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The Current electrical device could be a kind of “instrument transformer” that’s designed to provide AN electricity in its coil that is proportional to the present being measured in its primary.

Current transformers, in conjunction with voltage or potential transformers, square measure instrument transformers. Instrument transformers scale the big values of voltage or current to tiny, standardized values that square measure simply to handle for instruments and protecting relays. The instrument transformers isolate measuring or protection circuits from the high voltage of the first system. A current electrical device provides a second current that’s accurately proportional to the present flowing in its primary. the present electrical device presents a negligible load to the first circuit.


Current transformers scale back high voltage currents to method|a far lower worth and supply a convenient way to safely observance the particular electrical current flowing in AN AC line employing a normal meter.

Types of the current transformer (CT) :

There are Mainly two type of Ct

  • Oil-insulated Current Transformers
  • SF6 insulated Current Transformers

Oil-insulated Current Transformers :

  1. Excellent insulation stress control using a finely graded bushing.
  2. Safety.
  3. Special core class ratings TPS, TPX, TPY, and TPZ are available.
  4. Low reactance bar-type primary permits high short circuit currents and high primary current.
  5. Essentially unaffected by stray external magnetic fields.
  6. Excellent seismic performance due to optimized designs and porcelain.
  7. Exclusive use of corrosion resistant materials.


Types of OICT :

  1. Available from 40.5 kV up to 550 kV
  2. High-quality mineral oil insulation, PCB free, biological decomposition
  3. Porcelain or composite insulator
  4. Hermetically sealed stainless steel metallic bellows
  5. Head type design – low primary losses, low weight, and minimum oil volume
  6. Rated primary currents up to 5000 A
  7. Rated secondary current 0,5 A to 5 A
  8. Maintenance free during a long lifetime of more than 30 years
  9. High accuracy CT (0.1 %) over the wide extended range of primary current
  10. Explosion resistant by
    • High-reliability insulation system
    • Finely graded bushing
    • Bushing tube and connection to withstand high short circuit current
    • Shielded secondary windings

SF6 insulated Current Transformers :

  1. Explosion proof design with compressible SF6 insulation system and a rupture disc.
  2. Excellent seismic performance.
  3. Cast aluminum housing.
  4. Excellent short-circuit performance/capability.
  5. Perfect transient performance.
  6. Stable accuracy over a long period of time.
  7. Maintenance free.
  8. No insulation aging.
  9. Remote supervision of insulation condition by monitoring internal gas density.

Fig: SF6 Current Transformer

Types of SF6 ICT :

  1. Top core current transformer
  2. Identical principle design from 72.5 to 800 kV
  3. SF6 insulation
  4. Composite Insulator
  5. Multiple-turn primaries for small primary currents
  6. Perfect transient response
  7. Core changes after assembly can be carried out without impairing the high voltage insulation

Current Transformer Primary Turns Ratio :

Generally, current electrical devices and meters are used along as a matched try during which the planning of this transformer is like to produce a most secondary current reminiscent of a complete deflection on the ammeter.

Current transformers have the quality secondary rating of five amps with the first and secondary currents being expressed as a quantitative relation like 100/5. this suggests that the first current is twenty times bigger than the second current thus once a hundred amps are flowing within the primary conductor it’ll lead to five amps flowing within the secondary coil. A current electrical device of say 500/5, can turn out five amps within the secondary for five hundred amps within the primary conductor, a hundred times bigger.

A current electrical device, like several different electrical devices, should satisfy the amp-turn equation and that we grasp from our tutorial on double wound voltage transformers that this turns quantitative relation is equal to:

transformer turns ratio

current transformer turns ratio

For example, assume that this rating of the first winding is 100A. The coil has a quality rating of 5A. Then the quantitative relation between the first and also the secondary currents is 100A-to-5A, or 20:1. In alternative words, the first current is twenty times larger than the secondary current.

It ought to be noted, however, that a current electrical device rated as 100/5 isn’t a similar together rated as 20/1 or subdivisions of 100/5. this can be as a result of the quantitative relation of 100/5 expresses the “input/output current rating” and not the particular quantitative relation of the first to the secondary currents. Also, note that the number of turns and also the current within the primary Associate in Nursing secondary windings area unit connected by an inverse proportion.

Safety :

The secondary of a current electrical device shouldn’t be disconnected from its burden whereas current is within the primary because the secondary can arrange to continue driving current into an efficient infinite resistivity up to its insulation breakdown voltage and therefore compromise operator safety.


Current transformers square measure usually accustomed to monitoring high currents or currents at high voltages. Technical standards and style practices square measure accustomed make sure the safety of installations victimization current transformers.

For certain current transformers, this voltage could reach many kilovolts and will cause arcing. extraordinary the secondary voltage may additionally degrade the accuracy of the electrical device or destroy it. Energizing a current electrical device with Associate in Nursing electrical circuit secondary is comparable to energizing a voltage electrical device with a brief circuit secondary. within the 1st case, the secondary tries to provide Associate in Nursing infinite voltage Associate in Nursing within the second case the secondary tries to provide an infinite current. each situation will be dangerous and harm the electrical device.

Construction :

Bar-type current transformers have terminals for supply and cargo connections of the first circuit, and also the body of the present electrical device provides insulation between the first circuit and ground.

Current transformers, particularly those supposed for prime voltage station service, might have multiple faucets on their secondary windings, providing many ratios within the same device. this could be done to permit for reduced inventory of spare units, or to permit for load growth in Associate in Nursing installation.

A high-voltage current electrical device might have many secondary windings with identical primary, to permit for separate metering and protection circuits, or for affiliation to different types of protection devices. for instance, one second is also used for branch overcurrent protection, whereas a second winding is also employed in a bus differential protecting theme, and a 3rd winding used for power and current measuring.

Only world wide web unbalanced current produces a secondary current – this could be accustomed notice a fault from Associate in Nursing energized conductor to ground. Ring-type transformers typically use dry insulation systems, with a tough rubber or plastic case over the secondary windings.

Accuracy :

The accuracy of a CT is affected by a number of factors including:

  • Burden
  • Burden class/saturation class
  • Rating factor
  • Load
  • External
  • Temperature
  • Physical configuration
  • The selected tap, for multi-ratio CTs
  • Phase change
  • Capacitive coupling between primary and secondary
  • The resistance of primary and secondary
  • Core magnetizing current