According to the device and principle of operation, measuring a voltage transformer is slightly different from a power step-down transformer. The only difference is that the VTs are counted on very low power: The voltage transformer's normal operating mode is the no-load mode. Here you can know how to test a transformer using multimeter.
The VT consists of a steel core assembled from electrical steel plates, a primary winding, and one or two secondary windings. As a result of manufacturing, the required accuracy class must be achieved: in amplitude and angle. Three-phase voltage transformers with zero leads are made on a five-rod magnetic core so that in case of a short circuit on the high voltage side, the total magnetic flux is short-circuited along with the core steel (in case of a short circuit in the air, there is a high current, which leads to overheating of the transformer). Three-phase transformers with a three-strand magnetic core, based on the above reasons, do not have any external zero leads and are not used to record "short circuits on the ground." The less the secondary coil of a voltage transformer is loaded (in other words, the greater the resistance in the secondary circuit), the closer the actual Kt transformation factor is to the nominal value. This is especially important when connecting measuring instruments to the secondary circuit, as the transformer ratio affects the measurement's accuracy. Depending on the load, the same voltage transformer can operate in different accuracy classes: 0.5; 1; 3.
Types of voltage transformers
Voltage transformer to be grounded - single-phase voltage transformer, one end of the primary winding of which must be tightly grounded, or three-phase voltage transformer, the neutral of the primary winding must be tightly grounded (transformer with weakened insulation of one of the leads - single-phase VT type SNF or three-phase VT type NTMI and NAMI).
Non-earthed voltage transformer - a voltage transformer in which all parts of the primary winding, including clamps, are isolated from the ground to a level corresponding to the voltage class.
Cascade voltage transformer - voltage transformer, the primary winding divided into several sections connected in series, power transmission from which to the secondary windings is carried out by connecting and leveling windings. It is used in ultra-high voltage units (110kV and above).
Capacitive voltage transformer - voltage transformer containing capacitive divider.
Two-winding transformer - voltage transformer having one secondary winding.
Three-winding voltage transformer - voltage transformer having two secondary windings: the main one and the additional one. Most VTs are three-winding.
The optoelectronic voltage transformer is an experimental type of VT; unlike a conventional (electromagnetic) voltage transformer, it operates on a different physical principle. It is intended for measurements in ultra-high voltage settings (750 kV and more) because electromagnetic VTs for such high voltages are too cumbersome.