Current Transformers

A current transformer (CT) is 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.

A current transformer (CT) operates based on the principle of electromagnetic induction. Current transformers consist 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) flows through the primary winding, it generates a magnetic field in the core. This magnetic field induces a proportional voltage in the secondary winding, which can be measured using instruments or protective relays. The fundamental principle is that the CT steps 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, a standard current transformer (CT) 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 flows through the primary circuit, typically a conductor or power line passed through the CT's core. The primary current is the current that needs to be measured or monitored. CTs are designed to step down this primary current to a proportional, reduced secondary current. The secondary current is the output of the CT, and its magnitude is directly proportional to the primary current, as determined by the CT's transformation ratio. This step-down process ensures that the secondary current is safe and manageable for measurement or use in various applications, such as metering, protective relaying, and control systems.

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

1. Wound Current Transformers: These are the most common type of CTs. They consist of a primary winding, a secondary winding, and a magnetic core. The primary winding is connected in series with the circuit carrying the primary current to be measured. Wound Current Transformers are versatile and widely used in various applications, including metering and protective relaying.
2. Bar-Type Current Transformers: Bar-type CTs consist of a solid metal bar, usually made of copper or aluminum, with a secondary winding wrapped around it. They are often used in high-current applications and are known for their robust construction and reliability.
3. Bushings or Window-Type Current Transformers: These CTs are designed to be installed on high-voltage equipment, such as transformers and circuit breakers. They have a hole or window through which the primary conductor is passed. This design makes them suitable for retrofitting existing equipment without interrupting power flow.
4. Resin-Cast Current Transformers: Resin-cast CTs have their windings encapsulated in epoxy resin, protecting against environmental factors like moisture and dust. They are commonly used in outdoor and high-voltage applications where durability and reliability are crucial.
5. Low-VA (Volt-Ampere) CTs: These current transformers are designed for applications where a low burden (connected load) is required, such as protection relays. They have a high turn ratio to provide a small secondary current, reducing the burden on the measuring device.
6. Split-Core Current Transformers: Split-core current transformers have a hinged design, allowing them to be easily installed around existing conductors without disconnecting the circuit. They are commonly used in retrofit and maintenance applications.
7. Zero-Sequence Current Transformers: Also known as ground CTs, these are used to measure zero-sequence currents in three-phase power systems. They are crucial for ground fault protection and detecting imbalances in the system.
8. Summation CTs: These CTs are used to sum the currents from multiple primary conductors into a single secondary winding. They are often used in applications where the total current of several parallel conductors needs to be measured accurately.
9. Protection CTs: Designed for protecting relaying applications, where fast and precise detection of overcurrent and fault conditions is essential. These current transformers meet strict accuracy and performance standards.
10. High-Frequency Current Transformers: Designed to accurately measure high-frequency currents, often encountered in power electronics and high-frequency applications.