A standard current transformer consists of a primary coil, a secondary coil, a core, insulation supports, and output terminals. The core is made of stacked silicon steel sheets. For high-precision standard current transformers, the core material typically uses high-permeability, low-loss permalloy, nanocrystalline alloys, and other high-performance magnetic materials. The winding turns are precise, uniformly distributed, and manufactured with excellent craftsmanship. The primary coil is connected in series with the main circuit, and when the measured current I1 passes through it, it generates an alternating magnetic flux within the core, inducing a corresponding secondary current I2 in the secondary coil. If excitation losses are negligible, then I1n1 = I2n2, where n1 and n2 are the number of turns in the primary and secondary coils, respectively. The current transformer's transformation ratio K = I1/I2 = n2/n1. Because the primary coil of the current transformer is connected in the main circuit, the insulation material between the primary coil and ground must be compatible with the primary line voltage to ensure the safety of the secondary circuit and personnel. The secondary circuit consists of the secondary coil of the current transformer, instruments, and the current coil of a relay connected in series.
The accuracy class of standard current transformers is much higher than that of ordinary industrial current transformers. Common classes include 0.01, 0.02, 0.05, 0.1, and 0.2 [7]. Current transformers of class 0.1 and above are mainly used in laboratories for precision measurement or as standards to test lower-class current transformers, hence they are also called standard current transformers. They are mainly used in controlled environments such as laboratories or metrology rooms as metrological standard instruments for value transfer, verification, calibration, or precision measurement of other working current transformers. In power supply lines, the current varies greatly, ranging from a few amperes to tens of thousands of amperes. To facilitate measurement by instruments and meters, it is necessary to convert it to a more uniform current. In addition, the voltage on power supply lines is relatively high, and direct measurement would be very dangerous for instruments, meters, and people. For this reason, current transformers are designed to play the role of current conversion and electrical isolation.