Understanding The Types and Working Principles of Current Transformers

Understanding The Types and Working Principles of Current Transformers

Current transformers play a crucial role in power systems. But what are these devices exactly? What is their working principle? Also, how many types of current transformer are available? If you cannot answer these questions, you must read this comprehensive blog post by our expert team. It will help you learn what is a current transformer, its types, and its working principles in detail. So, start reading below. 

What is a Current Transformer?

Current transformers measure electric current in a power system by converting high current into a smaller and more manageable current value. These transformers transform current into a value that can be safely measured. Usually, current transformers are used in electricity metering. They are also used in protective relaying and for other applications needing accurate, current measurements. These instruments aim to step down the secondary current and monitor the primary current. 

What is the Use of Current Transformer?

Current transformers are used for measurement, protection, and control in electrical substations with high-voltage and electrical grid. They are installed inside switchgear, in apparatus bushings, and in free-standing outdoor current transformers. 

Advantages of Current Transformer

Below are some of the most significant advantages of current transformer

High Precision

Current transformers are highly appreciated for their exceptional accuracy in measuring current. These devices provide precise and reliable readings under varying load conditions. 


Another advantage of a current transformer is its versatility. It is used for various applications, from monitoring power consumption to protecting electrical equipment in renewable energy systems and industrial processes. 

Safety Isolation

These devices offer electrical isolation between the secondary and primary circuits. This isolation ensures safety and prevents high currents from reaching monitoring and measuring equipment. 


Current transformers have a robust design, making them long-lasting and able to withstand harsh environmental conditions. 

Easy Installation

These devices are also known for their easy installation and simple operation. They are also easy to maintain. 

How Does a Current Transformer Work?

How does a current transformer work? A current transformer uses a different operating principle than a typical voltage transformer. It has two windings, the same as a voltage transformer. AC is produced in the secondary winding. When this induced AC is supplied to the primary winding, it produces an alternating magnetic flux. The load impedance in these devices is relatively low. Therefore, it works in a short circuit. The current in the secondary winding depends on the primary winding current. However, it is independent of the load impedance. 

Effect of Open Secondary Windings of a Current Transformer

The secondary winding of a current transformer connects to its burden under normal operating conditions and is permanently closed. When the current flows through the primary windings, it also flows through the secondary ones, turning the amperes of each equally and oppositely. The secondary turns are 1% less than the primary ones and 2% less than the difference in the magnetizing core. So, if the secondary winding is open and current flows through the primary ones, then the secondary current does not let demagnetizing of flux happen. 

The absence of the counter-ampere turns of the secondary winding results in an unopposed primary MMF that sets up an abnormally high flux in the core. This high flux results in subsequent heating. It also induces high voltage across the secondary terminal. This voltage causes the insulation breakdown and loss of accuracy. Therefore, the secondary winding of a CT must never be opened when the primary winding is carrying the current. 

Types of Current Transformer

Below are the different types of current transformer

Indoor Current Transformer

Low-voltage circuits usually use Indoor CTs. The first type of CTs are the indoor current transformers, which are further divided into the following types. 

Wound Type

The wound-type indoor CTs have two different windings depending on their design. These windings are present on a magnetic steel core. Wound-type CTs are most similar to normal CTs. 

Bar Type

Bar-type CTs have their primary and secondary cores made of bars. Their major bar is a crucial component. Bar CT’s magnetism in its core can reduce accuracy. These transformers do not have primary winding. So, a window-type transformer can be placed near the primary conductor. 

Toroidal Type

Also known as window-type, these CTs have a conductor that carries the primary load current through the core’s opening instead of the primary winding. 

Outdoor Current Transformer

Another type of CT is the outdoor CT. These are generally used for high-voltage circuits like switchyards and substations. Outdoor CTs are of the following two types. 

Gas-Insulated Type

Gas-insulated CTs insulate using a gas like SF6. They are typically used in power systems that incorporate high voltage and are sealed to prevent them from environmental conditions. Gas-insulated CTs have a compact design, which is suitable for systems with space limitations. 

Oil-Filled Type

As the name suggests, these CTs are filled with oil. They are usually used in high-voltage power systems. The oil in these CTs provides insulation and cooling. It also protects the CT from dust, moisture, and other environmental conditions. 

Bushing Current Transformer

Next on the list of types of current transformers are the bushing current transformers (CTs). These CTs are comparable to the bar type as they have their secondary and core placed near the primary conductor. The secondary wind of these CTs can be made into a circular core. They are usually linked to high-voltage bushing in generators, power transformers, switchgear, and circuit breakers

Portable Current Transformer

Portable CTs are high-precision transformers used with high-precision ammeters and power analysers. They measure a current between 1000 and 1500 A. These CTs are available in different varieties and are mainly used to isolate measurement instruments from high-voltage circuits.


Summing Up

Reading this post must have informed you all about current transformers, starting from what is a current transformer to its various types. It plays an essential role in power systems. However, they can function their best only when you ensure they are high quality. That is why you must check out Schneider eShop. We offer all kinds of premium quality current transformers. Also, shopping online from us allows you to save time and experience convenience. 

Frequently Asked Questions

Q1. Why is Transformer Rating in kVA?

Ans: Transformer ratings are in kVA because they are independent of the power factor. Transformers are designed to handle the total apparent power. The apparent power includes both real power and reactive power. So, transformer ratings are in kVA to provide a consistent capacity measurement without the need to take varying power factors into account. 

Q2. What Current do Transformers Need?

Ans: Transformers use alternating current (AC) to operate. They use AC because they rely on the electromagnetic induction principle. So, AC provides the changing magnetic field required for this principle, allowing the transformer to transfer energy between the primary and secondary windings to step up and step down voltage as needed. 

Q3. Can Current Transformers Measure Direct Current?

Ans: No, current transformers cannot measure DC. This is because they work on the principle of electromagnetic induction and AC provides the changing magnetic field for this principle. DC is a constant flow of electricity. So, it does not create a varying magnetic field for electromagnetic induction.