powerhouses, industries, grid stations, control rooms in industries for metering & analyzing the flow of current in the circuit and also for protection purposes.


1). What is the difference between CT and PT?

The CT changes the high current value into low current value whereas the PT changes the high voltage value into low voltage.

2). Is the current transformer a step-up transformer?

In principle, CT is a step-up transformer

3). Why is CT connected in series?

The CT is connected in series through the line to change the line current to the typical 1/5 amperes appropriate for the meter otherwise relay. These transformers are used to calculate huge current which flows throughout a conductor.

4). What is the CT ratio?

It is the ratio of primary current i/p to secondary current o/p at full load

5). Why CT is used in substation?

This transformer is used for measurement & protection purposes in substation

Thus, this is all about an overview of the current transformer which includes its definition, working principle, construction, different types, errors, and applications. Here is a question for you, what is an instrumentation transformer?

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