Current Transformers - A Definitive Guide!

What Is A Current Transformer?

A current transformer is a device that helps in measuring alternating current. It converts high primary current to a low, safer level secondary current that is measurable.

A current transformer can be used to multiply or reduce alternating current. A typical current transformer is a series-connected electromagnetic device comprising an iron core, electrical grade lamination and copper wound coil.

Why Is The Current Transformer Used?

A current transformer is typically used when the voltage or current is too high to directly measure. In this case, the CT performs its usual function of lowering the current.

The resultant lower or secondary current is suitable for processing into electronic equipment. In this way, by reducing the high current into an equivalent lower current, the actual electrical current flowing in an AC transmission line can be safely monitored. Hence, a current transformer is an indispensable part of an electronic power system. 

CTs are also used to protect delicate measuring equipment. The current in the CT can be made significantly smaller than the current in the primary circuit being measured by increasing the number of windings.

How Does It Work?

The current transformer follows the principle of Faraday’s law of Electro-Magnetic Induction.
A current transformer is also referred to as low tension current transformer (LTCT) because it takes a low voltage and high current as input.

The CT comprises 2 windings – primary and secondary. The primary is wired to the power source, while the secondary is wired to the power distribution end. These two windings are twisted together around a single sealed magnetic iron circuit called the core, rather than being in electrical contact with each other.

The AC current flows through the primary winding. As per Faraday’s law, an equivalent magnetic flux is produced. This magnetic flux strikes the secondary winding and generates a proportionate alternating current.

The primary current flow is determined by a separate external load, while the secondary current is rated at 1A or 5A, making it suitable for measurement instruments.

Types Of Current Transformers

Current transformers are grouped into numerous types based on characteristics such as their intended purpose, circuit voltage, mounting manner, and so on.

1. Indoor Current Transformers

Wound type, bar type, and window type transformers are the most common types of transformers used in low voltage circuits. Wound-type transformers have both primary and secondary windings, just like regular transformers

2. Outside Current Transformers

These are typically utilized for circuits with substantially greater voltages, such as switchyards and substations. These CTs are insulated with either oil or SF6 gas.

3. Bar Current Transformer

The primary winding of a bar-type current transformer is the actual cable or busbar of the primary circuit, which is equivalent to one turn. Higher-insulation bar types are available and are typically attached to the current-carrying device.

4. Wounds Current Transformer

This type of transformer has a primary winding on the core that is greater than one full round. The wound current transformer’s primary and secondary windings are separated by one or more turns encircling the core and are isolated from one another. Taps on the secondary winding allow them to be built as multi-ratio CTs. The wound type performs admirably across a broad working range.

The Difference Between CT And PT

1. Connection

The circuit is connected in series with a current transformer, allowing full line current to flow through the winding. Whereas a potential transformer is connected in parallel with the circuit, resulting in full line voltage across the winding.

2. Function

A current transformer decreases a high current to a safe and measurable level. Large primary currents are converted to little 1A/5A currents that can be measured with an ammeter.

Potential transformer measures and decreases high voltage values to lower ones. The high voltage is converted to a typical secondary voltage of 100V or less.

3. Windings

The primary winding of a current transformer has fewer turns and carries the current to be measured. The main/primary winding of a potential transformer has several turns and conveys the voltage to be measured.

The secondary winding of a current transformer has a significant number of turns on the secondary side and is connected to the instrument’s current winding. The secondary winding of a potential transformer contains a modest no. of turns on the other side and is connected to the device.

4. Value Of The Input

The input value of a current transformer is constant current, whereas the input value of a potential current transformer is constant voltage.

5. Winding Range

The range of a current transformer is 1A or 5A, while that of a potential transformer is 110V.

Factors to Consider While Selecting CT

1. Ascertain that the current transformer’s primary range encompasses the application need.

2. It’s crucial to know which outputs your metering equipment can handle.
 
3. The CT you use must be able to fit around the conductor you’re going to measure. At the same time, over sizing a CT to accommodate a small conductor may not be cost-effective or space-efficient.
 
4. Determine the typical application of a current transformer that is most appropriate for its intended use.
 
5. Determine the rated power, which specifies a current transformer’s ability to “push” the secondary current through a load within error limits

Applications Of Current Transformers

  • These transformers are used to monitor electric power in power plants, factories, grid stations, and industrial control rooms for metering and evaluating current flow in circuits and protection.

  • It is utilized throughout the frequency response cycle to calculate harmonics and monitor power quality.

  • It can be used in substation converters, HVDC projects, AC filters, and DC filters.

  • In high-voltage mains and substations, as a protection device.

  • It is utilised in capacitive banks as an integrated protection module.

  • CTs with tiny footprints are used to detect current overloads, identify faults, and separate current feedback signals on printed circuit boards. 

  • In many three-phase systems, larger devices are utilised to measure current or voltage.

Beemet is one of the top manufacturers of Current Transformers in India. Beemet has helped define a unique market for quality analog and digital measuring instruments used in the manufacturing and service industry.

In addition to CTs, we also manufacture other electrical measuring instruments such as ammeters, shunts, voltmeters, panel meters, etc.