Current Transformers: Working Principle and Classification


Current transformers are essential components of a power system. A transformer is an equipment that uses electromagnetic induction to transfer electric power from one circuit while maintaining its frequency. The main function of these electronic devices is to step up and step down the voltage. This guide will cover transformer basics, such as its working principle and classifications.  

What is a Current Transformer?

Current transformers are ‘instrumentation’ transformers used to reduce high values of current to lower values. In electrical engineering, quantities with high values are supposed to be calculated frequently, and that’s what’s current transformers do. They do so by isolating the instrumentation devices from high voltages and currents.  

Why Are Current Transformers Used?

Current transformers are mainly used to measure current and survey the operation of the grid. These devices are justified for the following reasons:

  • They isolate the protection system from high voltages and currents, leading to a drop in the size and cost of protection equipment. 
  • Since these devices’ output is standard (1A or 5A), there’s no need for protective gear. 

The Working Principle of a Current Transformer

The operating principle of current transformers is based on electromagnetic induction law. The external network’s voltage is supplied to the primary power winding and overcomes its total resistance with a specific number of turns. Consequently, there’s the appearance of a magnetic flux trapped by the magnetic circuit around the coil that is perpendicular to the current direction. Therefore, the electric current lost during the conversion is insignificant.

The electromotive force helps stimulate the magnetic flux at the meeting point of the secondary winding switches when placed perpendicularly in a series. A current flux occurs under the control of EMF, which is required to resolve coil impedance and output load. At the secondary winding source, simultaneously, a voltage decrease is recorded.

Classification of Current Transformers

Current transformers are classified depending on the function or build. Here’s a brief understanding:

Function-Based Classification

Measurement CT refers to current transformers employed for metering and indicating circuits. One of the important features of these devices is that their saturation point is low. Therefore, when there’s a fault, the measurement device linked to it won’t be destroyed by the secondary currency since the core will saturate.  

Protection CT refers to current transformers used together with protective devices. These types of transformers are used to sense fault currents in the system and communicate it to relays. They run on currents values higher than the rated values. Therefore, their saturation point is high. 

Construction Based Classification

  • Wound Current Transformers 

These transformers have their primary winding linked physically in series with the conductor that carries the measured current flowing in the circuit.

  • Toroidal Current Transformer

They are also called window transformers. These current transformers don’t have a primary winding. The line with the current flowing in the network is threaded through a window in the device. Those with a “split core” can be opened, installed, and closed while the circuit is still connected. 

  • Bar-Type Current Transformers

These transformers use the actual cable or bus-bar of the main circuit like the primary winding, which is equal to a single turn. They’re insulated from the high operating voltage. 

Important Tips When Dealing with Licensed Professionals

When hiring an electrician, check if they have all the necessary qualifications and a bit of experience. An experienced electrician will ensure you get the highest standard of workmanship and find the best electrical solutions, including requirements for current transformers.

You can rely on your close friends’ recommendations when hiring a professional electrician to avoid spending too much time interviewing a long list of potential applicants. 

In most cases, an inexperienced electrician is likely to spend more time diagnosing and repairing an issue than an experienced one. Dealing with a high-quality contractor may see your repair completed within a day and done right the first time.