Current Transformer

Current Transformer – Need, Working, Types, and Losses

Electrical Substation, Electrical Transformer No Comments

Current Transformers (CT) are the devices in which the secondary current used for calculation is proportional to the primary current produced by the electrical network. For this purpose, the primary current is tapped from the transmission line and stepped down in the secondary for measurement purposes.

In series with the current lead, the connection to the primary winding circuit is carried out. In the form of measurement instruments and various relays, the secondary winding is linked to any load.

A proportional relationship, referring to the number of turns, between the currents of both windings exists. The separation between the windings in a high voltage transformer is based on the maximum operating voltage.

As a rule, one of the ends of the secondary winding is grounded, so the winding and ground potential would be essentially the same.


Current transformers fall under the category of special accessories used in combination with various measuring devices and relays in AC circuits.

The main purpose of transformers is converting any current value to required values which is most convenient for measurements and supplying power to protective devices with low internal resistance. Due to such isolation, the maintenance electricians are protected from high voltage shock.

Instrument current transformers are designed for high voltage electrical circuits where the direct connection of the measurement instruments is not feasible. Their main function is to pass the electrical current to the measurement devices connected to the secondary winding.

An important function of transformers is to control the flow of current passing through the circuit. During connection to power relays, routine checkings of circuit networks and proper grounding are performed. When the flow of current reaches over the limit protective device is activated shutting down the transformer for avoiding further damage.


The current transformers are designed to perform two main functions: Safety and Measurement. Both functions can also be combined in certain devices.

  1. Instrument transformers are mounted in a high voltage line where measuring instruments
    can not be connected directly. Therefore, only the secondary winding of the transformer is connected to the measuring instruments like ammeters, voltmeters, and other metering instruments.
  2. Protective transformer devices primarily transmit the received measurement information to control and protection devices. With the help of protective transformers, an alternating current of any value is converted into an alternating current with the most suitable value, which provides power to the relay protection devices.

Related: Power Transformer Working Principle, Construction and Types


The principle of operation of the current transformer is based on the law of electromagnetic induction. The transmission line voltage is supplied to the primary winding with a certain number of turns and overcomes its impedance.

This leads to the appearance of magnetic flux around the coil, which is captured by the magnetic circuit. This magnetic flux is perpendicular to the current direction.

As the secondary winding turns, located perpendicularly, are crossed, the electromotive force is activated by the magnetic flux. The loss of electrical current during the conversion would be negligible.

There is a current under the control of EMF, which is forced to overcome the impedance of the coil and the output load. At the same time, a decrease in voltage is observed at the output of the secondary winding.


All the current transformers can be categorized according to their features, method of use, and technical characteristics.

Based on the function performed,

  1. Measuring CT: These current transformers have high accuracy as they need to continue sensing currents at very high fault values. Since they have a very saturation limit designed in such a way that current measuring instruments are not destroyed by high current.
  2. Protective CT.
  3. Intermediate CT.

Based on the type of construction,

  1. Wound Type CT: A wound type consists of the primary winding and secondary winding either on separate legs of magnetic cores or central core. Which completely depends on design and usage requirements.
  2. Bar Type CT: A Bar type consists of a single bar of the required size and material type which is used as primary winding and equivalent to a single turn. A bar-type operate on the same principle of toroidal current transformers but have a fixed bar as the primary conductor.
  3. Toroidal CT: A toroidal type has no primary winding since this type of transformers are usually installed around a conductor which is carrying current flowing through a window into a transformer made of the wounded toroidal magnetic core.

In addition, there are high-precision, multi-transformation laboratory current transformers.


Invariably, the transition of energy from the primary to the secondary is followed by losses. There are no moving elements in the transformer and thus no mechanical losses.

However, there are copper losses due to the presence of electrical resistance in the windings, as well as magnetic losses in the core steel due to the resulting eddy currents and magnetization reversal.

Due to these losses, all the energy is not transferred from the primary to the secondary winding.


  1. At times, a CT is attached to a load that has malfunctions and even emergency situations. As a rule, this is due to breaches of the electrical resistance in the winding insulation and reduction in conductivity of the windings under the influence of elevated temperatures.
  2. Accidental mechanical stress or poorly done installation has a negative effect.
  3. Insulation damage most frequently occurs during the operation of the equipment, causing short-circuit overturns of the windings, which greatly decreases the transmitting power.
  4. Leakage currents can result from accidentally generated circuits or even short-circuit conditions.
  5. In order to avoid emergencies, experts regularly inspect the entire operating system using thermal imaging equipment. This makes it easier to remove contact defects easily and avoid overheating of the machinery.


  1. A range of their models can be used both in small electronic devices mounted in a small case and in energy devices with a scale of several meters. They are graded on the basis of operational characteristics.
  2. For measuring the transmission line parameters, CTs are used.
  3. For power system protection, current transformers are employed with protective relays.
  4. Heavy EI transformer laminated cores could be also large CTs for power transmissions.

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