What Are Current Transformer Types?

A current transformer is a type of instrument transformer that transforms alternating current (AC) from high to low value.

This current is proportional to the current measured in primary windings. These types of transformers are used for safety measures.

The primary winding of the current transformer consists of a limited number of turns. While the secondary winding consists of a large number of turns.

There are three different current transformer types in use. Let’s discuss in detail.

Applications and usage

CTs are commonly used in many electrical applications. Such as switch-gears, control panels current meters and current measurement devices, such as clamp meter which are the most famous application because almost every electricians needs it in his daily work.

Current Transformer types

The CT consists of three types. I.e.Wound transformer, Bar type transformer, and Toroidal CT. Each one is described in detail.

Wound Transformer

One of the CT types is the wound transformer. In this type of transformer, the primary winding is located inside the transformer.

Normally the primary winding consists of limited or sometimes a single turn. The primary winding is then connected in series with the conductor measuring current.

It Acts as an Ammeter and can measure the current ranges from 1 to 100 Amperes.

Therefore, the value of current in the secondary winding is dependent on the ratio of the number of turns of the transformer.

Wound current transformer is a modified window transformer. 

Bar Type current transformer

bar current transformer
Алексей Намидов, Current transformer TPK 10, CC BY-SA 3.0

This type  consists of a secondary winding only. While the transformer on which the transformer is mounted acts as a primary winding.

Here the primary winding also consists of a single turn. Full type insulation is provided from the high operating voltage of the system.

Bar types Transformers are usually bolted to the current-carrying device.

Toroidal CT

window current transformer
window current transformer

This type of transformer does not include primary windings. The current-carrying wire is attached through a hole with or sometimes we call it a window of the transformer.

Toroidal current types of transformer are mostly in use.

The main reason for its maximum usage is that this type of transformer has a symmetrical shape.

Due to the symmetrical shape, the flux leakage is very low. And thus a less amount of electric interference occurs in the transformer.

Some toroidal current transformers have split cores. It can be used to install, open or close the circuit without disconnecting the attached circuit.

What is neutral current transformer and where do we use it?

A transformer’s star winding uses a current transformer called a neutral current transformer (NCT) (Since only the Star winding has a neutral).

This NCT is mostly used to gauge current flow via the transformer’s neutral, which is what we’re interested in.

This NCT’s secondary is linked to safety relays such as the standby earth fault relay and the limited earth fault relay, among others.

Keep in mind that the fault current passes via the transformer’s neutral (fault conditions) in imbalanced situations.

Use of neutral current transformer

  • Only loads that generate neutral current necessitate a neutral CT.
  • When a 4-wire system is utilized with an electronic-trip circuit breaker that offers a ground-fault option (alarm or trip), a neutral CT is necessary. For precise sensing, neutral current must be taken into account along with phase current.
  • When utilised on a 4-wire system, the GFM modules additionally need a neutral CT.

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