Current transformer is a measurement and protection device used in power systems. Its main function is to convert high voltage or high current into standard low voltage and small current signals in proportion, in order to facilitate the use of measuring instruments, relay protection devices, and automation control equipment. It works based on the principle of electromagnetic induction and is usually composed of a primary winding, a secondary winding, an iron core, and an insulation structure. The primary winding is connected in series in the tested circuit, and the load current flows through it; Connect the secondary winding to measurement or protection equipment, and output standardized current signals (commonly 5A or 1A). Due to the fact that the secondary side must be reliably grounded and open circuits are not allowed, otherwise high voltage may be generated, endangering equipment and personal safety. Therefore, strict adherence to operating procedures is required during operation.

The core characteristics of current transformers include transformation accuracy, accuracy level, rated capacity, and saturation characteristics. According to different purposes, it can be divided into two categories: measuring and protective. Measuring current transformers require high accuracy within the normal working range to ensure the accuracy of energy measurement and monitoring data; The protective current transformer needs to be able to accurately reflect the fault current under abnormal working conditions such as short circuits, and support rapid action of relay protection devices. In addition, according to the installation method, it can be divided into various types such as through type, pillar type, sleeve type, and portable, which are suitable for different scenarios such as substations, distribution cabinets, and generator sets.

In practical applications, the selection of current transformers should comprehensively consider factors such as system voltage level, load current, short-circuit current level, accuracy level, and secondary circuit load. At the same time, in order to avoid transmission distortion caused by magnetic saturation, especially in high short-circuit current situations, products with sufficient dynamic thermal stability and anti saturation ability should be selected. The development of modern smart grids has also promoted the application of electronic current transformers (such as optical current transformers). These new types of transformers utilize Faraday magneto-optical effect or Roche coil principle, and have advantages such as wide bandwidth, no magnetic saturation, small size, and insulation performance. They are gradually replacing traditional electromagnetic products and playing a key role in digital substations.