Current transformer- definition, basic function, composition, types & rating.

CT or Current Transformer is an important switchgear device. Its sole purpose is to measure current for measurement or sensing purpose.

What is current transformer or CT:

Instrument transformer used in conjunction with ammeters and over current relays are called current transformers.Current transformers lower the measured current to a reasonable value that can be used in measurement and with in protective relaying.

In Current transformer primary current is independent of secondary current; hence primary current is the dominant factor in CT.

Current transformers have two basic functions:

  • adapting the MV current value at the primary to the characteristics of the metering or protection devices by supplying a secondary current with a reduced, but proportional current value
  • isolating power circuits from the metering and/or protection circuit.

The primary is series-mounted on the MV network and subject to the same over-currents as the latter and withstands the MV voltage.

The secondary generally has one of its terminals connected to earth. The secondary must never be in an open circuit to avoid lethal shock as high voltage induced in secondary open terminal in this case.

Composition and types of current transformers

Current transformer composition

A current transformer comprises a primary circuit and a secondary circuit connected via a magnetic core and an insulating coating system in epoxy-silica or any other suitable material by manufacturer.

Current transformer types:

wound: when the primary and the secondary have a coil wound on the magnetic circuit
slip-over: primary made up of a conductor that is not insulated from the installation
core: primary made up of an insulated cable.

wound type and closed core type current transformer.bmp
wound type and closed core type current transformer.

Current transformer ratings-

Rated primary current (Ipn) -Rms value of the maximum continuous primary current. Usual values are 25, 50, 75,100, 200, 400, 600 A.
Rated secondary current (Isn) – This is equal to 1 A or 5 A.
Rated transformation ratio -Kn = I rated primary / I rated secondary (e.g.: 100 A / 5 A)

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