Current Transformer, also called as CT,  is an essential equipment in the Power System, mainly in Transmission & Distribution Systems.

What is a Current Transformer (CT)?

  1. Defination: It is an Instrument Transformer, which replicates the Primary Current, but at much lower values, which can be measured by Relays/Meters.
  2. Working Principle: It is a Step-up type Transformer and so the High Secondary Voltage results in reduced or Lower Secondary Currents.
  3. Location: The Primary of a CT is placed in Series with the Conductor, whose current needs to be measured. In the Power System, it is located 
  4. Remarks: Never keep CT Secondary Open.

    What are the Types of CT’s?

    On the basis of Tank,  CT’s are classified as

    1. Live Tank type CT: The Core and the Secondary Winding are in the Live Tank Portion.
    2. Dead tank type CT: The Core and the Secondary Winding are in the Dead Tank Portion of the CT.


      What are CT Specifications?

      These specifications are given on the name plate and are also called as Name Plate Details of the CT.

      1. Rated Primary Current: The value of Primary Current which appears on the Name Plate of the Transformer and on which the performance of the CT is based.
      2. Rated Secondary Current: The Current in the Secondary Circuit, on which the performance of the CT is based. Typical values of Secondary Current are 1A or 5A.
      3. Rated Burden: It is the apparent power of the Secondary Circuit in Voli-amperes, expressed at the rated secondary current and at a specific Power Factor (0.8 for almost all standards). It is termed as “Burden” to distinguish it from the Primary Load.
      4. Accuracy Class: For each class, the  standards define a maximum allowable current and Phase Displacement Errors for different Load Conditions.

        Some Important Features:

        Please find below, some Important CT Functions:

        1. Primary current is not controlled or determined by the conditions of the Secondary Winding Circuit.
        2. Secondary Current is substantially proportional to Primary Current (under normal operating conditions)  and differs in Phase from the Primary Current ideally by Zero Degrees for an appropriate direction of the connections.

          CT Testing:

          There are following CT Tests:

          1. Insulation Resistance(IR) Test: Low IRValue indicates abnormal conditions such as presense of moisture, dirt, dust , crack in insulator of CT and degradation of Insulation. 
          2. Tan Delta:
          3. Knee Point Test:
          4. Polarity Tests:
          5. Ratio Test:


            Please find Details Article on CT Testing below:

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