Directly measuring large currents or high voltages is quite difficult. In AC circuits, special transformers are commonly used to convert large currents into smaller currents and high voltages into lower voltages before measurement. These conversion devices are called current transformers and voltage transformers. The advantages of using transformers are that they isolate the measuring instruments from high voltage, ensuring the safety of the instruments and personnel; they can expand the measurement range of the instruments, facilitating standardization; and they can reduce energy consumption during measurement. Therefore, transformers are widely used in AC voltage, current, and power measurements, as well as in various relay protection and control circuits.
Instrument transformers are used in power systems as signal sources for measurement, control, indication, and relay protection circuits. They can insulate instruments and relays from the high-voltage, high-current measured circuits, allowing the specifications of instruments and relays to be much smaller and more standardized than those used for directly measuring high-voltage, high-current circuits. Instrument transformers are mainly used for measuring high voltage and high current, and are also called instrument transformers.