A current transformer (CT) is a type of instrument transformer used to measure alternating current (AC) in electrical power systems. It is designed to produce a secondary current proportional to the primary current passing through it, thereby allowing for accurate measurement and monitoring of high-current circuits without directly connecting measuring instruments to the high-current conductors.

Here's how a current transformer typically works:

1. Primary Circuit: The primary circuit of a current transformer is the high-current circuit being measured. It consists of one or more turns of a conductor (typically a busbar or power cable) through which the current flows. The primary circuit is connected to the primary winding of the current transformer.

2. Primary Winding: The primary winding of the current transformer is connected in series with the primary circuit. It consists of a few turns of a heavy-duty conductor to carry the high primary current. The number of turns in the primary winding determines the transformation ratio of the current transformer.

3. Core: The primary winding is wound around a magnetic core made of a highly permeable material, such as silicon steel. The core serves to concentrate the magnetic flux generated by the primary current, thereby inducing a proportional voltage in the secondary winding.

4. Secondary Winding: The secondary winding of the current transformer is connected to the secondary circuit, which typically consists of measuring instruments, protective relays, or other monitoring devices. The secondary winding has a large number of turns relative to the primary winding, resulting in a secondary current that is proportional to the primary current but scaled down in magnitude according to the transformation ratio.

5. Output: The secondary current produced by the current transformer is used as the input signal for measuring instruments or protective relays. The secondary current is typically standardized at a specific value, such as 5 amperes or 1 ampere, for compatibility with standard measuring instruments and relays.

Current transformers offer several advantages for measuring high currents in power systems, including:

• Electrical isolation between the primary circuit and the secondary circuit, ensuring safety for measuring instruments and operators.
• Accuracy and linearity over a wide range of primary currents and operating conditions.
• Ability to measure high currents without requiring high-current measuring instruments, which may be expensive or impractical.

Current transformers are commonly used in electrical substations, power distribution systems, industrial plants, and commercial buildings for metering, monitoring, and protection purposes. They play a crucial role in ensuring the reliability, safety, and efficiency of electrical power systems.