A Complete Guide to Current Transformer

What is Current Transformer? A Current Transformer (CT) is a transformer…

What is Current Transformer?

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A Current Transformer (CT) is a transformer used to measure the current flowing through high current circuits.

It is connected in series with the circuit whose current is to be measured and works by producing an accurately proportioned reduced current, which can then be easily measured by an ammeter.

The primary winding of the CT has only one turn or a few turns and it carries the full load current of the circuit whereas, the secondary winding consists of many turns and carries only a small fraction of the primary current.

This small secondary current is proportional to the primary current.

The ratio between primary and secondary currents is known as transformation ratio and it remains constant over a wide range of load currents. By measuring this secondary output, it is possible to calculate the exact value of primary current in each phase.

Therefore, Current Transformers are widely used in electrical power systems for measuring, protection and control purposes.

Construction of Current Transformers

The construction of current transformers is relatively simple. A current transformer consists of a primary winding of the current transformer and a secondary winding.

The primary winding is connected in series with the line carrying the primary current flow, while the secondary winding has an output proportional to the primary current. 

This output is usually expressed as a number of turns in relation to the number of turns in the primary winding, which is known as the current ratio.

By making use of this ratio, it is possible to measure currents that are too large for direct measurement.

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The accuracy of these measurements relies on having a precise knowledge of both the primary and secondary winds, as well as their number of turns. With these elements in place, it is possible to accurately measure any given level of primary current with a ct.

Types of current transformers

There are three basic types of current transformers: wound, toroidal and bar.

1. A Wound Current Transformer has its primary winding connected directly in-line with the line carrying the current to be measured. The magnitude of the secondary current is dependent on the number of turns ratio of the transformer.

2. A Toroidal Current Transformer has no primary winding. Instead, the circuit carrying the current flows through a gap or opening in the toroidal transformer. Some varieties of this type of transformer have an aperture which can be opened and closed without having to disconnect the line it is connected to.

3. Bar-type Current Transformer – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.

Working of current transformers

A Clampon (also referred to as Clipon) is an amperemeter based on a current transformer. Its laminated core is crafted in a manner that allows it to be released by a slight press – thus, opening the transformer’s core.

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A busbar or feeder whose current measurement is required are inserted in the opened core and the trigger released to close the core.

The current carrying conductor or feeder acts as a single turn primary while the secondary winding is connected across the standard ampere-meter. 

Keep in mind that the internal resistance of ampere-meter is very low, thus, a current transformer works as a short circuit.

If for some reasons, the ampere meter is disconnected from the secondary windings, it is necessary to short circuit the secondary winding.

In other words, the secondary of the current transformer should be never left open under any circumstances. In short, it should be never operated at no-load condition when the current is flowing through the primary. Otherwise, it will be like operating a voltage transformer in short-circuit condition which leads to damage to the insulation as well as the chance of danger of electric shock to the operator.

Current transformer advantages and disadvantages

A current transformer (CT) is an instrument transformer used to measure alternating current. It produces a reduced voltage in its secondary coil, proportional to the current flowing through the primary winding.

The advantages of using a CT include increased safety due to reduced voltage levels, improved accuracy and resolution when measuring high currents and a wide range of uses including monitoring power lines, testing electrical equipment, and measuring electricity consumption.

One of the main disadvantages of CTs is their inability to accurately measure DC currents. Additionally, as CTs are usually installed outside or in hazardous areas, they must be regularly checked for signs of damage such as corrosion or overheating.

Although CTs may require more maintenance than other measurement devices, their precision and reliability make them a suitable choice for many applications that involve alternating current measurements.


Current transformer applications

CTs are commonly used to measure the current flow in high voltage circuits, or for monitoring and protection purposes. They can also be used for metering purposes, such as measuring total electricity consumption and cost information in domestic and commercial applications.

Furthermore, they can be used to monitor industrial processes, helping ensure safe operation of equipment and machinery. In short, CTs offer a range of applications where accurate measurement of current flow needs to be taken.

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