The voltage transformer [1] (also known as PT, or VT) is similar to a transformer and is an instrument used for voltage transformation. However, the purpose of voltage transformation by transformers is to facilitate the transmission of electrical energy, so they have large capacities, usually measured in kilovolt-amperes or megavolt-amperes; while the purpose of voltage transformation by voltage transformers is mainly to supply power to measuring instruments and relay protection devices, to measure the voltage, power and energy of the line, or to protect valuable equipment, motors and transformers in the line in case of faults. Therefore, the capacity of voltage transformers is very small, usually only a few volt-amperes, a few tens of volt-amperes, and the maximum does not exceed one thousand volt-amperes. The entry provides an introduction to its basic structure, working principle, main types, connection methods, precautions, abnormalities and handling, as well as ferromagnetic resonance, etc.