Current Transformer

Current Transformer is an essential part of power system. The basics of current transformer including construction, applications, working principle are going to be discussed in this article.

What is Current Transformer & Why is it used?

​Current Transformer is an ‘instrumentation’ transformer which steps down high values of currents to lower values.

  1. It isolates the protection system from high voltages and currents, resulting in the reduction of the and cost of protection equipment
  2. The output of a Current Transformer is standard (i.e. 1A or 5A) which eliminates the need for protection equipment relays having diversified operating values.

Construction of CT (Current Transformer):

Construction of current transformer
Construction of current transformer

A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. The secondary coil of the current transformer is made up of a large number of turns of fine wire having a small cross-sectional area. Can you guess what the current rating of a normal Transformer is? Click here for continue reading.

Working Principle:

The alternating current in the primary windings induces magnetic flux in the core which is transferred to the secondary windings and induces alternating current there as well. These transformers are basically step-up transformers i.e. stepping up a voltage from primary to secondary.


Based on Function:

  • Measurement CT:

Current Transformer used for metering and indicating circuits are popularly termed as Measuring CT. They have a low saturation point.

  • Protection CT: ​

Current Transformer used in conjunction with protective devices is termed as Protection CT. Since it operates on current values higher than its rated value, its core has a high saturation point.

Based on Construction: ​

  • Bar Type Current Transformer:

This type of current transformer uses 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

  • Wound Current Transformer: ​

The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit.

  • Toroidal/Window Current Transformer:

These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Can you guess the advantages of a Transformer possessing a “Split core”?

Connections of CTs: ​

A CT is quite simple to connect in a single phase system, but for three phase system, there are two ways in which a CT can be connected. A Star (Wye) Connection and Delta Connection.

It is a normal practice to connect the CTs in delta if it is on the transformer side connected in wye and vice-versa.

Polarity of CT: ​

Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way transformer leads are brought out of the case.

Grounding of CT:

The grounding of the current transformer is very important for the safety and correct operation of the protective relays.

Burden of a CT: ​

Current transformer burden is defined as the load connected across its secondary. It is generally expressed in VA(volt-ampere).

Burden of current transformer
Burden of current transformer

Learn more about it comprehensively here.

CT Ratio:

The CT ratio is the ratio of primary current input to secondary current output at full load.

Transformer Ratio =Primary Current/Secondary Current

Errors in a CT:

The current transformer has two errors — ratio error and a phase angle error.

Current Ratio Errors​

It is mainly due to the energy component of excitation current and is given as

Ratio Error =KtIs / IpIp

Where Ip is the primary current, Kt is the turn ratio and Is is the secondary current.

Phase Angle Error

In an ideal current transformer, the vector angle between the primary and reversed secondary current is zero. But in an actual current transformer, there is a phase difference between the primary and the secondary current because the primary current has also supplied the component of exciting current.

Please visit our detailed blog to know comprehensively about the Phasor diagrams of an ideal and actual CT as well as Model of a conventional CT.

Arc Flash studies are the first step towards a safe environment without the fear of equipment malfunction or system failure. Maintaining a well equipped power transmission network requires an efficient Arc Flash Analysis executed by a professional Power Systems Engineer.

Let us know if you have any queries regarding this topic and do provide us with your feedback in the comments.

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