What is the Working Principle of Current Transformer?

Medium-Voltage (MV) and High-Voltage (HV) systems use current transformers (CT) to provide a representation of electrical output to protection units, relays and metering equipment. These are built to provide a secondary winding current that is proportional to the primary winding current. In our previous article, we covered the fundamentals of current transformer construction and operation. Here we shall discuss some common current transformer configurations and their uses.

Current Transformer:

A transformer is a device that uses electromagnetic induction to move electrical current from one circuit to another without changing the frequency of the current. There are two basic kinds of transformers, the core type and the shell type. The two most common functions are voltage stepping down and stepping up. Elmex Controls Pvt. Ltd. is one of the most prominent suppliers of Ring Type Current Transformer in India.

As these transformers can detect voltage, current, power, and energy, they are used for quantitative measurements. Multiple devices employ them in tandem, including ammeters, voltmeters, wattmeters, and energy metres. There are two primary types of transformers, and they are potential transformers and current transformers. In this article, we'll dive deep into the topic of current transformers, covering everything from their description and classification to its various uses.

An AC (Alternating Current) is generated across the secondary winding of a current transformer, an instrument transformer. Due to its configuration in series with the circuit used to measure electricity's many characteristics, this is also being presented as a series transformer. Here, the secondary section's current is linked to that of the first. These are typically used to convert high-voltage inputs into lower-voltage outputs.

Working Principle:

When compared to the operation of a transformer that measures voltage in accordance with standards, the current transformer’s operating principle is somewhat distinct. The regular voltage transformer has two windings, just like this one does, but it also has two windings. When AC is supplied all the way through the primary section, it allows for the production of alternating magnetic flux, which in turn causes AC to be stimulated all the way through the secondary section. With this particular configuration, the load impedance is a very low value. As a result, this apparatus functions in conditions that involve a short circuit. Therefore, the current that flows through the secondary section is determined by the current that flows through the primary winding; it is not determined by the impedance of the load.

Types of Current Transformers:

There are many different types of current transformers available on the market. In this piece, we will discuss the primary categories that fall under this category. There are four primary types of current transformers, and each of these types has the following components:

• Indoor CT:

Current transformers are frequently employed in low-voltage circuits inside buildings. You can categorise them as windows, wounds, or bars. Like the fundamental type, the wound type has two windings—a primary and a secondary. Their high precision and primary ampere turn values make them ideal for usage in summing cases.

• Outdoor CT:

High voltage circuits, such as those found in switchyards and substations, sometimes need the use of current transformers installed outdoors. You may get these in either an SF6 gas-insulated or oil-filled version. When compared to oil-filled transformers, the SF6 insulated type of current transformer is far more portable.

• Bushing CT:

This sort of transformer is comparable to the bar form in that the core as well as secondary section are located in the primary conductor section of the transformer. It is possible to twist the secondary portion of the transformer into a circular shape, which is very different from the annular shape of the core. It is fastened to the high-voltage bushing that is located across the circuit breakers, switchgear, as well as power transformers, unless generators are present.

• Portable CT:

These transformers are typically used in power analyzers and other devices that require a high degree of accuracy. Multiple varieties of these transformers are commercially available. The range of available current transformer types for portable use is roughly between 1000 A and 1500 A.

• Standard Measuring CT:

With the help of ammeters and standard measuring current transformers, excessive currents can be safely regulated and reduced to a more manageable 1 A or 5 A. The V. A condition of the current transformer is matched with the V. A reading from the ammeter or other measuring device.

For this purpose, a 200/5 FSD current transformer and a zero-to-200 A spinning iron ammeter are used. Once the current transformer's output reaches 5 A, the ammeter is recognised and FSD is triggered.

• Metering CT:

Continuously sensing the current and functioning exactly within the rated current level are two essential functions of a metering current transformer. The accuracy class specifies the maximum allowable phase displacement and the maximum allowable current inaccuracy. Classes of accuracy range from 0.1 to 1, with 0.5 and 0.1 being the most common.

Phase shift causes inaccurate readings in power, energy, and power quality metres. However, the phase error in the current can now be calibrated out thanks to the advent of electronic energy and power metres.

• Protection CT:

The overcurrent behaviour of a protective current transformer is simulated. Because of this, even in high-current circumstances, the specific relays can exert exact control over the fault currents. During a fault condition, the secondary output is used to power a protection relay that cuts power to the faulty portion of the energy circuit. This CT's core material typically consists of silicon steel, which has a high saturation level. Along with Ring Type CT, the Protection CT is also one of the widely used current transformers in India.

• Wound Primary CT:

In this configuration, the primary winding and the conductor that governs the current are essentially connected in series. The main portion is constructed inside the gadget and has one rotation. To sense currents between 1A and 100A, wire wrapped CT types can be used.

• Bus Bar CT:

In this configuration, a single turn of the basic circuit's bus bar serves as the principal section. Therefore, there are only secondary windings in the bar transformer. Insulation between the primary circuit as well as the ground is provided by the body of the current transformer itself. These transformers can operate at the greatest transmission voltages thanks to their use of porcelain bushings and oil insulation.

• Ring Type CT:

In this configuration, the secondary coil of the current transformer has only a low rating of insulation and is placed over a bus bar or an insulated wire. It is recommended to pass the primary wire through the ring more than once in order to reach non-standard levels or for other specific objectives. The windings are often constructed of copper, and the core is typically laminated silicon steel. Elmex Controls Pvt. Ltd. is an excellent as well as the most trusted manufacturer and supplier of Ring Type CT in India.

Elmex, a well-known electrical wire termination technology company in India and a notable manufacturer of terminal blocks, was formed in 1963. Elmex's production plant is ISO 9001:2015 certified.