In power generation, transformation, transmission, distribution, and consumption lines, currents vary widely, ranging from a few amperes to tens of thousands of amperes. For ease of measurement, protection, and control, it's necessary to convert these currents to a more uniform value. Furthermore, the voltage on these lines is generally quite high, making direct measurement extremely dangerous. Current transformers serve to transform current and provide electrical isolation.
For pointer-type ammeters, the secondary current of current transformers is mostly in the ampere range (e.g., 5A). For digital instruments, the sampled signal is typically in the milliampere range (0-5V, 4-20mA, etc.). Miniature current transformers have a secondary current in the milliampere range and primarily act as a bridge between the larger transformer and the sampler.
Miniature current transformers are also sometimes called "instrument current transformers." ("Instrument current transformers" also refers to high-ratio precision current transformers used in laboratories, generally to extend the range of instruments.)
Similar to transformers, current transformers operate based on the principle of electromagnetic induction; however, transformers transform voltage, while current transformers transform current. The winding of a current transformer connected to the current being measured (with N1 turns) is called the primary winding (or primary side winding); the winding connected to the measuring instrument (with N2 turns) is called the secondary winding (or secondary side winding).
The ratio of the primary winding current I1 to the secondary winding current I2 is called the actual current ratio K. The current ratio when the current transformer operates at its rated current is called the rated current ratio of the current transformer, denoted by Kn.
Kn = I1n / I2n The function of a current transformer (CT) is to convert a large primary current into a smaller secondary current through a certain transformation ratio for protection, measurement, and other purposes. For example, a current transformer with a transformation ratio of 400/5 can convert an actual current of 400A into a current of 5A.