what is CT(current transformer)? - Electronicsinfos

What is CT(current transformer)?

    what is CT(current transformer)? - Electronicsinfos

    A Current Transformer (CT) is a type of instrument transformer designed to measure alternating current (AC) in electrical systems. It consists of a primary winding, which carries the current to be measured, and a secondary winding which generates a proportionally reduced current, which is used for measurement or relay purposes. 

    CT (current transformer) working principle

    CTs operate on the principle of electromagnetic induction. When an alternating current flows through the primary winding, a magnetic field is created around it. The secondary winding, which is connected to measuring or protective devices, is wound around a magnetic core. According to Faraday's law of electromagnetic induction, the magnetic field induced by the primary current in the core causes a proportional current to be induced in the secondary winding. The secondary current is typically reduced in magnitude compared to the primary current, making it suitable for measurement or protection devices.

    CT (current transformer) wiring diagram 

    CT (current transformer) wiring diagram

    CT(current transformer) Formula

    The current transformer (CT) is an electrical device used to measure alternating current (AC) in power systems. It works on the principle of electromagnetic induction. The formula for a current transformer is given by:

    Iprimary / Isecondary = Nprimary / Nsecondary

    I primary represents the primary current flowing through the primary winding of the current transformer.
    I secondary represents the secondary current flowing through the secondary winding of the current transformer.
    N primary represents the number of turns in the primary winding.
    N secondary represents the number of turns in the secondary winding.

    CT (current transformer) Types

    There are various types of CTs designed for specific applications

    • Bar-type CTs
    • Wound CTs
    • Window-type CTs 

    Bar-type CTs

    These CTs are commonly used for primary currents ranging from a few hundred amperes to a few thousand amperes. They consist of a single primary turn and are primarily used in switchgear and industrial applications.

    Wound CTs

    These CTs have multiple primary turns, allowing them to handle higher primary currents. They are extensively used in power generation and transmission systems.

    Window-type CTs 

    These CTs have a split-core design, which allows them to be easily installed around existing conductors without interrupting the circuit. They are commonly used in retrofit applications or in situations where it is not feasible to disconnect the circuit.

    CT (current transformer) ratio table

    CT (current transformer) ratio table

    CT (current transformer) Application


    CTs are widely used for accurate measurement of current in power distribution systems. They enable the conversion of high primary currents into proportional low secondary currents, which can be safely and precisely measured by ammeters or energy meters.


    CTs are an integral part of protection schemes in electrical systems. They provide current inputs to protective relays, which monitor the magnitude and direction of current flow. In abnormal current conditions such as short circuits or overloads, the relays activate protective devices like circuit breakers to isolate the faulty section and prevent damage.

    Power Quality Analysis

    CTs are used in power quality monitoring systems to analyze and record current waveforms. By capturing detailed data on current harmonics, power factors, and distortion, CTs help in identifying and resolving power quality issues, ensuring efficient and reliable operation of electrical systems.

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    Accuracy Class of CT(Current Transformer)

    Three phase to six-phase transformer

    Mutual flux

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