What is a Current Transformer or CT?

A Current Transformer (CT) is an electrical device primarily used in power transmission networks for sampling high-voltage currents. These transformers have the capability to convert high and powerful currents into lower levels while isolating the networks. As they transmit high-power currents, the use of current transformers for managing and controlling these currents is crucial.

Recognized as a fundamental component in measuring currents in industrial electrical panels, this device plays a vital role in protecting and ensuring the safety of isolated power system components. Additionally, these equipments ensure optimal performance and safety of power systems.

The importance and application of CTs in network protection and current measurement are significant. CTs connect their outputs to measuring devices such as megohmmeters and ammeters, enhancing the effectiveness and accuracy of measurement information.

Current transformers with measurement cores must accurately match the nominal and normal current limits of the network to correctly interpret measurement data. This assurance ensures that the obtained information from the currents is accurate and reliable.

For the protection and control of power networks, the outputs of current transformers are connected to protective relays. These relays require accurate current information and are responsible for tasks such as creating ground connections, controlling current, and establishing distance in the network. This effective interaction enhances the security and efficiency of power networks.

Review of the Structure and Components of Current Transformers

Every electrical device has a unique structure, and a Current Transformer (CT) is no exception, possessing its own distinctive structure. The CT comes in two different core structures: Core-Balanced (Korbala) and Core-Permeable (Korpaein), where the term “core” refers to the central component. In the Core-Balanced structure, the current core is positioned at the top and includes the following elements:

1. Secondary winding

2. Core

3. Outer clasp

4. Insulating material

5. Low-pressure terminals

6. High-pressure terminals

How is a Current Transformer Installed?

These transformers are installed in two forms: internal and external. External installation is done in AIS (Air Insulated Switchgear) substations, while internal installation is carried out in GIS (Gas Insulated Switchgear) substations. During the installation process, the secondary CT circuit is connected to the ground. This connection must occur at a single point, as otherwise, without a load on the secondary transformer, the risk of explosion exists.

During the installation and connection of these transformers, observing necessary precautions is vital. Ensure that the direction of the electrical current is correctly maintained, and the current is sent towards the measurement and protective equipment. Incorrect orientation of the CT may lead to incorrect power measurement on the meter.

Exploration into the Diverse World of Current Transformers

After familiarizing ourselves with the definition and concept of current transformers, let’s delve into the differentiation of various parts and types of this crucial equipment in the electrical domain.

Types of Dry and Oil-Filled Current Transformers:

Current transformers, based on their construction, can be categorized into two groups: dry and oil-filled. This classification is based on the type of environment surrounding the transformer core.

Instrument Transformers:

Measurement transformers or CTs, as advanced tools in the electrical field, garner attention. Primarily used for measuring voltage and current in electrical equipment, especially in cases where the measurement of higher and more complex voltages is required, these transformers have a structure consisting of two cores made from primary and secondary windings.

Protective Current Transformers:

Protective current transformers serve the purpose of safeguarding power sources, equipment, and relays. If the current exceeds 10 times the nominal current, these transformers rapidly and instantly become active. Various types of protective transformers encompass transformation ratio, accuracy class, permissible voltage, magnetic curve, and dividing power. Each of these protective transformers plays fundamental and essential roles in preventing electrical issues and accidents, operating with precision and high efficiency.

Intervention Current Transformer:

Advanced intervention current transformer technology is employed to reduce current measurement to a controllable level and is widely used in precise current measurement and circuit protection.

In the secondary section of this transformer, alternating current corresponds directly to the primary alternating current. This type of transformer is well-suited for situations requiring very high and accurate current measurements. The intervention technology, by applying principles of adaptation and coordination with various current conditions, enables protection and improved control within the measurement range.

Medium and Low-Pressure Current Transformers:

These types of transformers are produced in open spaces or using different wire coils to convert input currents into standard values.

High-Core Current Transformer:

The high-core current transformer is an advanced type of transformer that, by having the core at the top of its structure, brings about special performance. In this type of transformer, the current path passes through a complex ring inside which the secondary winding wire is wound. The winding wire inside this transformer is placed within an oil-insulated insulation, enhancing its performance and stability.

Low-Core Current Transformer:

In low-core current transformers, the winding is placed inside a bushing, and the lower part of the transformer is permanently housed in an oil-filled tank. This winding is coated with insulating paper and immersed in an oil environment. The circular windings around this winding become complex, and as the current passes through it, the transformer heats up. This type of transformer is typically subject to sudden vibrations during operation. Therefore, it is not recommended for use in earthquake-prone areas.

Core-Balanced CT Transformer:

Core-Balanced CT transformers are intelligent tools used to detect leakage faults in power grids. In single-phase circuits, the phase and neutral wires pass through the Core-Balanced CT, while in three-phase circuits, all phase wires along with the neutral pass through this transformer. This unique feature makes these transformers highly efficient for accurate current sampling, with a transformer ratio of 1 to 1000.

A prominent advantage of current transformers is their ability to directly send alarms from problematic sections. This feature enables quick and precise detection and resolution of errors. By utilizing Core-Balanced CT transformers, preventing serious damage to the power grid becomes a primary priority.

Understanding the Specifications and Characteristics of Current Transformers

Current transformers possess unique features that are highly important and require careful consideration. These characteristics include:

– Rated Current (Primary Current): The maximum current that a current transformer can handle.

– Rated Voltage: The maximum working voltage that the current transformer can withstand.

– Accuracy Class: The precision of current measurement by the transformer, varying based on different applications.

– Magnetization Curve: A curve indicating the relationship between primary and secondary current in the presence of a magnetic field.

– Power Rating: The ability of the transformer to transmit current considering various characteristics.

– Transformation Ratio: The ratio of primary current and voltage to secondary current and voltage.

Selecting a Current Transformer:

For optimal selection of a current transformer, keep the following in mind:

– System Specifications: Information such as the maximum system voltage, system frequency, rated current, and type of system short circuit.

– Intended Use: Determine the type of transformer based on whether it is for protection or measurement purposes.

– Application and Environment: Environmental conditions such as installation altitude, maximum temperature, etc.

– Transformer Type: Identify the type of transformer, including current transformers and voltage transformers.

– Accuracy Class: Determine the required measurement accuracy based on the transformer’s application.

– Environmental Conditions: Consider environmental conditions such as installation altitude and maximum temperature.

Ways to Choose the Best Current Transformer

By considering the mentioned factors, selecting the best current transformer for your specific needs will be simpler.

Are you looking to find the best current transformers from reliable sources? Shaili Electric proudly offers all current transformer products from prominent brands such as Schneider Electric. These brands, due to their outstanding quality and reputation in the electrical industry, stand as an unparalleled choice for engineers and users in this field.

To purchase and choose the best current transformer, visit the Shaili Electric website and explore our product catalog. Here, you can benefit from the diversity and various types of current transformers and easily select the suitable product for your needs.


In this article, we delved into a detailed examination of the most specialized current transformers. These devices play a crucial role not only in measuring current but also in protecting equipment and relays. The unique features and structures of these transformers require your precise understanding.

These transformers are categorized into various types, including precision instruments, protective transformers, high-core, low-core, low-pressure, etc. For a comprehensive understanding of these vital equipment, it is recommended to familiarize yourself precisely with the features and applications of each category.

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