What is CT in Electrical? – Construction, Working | New Topic [2024]

In this note, we are going to learn about a topic called “What is CT in Electrical or Current Transformer?“, also we learn about its construction, working, specifications, and applications as well. Welcome to Poly Notes Hub, a leading destination for Engineering Notes.

Author Name: Arun Paul.

What is CT in Electrical?

A Current Transformer (CT) is a device used in electrical engineering to precisely measure alternating current (AC) by converting high currents to a smaller, more manageable level. It is made up of a primary winding that carries the current to be measured and a secondary winding that connects to a measuring device or relay. The secondary winding usually has many more turns than the primary winding, resulting in a corresponding current reduction. CTs are critical components in power systems for a variety of applications, including metering, protective relaying, and control. They provide precise current measurement and electrical isolation between high-voltage circuits and measuring instruments.

Construction of Current Transformer

Below we listed some important parts of CT –

• Gas Cushion: Some CTs include a gas cushion, typically sulfur hexafluoride (SF6), to offer insulation and to extinguish arcs that may arise during high-voltage faults. The gas cushion serves to reduce flashovers and preserve dielectric strength between the primary and secondary windings.
• Magnetic Core: The magnetic core is frequently composed of high-permeability materials such as silicon steel or nickel-iron alloys. It creates a low-reluctance channel for the magnetic flux produced by the primary current. The core’s design and material selection are critical for achieving precise current transformation while minimizing losses.
• Porcelain Insulator: Porcelain insulators are often utilized to provide electrical insulation and mechanical support for the primary and secondary windings of the CT. These insulators are engineered to endure high voltages and extreme temperatures, ensuring the CT’s dependability and lifespan.
• Secondary Terminal Box: The secondary terminal box houses the secondary winding and provides connectors for connecting the CT to measurement equipment or relays. It may also provide facilities for grounding and insulation coordination. The secondary terminal box is typically intended to protect the secondary winding and provide quick connections for external electronics.

Working Principle of CT

The functioning principle of a Current Transformer (CT) in electrical systems is based on electromagnetic induction. When alternating current (AC) travels through the CT’s primary winding, it creates a magnetic field around it. This magnetic field produces a proportionate voltage in the secondary winding in accordance with Faraday’s law of electromagnetic induction.

The magnetic flux in the core varies in proportion to the AC current in the primary winding. This fluctuating magnetic flux generates an electromotive force (EMF) in the secondary winding, resulting in a secondary current. The secondary current is directly proportional to the primary current, however it is often significantly smaller in magnitude due to the turn ratio between the primary and secondary winding.

The CT’s design ensures that the secondary current matches the primary current in magnitude and waveform. This enables precise measurement and monitoring of the primary current without directly connecting measuring instruments to the high-voltage circuit. CTs also provide electrical isolation between the primary and secondary circuits, which improves safety and prevents damage to measurement instruments.

Specifications of Current Transformers (CTs)

Below we listed some specifications of Current Transformers or CTs –

• Current Ratio: The primary current to secondary current ratio, usually represented as a percentage or fraction. As an example, a CT with a ratio of 100:5 indicates that the secondary winding will generate 5 amps for every 100 amps in the primary winding.
• Accuracy Class: Indicates the CT’s accuracy in percentage terms under given settings. Common accuracy classes are 0.1, 0.2, 0.5, 1.0, and 3.0, with lower values indicating more accuracy.
• Burden: The maximum load that can be supplied to the secondary winding while maintaining CT accuracy. It is often measured in volt-amperes (VA) or ohms.
• Frequency: The frequency range in which the CT is intended to operate. Most CTs are intended to operate at 50 or 60 Hertz, but specialist CTs may be developed at various frequencies.
• Insulation: Shows the highest voltage that the CT is capable of withstanding without breaking down between windings and between windings and the core. Class 0.6 kV, Class 1.2 kV, and Class 3.6 kV are examples of common insulation classes.
• Physical Dimensions: Dimensions of the CT include its length, breadth, and height, as well as the primary and secondary terminal sizes.
• Continuous Thermal Current: The highest continuous current that the CT is capable of handling without going over its temperature threshold.

Applications of Current Transformer

Below we listed some applications of current Transformer –

1. Metering and Billing
2. Protective Relaying
3. Power Quality Monitoring
4. Energy Management Systems
5. Fault Detection and Diagnosis
6. Load Monitoring and Control
7. Renewable Energy Systems
8. Industrial Automation and Process Control