Current Transformers

Current transformers are an electrical device that safely and accurately measures electrical current in power systems and circuits. It operates on the principle of electromagnetic induction, reducing high primary currents to proportional, lower secondary currents. CTs provide:

• Electrical isolation between the primary (high-current) and secondary (low-current) circuits.
• Making them essential for tasks like metering energy consumption.
• Protective relaying.
• Monitoring current in industrial processes.

They ensure equipment and personnel safety while delivering precise current measurements, serving as a fundamental tool in electrical engineering and power distribution.

Current Transformers worked based on the principle of electromagnetic induction. Current transformers consisted of a primary winding, typically a single turn or a few turns of a conductor, and a secondary winding, with many more turns, wound around a magnetic core.

When an alternating current (AC) flowed through the primary winding, it generated a magnetic field in the core. This magnetic field induced a proportional voltage in the secondary winding, which could be measured using instruments or protective relays. The fundamental principle was that the CT stepped down the current, making it safe and convenient to measure while maintaining a proportional relationship between the primary and secondary currents, allowing for precise current measurements and protective actions in electrical systems.

No, standard current transformers (CTs) designed for measuring alternating current (AC) cannot be used for direct current (DC) measurements. CTs operate based on the principle of electromagnetic induction, which relies on the changing magnetic field produced by AC currents to induce voltage in the secondary winding. In contrast, DC currents generate a constant magnetic field that does not change in direction or magnitude. As a result, a standard AC CT cannot accurately measure DC currents since there is no changing magnetic field to induce voltage in the secondary winding. To measure DC currents accurately, specialized DC current transducers (DCCTs) are used, designed with different core materials and construction to work effectively with constant DC currents.

In a current transformer (CT), the primary current flowed through the primary circuit, typically a conductor or power line passed through the CT’s core. The primary current was the current that needed to be measured or monitored. CTs were designed to step down this primary current to a proportional, reduced secondary current. The secondary current was the output of the CT, and its magnitude was directly proportional to the primary current, as determined by the CT’s transformation ratio. This step-down process ensured that the secondary current was safe and manageable for measurement or use in various applications, such as metering, protective relaying, and control systems.

The specific choice of current transformers depended on the application’s requirements, including current magnitude, accuracy, installation constraints, and environmental conditions. Selecting the correct type of CT was crucial to ensure accurate current measurement and protection in electrical systems.

 

Wound Current Transformers

 

These were the most common type of CTs. They consisted of a primary winding, a secondary winding, and a magnetic core. The primary winding was connected in series with the circuit carrying the primary current to be measured. Wound Current Transformers were versatile and widely used in various applications, including metering and protective relaying.

 

Bar-Type Current Transformers

 

Bar-type CTs consisted of a solid metal bar, usually made of copper or aluminum, with a secondary winding wrapped around it. They were often used in high-current applications and were known for their robust construction and reliability.

 

Bushings or Window-Type Current Transformers

 

These CTs were designed to be installed on high-voltage equipment, such as transformers and circuit breakers. They had a hole or window through which the primary conductor was passed. This design made them suitable for retrofitting existing equipment without interrupting power flow.

 

Resin-Cast Current Transformers

 

Resin-cast CTs had their windings encapsulated in epoxy resin, protecting against environmental factors like moisture and dust. They were commonly used in outdoor and high-voltage applications where durability and reliability were crucial.

 

Low-VA (Volt-Ampere) CTs

 

These current transformers were designed for applications where a low burden (connected load) was required, such as protection relays. They had a high turn ratio to provide a small secondary current, reducing the burden on the measuring device.

 

Split-Core Current Transformers

 

Split-core current transformers had a hinged design, allowing them to be easily installed around existing conductors without disconnecting the circuit. They were commonly used in retrofit and maintenance applications.

 

Zero-Sequence Current Transformers

 

Also known as ground CTs, these were used to measure zero-sequence currents in three-phase power systems. They were crucial for ground fault protection and detecting imbalances in the system.

 

Summation CTs

 

These CTs were used to sum the currents from multiple primary conductors into a single secondary winding. They were often used in applications where the total current of several parallel conductors needed to be measured accurately.

 

Protection CTs

 

Designed for protecting relaying applications, where fast and precise detection of overcurrent and fault conditions was essential. These current transformers met strict accuracy and performance standards.

 

High-Frequency Current Transformers

 

Designed to accurately measure high-frequency currents, often encountered in power electronics and high-frequency applications.